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Biomass based energy intermediates boosting biofuel production

This project has received funding from the European Union’s Seventh Programme for research, technological development and demonstration under grant agreement No 282873

Deliverable

Deliverable 5.8

Study on energy carrier use for entrained flow gasification

Workpackage: WP5 Deliverable No: D5.8 Due date of deliverable: 30/09/14 Actual date of delivery: 06/03/15 Version: Final / Vers.1.2 Responsible: KIT Authors: Prof. Dr. Edmund Henrich, Prof. Dr. Nicolaus Dahmen,

Andreas Niebel Contact: [email protected] Dissemination level: PU-Public

D5.8 / Study on energy carrier use for entrained flow gasification page 2/38

Publishable Summary For biomass gasification, a variety of technologies exist. For large-scale syngas generation with downstream synthesis of organic chemicals or synfuels, pressurized entrained flow (PEF) gasification has emerged as the preferred technology. The technology is flexible and can accommodate many different feedstocks, but at the expense of more technical effort for gasifier feed preparation.

The various feed preparation methods and feed properties as well as the gasifier feeding systems are described in context with the other interacting steps in the process network.

Gases and liquids (fluids) either with or without an entrained or suspended pulverized fuel are suitable feed forms. The fluid feed can be continuously transferred with pumps or compressors into a highly pressurized gasification chamber. Immediately at the gasifier inlet the fuels are mixed in special nozzles with pure oxygen (and steam) as the gasification agent; liquid and slurry fuels are atomized simultaneously. PEF gasification proceeds at high temperatures >1000 °C and high pressures up to 100 bar or more in a gasifier flame in the course of a second. The total residence time in the gasification vessel is only few seconds and the gasifier volume is correspondingly small. Solid or liquid fuels must be present as small particles or droplets with a sufficiently large surface area for complete conversion in few seconds. Ash is removed as molten slag and their melting behavior determines the minimum gasification temperature.

In biofeedstocks – mainly lignocellulose like wood or straw – the cellulose fibers prevent direct milling to a suitable powder and generate fiber muddles. A suitable PEF gasifier feed can be prepared from biomass pyrolysis products; preferred processes are fast pyrolysis or torrefaction. Biomass pyrolysis destroys cellulose fibrils and the chars are brittle and easily pulverized. The pulverized chars can be transferred to a pressurized on-site gasifier either with an inert gas as a dense char particle stream from a pressurized fluidized bed or as a slurry, after char suspension in the pyrolysis liquids or any other combustible (waste) liquid or even as a water slurry, as it is already practiced with pulverized coal. Slurries, especially bioslurries, are not only a suitable PEF gasifier feed form, but also a storage and transport form with a ca. 10 times higher energy density compared to the initial biomass. Bioslurry transport from many regional pyrolysis facilities to a large and more economic gasification/synthesis plant is a unique feed preparation and handling characteristic of the KIT bioliq® process.


Table of Content Publishable Summary ................................................................................................................. 2 Table of Content ......................................................................................................................... 3 Report ......................................................................................................................................... 4 1 A suitable gasifier for downstream synthesis of organic chemicals and synfuels .............. 4

1.1 Fixed-bed gasifiers ...................................................................................................... 5 1.2 Fluidized-bed gasifiers ................................................................................................ 5 1.3 Entrained flow gasifiers ............................................................................................... 6

2 Essential operating and design features of PEF gasifiers ................................................... 8 2.1 Operating conditions of PEF gasifiers ......................................................................... 8 2.2 Design characteristics of PEF gasifiers ..................................................................... 11 2.3 Interactions in the total PEF gasification/synthesis process network ........................ 14

3 Suitable feed preparation, feed properties and feeding systems for PEF gasifiers ........... 17 3.1 Gaseous fuel .............................................................................................................. 17 3.2 Liquid fuel, transfer and atomization ........................................................................ 17 3.3 Pulverised fuel, handling and transfer ....................................................................... 17

4 Experience from commercial PEF gasifiers ..................................................................... 20 4.1 Characteristics of commercial gasifiers ..................................................................... 20 4.2 Description of typical fuel feeding techniques .......................................................... 22 4.3 Status of the world gasification technology .............................................................. 26

5 Biomass conversion to PEF gasifier feed ......................................................................... 27 5.1 Focus on lignocellulosic feedstocks .......................................................................... 27 5.2 Torrefaction ............................................................................................................... 27 5.3 Fast pyrolysis ............................................................................................................. 27

6 Pilot projects for large-scale PEF gasification of biomass ............................................... 28 6.1 Lulea university, Sweden (formerly Chemrec): Black liquor feed ........................... 28 6.2 Feeding concept of the Choren Carbo-V process ...................................................... 29 6.3 KIT “bioliq” pilot facilities ........................................................................................ 29

7 Recent process developments ........................................................................................... 30 8 Experience from slurry gasification .................................................................................. 31 9 Summary and conclusions ................................................................................................ 34 10 Sources .......................................................................................................................... 36


Report

1 Asuitablegasifierfordownstreamsynthesisoforganicchemicalsandsynfuels

Gasification can convert almost all organic feeds with HHV > 10 MJ/kg into syngas, a mix of CO and H2, and consumes only 20-40 % of the O2 required for stoichiometric combustion. Syngas is a very flexible intermediate (platform chemical) and educt for the selective catalytic production of numerous valuable organic chemicals and fuels at certain temperatures and higher pressures [14],[15]. The alternative use as a fuel is syngas combustion in IGCC plants for the generation of electricity or high temperature process heat. All these technical possibilities are already applied commercially.

The main gasifier types are depicted in Fig. 1; essential design and operating characteristics are summarized in Tab. 1.

Fig. 1: Main gasifier types [8]

Tab. 1: Design and operating characteristics of the main gasifier types [14],[22]

fixed bed fluid bed entrained flow (EF) gasification conditions: fuel type solid solid powder, liquid, gas fuel size 10-1-10-2 10-2-10-3m ca. ≤ 10-4m pressure 1-30 bar 1-30 bar 1-100 (+) bar residence time 103-104s ca. 102 s 1-few s raw syngas purity low (tar, CH4) low (tar, CH4) high fuel/gas flow countercurrent mixed cocurrent design characteristics: reactor geometry cylinder cylinder cylinder reactor wall refractory refractory membrane, refractory bed material - sand (olivine) - carbon conversion >90% >95% >99% ash dry (solid) dry (solid) molten slag

Fuel spray(liquid, slurry,pulverized..)

Fixed bed gasifier Fluidized bed gasifier Entrained flow gasifier

Co-current

Counter current Stationary

Circulating

Downdraft

Updraft

Solid fuel Raw syngas

Ash (slag) Air (O2), steam

sand bed

solid fuel raw syngas

( fuel)

fluidising gassyngas recycle

ash

Bubbling bed

Solid fuel Raw syngas

Fuel

Fluidization medium

Ash

O2Solid or slurry fuel

(air)steam

Raw syngasMolten slag

Raw syngas

Fuel, air, O2, steam

- Raw syngas

AshFuel

Bed material recirculation

Ash (slag)

Solid fuel

Raw syngas

Air (O2)steam

Bed material

SlagFluidization medium

Fuel, air, O2, steam


1.1 Fixed‐bedgasifiersIn fixed-bed reactors, the feedstock is exposed to the gasifying agent in a packed bed that slowly moves from the top of the gasifier to the bottom, where the ash or slag is discharged. By moving through the reactor, the biomass passes through distinct zones of drying, pyrolysis, oxidation, and reduction. Usually the different types of fixed-bed reactors are characterized by the direction of the gas flow through the reactor and consequently are denoted as updraft, downdraft and horizontal (crossdraft) gasifiers. Depending on fuel and product gas application, a multitude of fixed-bed gasifier designs exist. On a small scale, fixed-bed reactors are used for district heat and power production up to fuel input capacities of 20 MWth. On a large scale, the updraft pressurized type has been successfully used since decades. Among all gasification technologies, the Lurgi pressurized fixed-bed gasifier is the economically most successful one. The gas of high calorific value generated by autothermal counter-current pressurized gasification, with a water steam/technical oxygen mixture being the gasification agent, is used for town gas, SNG and synthesis gas production for ammonia and FT-syntheses as well as being integrated in IGCC power plants. To study the influence of the operating pressure on the gas yield and composition, specific performance and thermal efficiency, a 10 MPa coal fired pilot plant (Ruhr100) was constructed and operated between 1979 and 1983. By increasing the pressure from 3 to 9 MPa, the methane yield was improved from 10.3 to 15.5%. At the same time, the oxygen demand in this plant was reduced by ca. 12%, due to the increased exothermic methane formation. The cold gas efficiency was raised from 70 to 80%. Based on the results, a detailed comparative study for SNG production for a plant design for 3 and for 9 MPa was performed. It turned out that lower investment costs and thus capital-dependent and maintenance costs are required for the high pressure alternative.

1.2 Fluidized‐bedgasifiersFor the production of synthesis gas for chemical syntheses fluidized-bed gasifiers with high carbon conversion efficiencies and low yields of hydrocarbons have been developed. In fluidized-bed gasifiers, fine fuel particles are rapidly mixed and heated e.g. by hot fluidized sand. Due to the intense mixing, the gasification reactions cannot be divided into local zones as in the case of fixed-bed reactors, but occur throughout the whole bed, leading to a uniform temperature distribution. The degree of fluidization can be small (bubbling fluidized bed, BFB) or high (circulating fluidized bed, CFB). The former reactors, which have a well-defined interface between the reaction zone of the fluidized bed and the freeboard above the bed surface, are commonly used because of their robust operation. In a CFB gasifier, there is no distinct interface between the fluidized sand bed and the freeboard; the entrained media and char are recycled back to the gasifier via a cyclone. The carbon conversion is considerably better than in BFB gasifiers, but operation is more complex und less robust. High temperature Winkler generators (HTW) exhibit high carbon conversion efficiencies and a low hydrocarbon formation. This has been achieved by two reaction zones in the reactor. In the BFB, the fuel is contacted with the main quantity of gasification agent, a mixture of steam and oxygen or air. Above the fluidized bed, an additional gasification agent is added to increase the temperature in the zone downstream of the gasification. These syngas generators have been commercially operated between 1956 and 1997 with feedstock capacities of up to 30 t/h and at pressures of up to 2.5 MPa. Efficient hydrogenating gasification to produce SNG was demonstrated with coal in a stationary fluidized-bed gasifier on pilot (7.5 t/h, 8 MPa) and semi-technical scales (300 kg/h of lignite, 10 MPa) within the prototype nuclear process heat project (PNP) in Germany. Helium of high temperatures of about 900 °C, generated in a high-temperature nuclear reactor (HTR), should be used for steam reforming to provide hydrogen in the subsequent coal gasification unit. Different types of coal were converted at around 920 °C to a methane-rich


raw gas with a carbon conversion degree of 65%. Within the same project, steam gasification of coal in an allothermal BFB reactor was studied on a semi-scale plant (230 kg/h). The required process heat should be generated in the HTR reactor, transferred to the gasification unit by 1000 °C hot helium, leading to an increased carbon efficiency of the overall process. In the test facility, electrically heated He was supplied to a tube bundle heat exchanger installed in the fluidized bed of the gasification reactor. Gasification temperature and pressure were 800-850 °C and 4 MPa, respectively. The first pressurized CFB pilot gasifier for biomass is operated in Värnamo, Sweden. The plant was run between 1996 and 1999 and has been shut down in 2000 after highly successful operation as a Biomass Integrated Gasification Combined Cycle (BIGCC) demonstration plant and a CHP gasifier plant. The feedstocks used successfully in the plant were different wood fuels, bark, straw and waste-derived fuels. Of the total fuel input of 18 MW, 6 MWel were fed into the public network, and 9 MWth were supplied to the district heating network of Värnamo during the EU funded Chrisgas project. In this plant, dried, comminuted wood fuel (e.g. wood chips) was fed by a lock-hopper and a screw into the air-blown CFB gasifier. Hot syngas carried the bed material up into a cyclone; solids returned to the bottom of the gasifier. Average gasification temperatures were slightly below 1,000°C at 1.8 MPa of operating pressure. Re-organized to the Växjö Värnamo biomass gasification centre (VVBGC) the plant has been rebuilt to produce clean synthesis gas for chemical syntheses starting in 2012.

1.3 EntrainedflowgasifiersIn fixed bed gasifiers the fuel particle size must be large enough to allow a free co- or countercurrent gas flow with little pressure drop through the gaps of the bed. The temperature in fixed and fluidized beds must be below the ash softening point somewhere below 1000 °C, because a sticky ash or molten slag would plug the gas flow channels. At lower gasification temperatures the raw syngas is contaminated with substantial amounts of unconverted tar vapours with high molar masses and boiling points and alkanes, especially methane and requires much effort for gas cleaning.

In order not to poison the sensitive synthesis catalysts, the syngas must be of high purity and free of dust and tar. Unlike most fixed-bed and fluidized-bed gasifiers, entrained-flow gasifiers (EF) are able to generate a gas practically free of tar with only little methane. Entrained flow gasifiers operate above 1000 °C, usually above 1200 °C, and the ash is removed as a molten slag. In a small gasifier volume with one or few s residence time, small solid fuel particles and little liquid droplets below ca. 10-4m size can be rapidly and completely gasified with > 99% carbon conversion. At higher pressures and partial pressures the gasification rate of solid particles accelerates with about the square root of the operating pressure (in the film diffusion regime!) and feed conversion can become as high as 99,9 %. At higher temperatures above about 1200 °C, tar vapours are efficiently gasified and an almost tar-free (e.g. benzene ≤ 100 ppmv) and low methane (≤ 0.5 % CH4) syngas is obtained for thermodynamic reasons. This simplifies the downstream cleaning and conditioning steps for raw syngas prior to the selectively catalysed syngas reactions for the production of organic chemicals and fuels like methanol, DME, Fischer-Tropsch-diesel etc. [21],[30]. Modern catalysts are sensitive to a number of poisons down to the trace level in the ppb range. Thus, a low-poison gasifier feed is desirable. Soon after the first application of gasifiers to coal conversion operated at atmospheric pressure, large scale operation of pressurized gasifiers at pressures between 2.5 to 4.0 MPa became state of the art.


The advantages motivating their development were: Increase in the reaction rate, (ii) Higher specific throughput, (iii) Increased methane yield at low temperature operation (for SNG production), (iv) Reduction of the gas volume to be treated, (v) Saving the work of compression for the subsequent use of the gas produced (gas turbine, methanol, ammonia, Fischer-Tropsch synthesis) Most syngas reactions are conducted at higher pressures – up to 100 bar or even more – for thermodynamic reasons. If the gasifier is already operating slightly above the downstream synthesis pressure, expensive intermediate syngas compression can be avoided. High pressure gasification is easily realized in pressurized entrained flow (PEF) gasifiers because of the short residence times and the corresponding small reactor dimensions, but at the expense of special feed properties, especially the use of pulverized solids. In order to guarantee a smooth integration into the total process chain, the choice of a suitable feed and feeding system must be based on a detailed knowledge of design and operating characteristics of PEF gasification and the downstream gas cleaning and synthesis steps.

PEF gasifiers are the preferred type for processes with downstream synthesis. The following script is therefore confined to production and handling of suitable feed forms for this gasifier type. The information in the script should enable the reader to propose a reliable sequence of operations and to select suitable equipment types for the conversion of a certain feedstock into a suitable PEF gasifier feed.

The main advantages and disadvantages of PEF gasifiers are briefly summarized as follows:

Advantages: 1. Short conversion time of one or few seconds due to high temperature and pressure and

small solid fuel particles ≤ 0.1 mm. This results in a small reactor volume. 2. Complete carbon conversion > 99% 3. Ash is removed as liquid slag 4. Clean and almost tar-free and low methane raw syngas;

this simplifies downstream gas cleaning 5. Membrane type PEF gasifiers allow rapid start-up and digest immediate shut-down without

damage. 6. No expensive intermediate syngas compression if p(gasifier) > p(synthesis) 7. High feedstock flexibility 8. High state of development, large 0.5 GW PEF-gasifiers are already in operation for coal

(Siemens EGT activities in China), 0.85 GW gasifiers are in development

Disadvantages: 1. At high syngas temperatures a significant percentage of the feed energy is converted into

less valuable sensible heat of raw syngas and lowers syngas efficiency. 2. PEF gasifiers require more effort for feed preparation, but are therefore flexible

“omnivores”. (Feed preparation efforts are usually overcompensated by process simplifications later on)

3. Membrane wall gasifiers (Shell, Siemens) have a higher heat loss through the membrane wall; only in large gasifiers (> 100 MW) losses are reduced to the percent range because of the lower surface-to-volume ratio in the gasifier chamber.

4. Poor heat recovery from the hot syngas with a simple and reliable water quench. With very expensive syngas coolers economic benefits are doubtful.


It should be clear that the preferred gasifier type will depend on the type of feed used as well as the application/use of the product gas.

2 EssentialoperatinganddesignfeaturesofPEFgasifiersFurther reading: [14],[15],[27],[26]

2.1 OperatingconditionsofPEFgasifiers

2.1.1 GasificationtemperatureAlmost all PEF gasifiers operate at temperatures above 1200 °C to get a molten slag with a sufficiently low viscosity in order of ca. 1 Pa·s for a sufficiently fast draining down the reactor wall by gravity. At such high temperatures and supported with efficient feed atomization, the thermodynamic equilibrium is quickly attained and easily estimated with the pressure-independent homogeneous shift reaction as the only key reaction:

CO + H2O ⇄ CO + H2

The disadvantage of a high gasification temperature is a higher O2-consumption and a reduction of the cold-gas or syngas efficiency. The lowest gasification temperature is usually given by the ash melting range, not by kinetic or thermodynamic restrictions. Black liquor gasification (see Lulea university, Sweden, section 6.1) is conducted at 950-1000 °C due to the much lower fusion range of the recovered mix of Na-salts of the cooking chemicals; this low temperature is an exception.

2.1.2 AshmeltingbehaviorFor slagging gasifiers the choice of ash melting ranges are of eminent importance for practicable gasification temperatures and efficiencies. For feedstocks with little ash and a high ash melting point, the addition of a flux could be reasonable to lower ash melting and gasification temperature. Prohibitive amounts of flux would be needed for high-ash fuels. Fig. 2 shows an oversimplified picture of the slag melting behavior, suited for rough estimates. A better representation is obtained from triangle diagrams with three basic constituents (see Fig. 3).

Conclusion: The ash melting behavior is of eminent importance for PEF gasifiers and must be considered already during feed preparation. Flux addition is possible as powder or solution and also in a separate gasifier feed line. Partial evaporation of constituents which reduce the slag melting point like common for potassium, must be taken into account. [26]


Fig. 2: Melting behavior of CaO/SiO2 slags (from [14])

Fig. 3: Slag melting behavior in the K2O-CaO-SiO2- triangle diagram (from [28])

2.1.3 GasificationpressurePressurised gasification has been developed from previous versions at atmospheric pressure; e.g. the Koppers-Totzek gasifier [14] developed during World War II is the precursor of the SCGP type. Most IGCC versions operate at pressures below 30 bar e.g. in the Wabash river, Buggenum and Puertollano plants (see chapter 4); PEF gasifiers for chemical synthesis operate usually at higher pressures of up to 80 bar or more.

At higher pressures the gasification rate increases with about the square root of pressure with a desirable further reduction of the equipment size. A synthesis of larger molecules from the small syngas molecules CO and H2 is favoured thermodynamically at higher pressures. Pressurizing already the gasifier at the beginning of the process is not only favourable for


accelerating the gasification, but also avoids more technical effort later on for intermediate gas compression prior to synthesis. Rather high pressures up to 80 bar have already been realized in PEF gasifiers for chemical synthesis applications in the Texaco gasifier (now GEE) with pulverized coal/water slurries and organic waste liquids [31].

Another feeding technique is a dense stream of entrained coal powder in compressed inert gases like N2, CO2 or recycled syngas. This “dry” feed mode is typical for IGCC plants up to 30 bar gasification pressure, but causes much dilution at higher pressures and needs technically more complex lock hopper systems.

Slurries or pastes can easily be pumped into pressure vessels up to 100 bar or more with various pump types like piston pumps or continuous screw, lobe and gear pumps or others which can digest corrosive or abrasive particles. A liquid must be efficiently atomized for fast gasification. The energy for efficient pneumatic atomization of liquids or slurries is supplied by the kinetic energy of the concurrent O2 gas stream with a relative velocity of > 100m/s. Volumes and velocity of the gasification agents O2 and eventually some steam are reduced in proportion with increasing pressure. In the same atomizing nozzle, the kinetic energy v² of

the O2- injection for slurry atomization becomes too small, because the energy is reduced with the square of the velocity. Since the O2-to-feed mass ratio is in the range between ½ to 1, the design and safe operation of the very narrow O2-nozzles for high pressure applications is a critical task.

2.1.4 ElementaryfeedcompositionandoxygenconsumptionThe feed is represented by the moisture and ash-free (maf) organic composition CxHyOz plus inert ash and moisture. The small amount of volatile heteroatoms N, S, P can be neglected to a first approximation, as well as trace impurities like heavy metals as potential catalyst poisons. The formula representation of lignocellulose (LC) is helpful for use in empirical chemical equations for mass and energy balances. A feed with the formula [CxHyOz + u (H2O) + v ash] requires (x+y/4-z/z) O2 for a complete stoichiometric combustion to CO2 and H2O, and (x-z-u) O2 for an allothermal gasification, an endothermal conversion reaction to CO and H2. The O2 needed for autothermal gasification at a given temperature in a PEF gasifier, can be found by trial and error in few iterative steps by variation of the O2 consumption, which is usually in the 30 ± 10 % range of stoichiometric combustion. The reaction enthalpy rH for autothermal gasification can be calculated from the reaction equation and the known, tabulated thermodynamic data. The reaction enthalpy rH increases with increasing oxygen consumption and must be just sufficient to heat the gasification products to a preselected temperature e.g. to 1200 °C and to cover some thermal losses.

The trial and error procedure starts by assuming a zero CO2 formation in the shift reaction and continues by iteration for finding a suitable x value, which reproduces the known equilibrium constant K:

K = ∗

∗ = K1(x=x1)……K2(x=x2)……..K3 etc…..;

x = CO2 partial pressure in bar


2.1.5 RawsyngascompositionandPEFgasifiermodelingModeling a PEF gasifier with known geometry means to predict the raw syngas composition from the known educt composition and the operating conditions [9],[20]. This can simply be done even with a pocket calculator, since at the high temperatures and pressures in combination with good liquid atomization and small fuel particles, the chemical equilibrium is attained in good approximation. Equilibrium conditions and O2 requirements can easily be estimated at a preselected temperature compatible with slag melting, usually above 1200 °C. High gasification temperatures > 1200 °C are thermodynamically out of the soot formation limits via the Boudouard reaction 2 CO ⇄ CO2 + C; or methane decomposition CH4⇄ C+ 2 H2; the equilibrium gas composition can therefore be calculated from the homogeneous water gas shift as the only key reaction:

CO + H2O ⇄ CO2 + H2 ; K = ∗

∗ = exp ( – 4.33) , T in K;

K (1500 K) = . = 0.3

In the usual temperature and feed composition range for gasification, the H2/ CO ratios in the raw syngas are about 0.5 or slightly above and the O2-consumption corresponds to ca. 30 ± 10 % of combustion.

2.2 DesigncharacteristicsofPEFgasifiers

2.2.1 ReactorgeometryandwallThe usual geometry of the hot gasification chamber is a vertical cylinder with thick pressure resistant steel walls. The inner surface is protected from corrosion and high temperatures either with a thick refractory wall or a thin membrane wall cooled with pressurized water. Both types are suited for gasification temperatures up to 1600 °C or even more. Refractory walls are suited for low-ash materials; membrane walls are preferred for high-ash materials up to 10% ash or more (see also Chemrec black liquor gasification, section 6.1 ).The ash melts and a viscous molten slag layer drains down the vertical gasifier wall by gravity and drops into a water cooling bath below. A water slurry with slag grains is then periodically removed from the gasifier bottom via a lock hopper system.

2.2.2 Membranewall(=coolingscreen)designA membrane wall consists of cooling pipes for pressurized cooling water with a temperature of 200-300 °C and is used to generate low temperature steam. A welded steel wall between the pipes generates a gastight containment. The inner surface is studded and covered with a castable refractory layer, usually SiC, to protect the wall from corrosion and erosion as is shown in Fig. 4 (right). Directly in contact with the membrane surface the slag solidifies. With increasing slag layer thickness, the temperature increases until at a temperature of normally above 1200 °C a honey-like slag with a viscosity of one or few Pa·s drains down at a slag thickness in order of about 1 cm. The section of the slag and syngas outlet at the bottom hole is cooled and controlled separately to prevent plugging. A narrowing syngas exit hole is detected by an increasing pressure drop of the gas flow and controlled by increasing the O2-flow and the temperature to melt away the slag. The high heat loss through the thin membrane wall is in the order of 0.1-0.2 MWth per m² membrane surface. In small pilot gasifiers of around ten MW, the surface-to-volume ratio is large and the heat loss amounts to > 10 % of the HHV of the feed and is unacceptably high. Only at a large technical scale of 200-1000 MW the heat loss reduces to several or down to one %, because of the smaller surface area per volume.


Fig. 4: Refractory solution (left) and Membrane wall construction (right) [29]

An advantage of a thin membrane wall compared to a thick refractory wall is the lower heat inventory, which allows rapid start up. During immediate shut down, e.g. in case of an emergency, the thin solidifying slag layer breaks off without causing damage by fusing with the refractory wall material. The necessity of operating at higher temperatures above the slag softening/ melting point range has also less desirable consequences: the O2-consumption increases and the energy efficiency of PEF gasification is reduced (see section 2.1.1 gasification temperature). [27],[26]

2.2.3 FeedtransferandheatingequipmentThe well- known heating and transfer operations for gases or liquids are not described. Slurry transfer: The selection of a suitable liquid pump type must take into account the viscosity and the abrasiveness of the particles in slurries as well as the final pressure. Screw, lobe and gear pumps or damped plunger pumps etc. are suited for the transfer of high viscosity liquids or slurries with abrasive particles. Previous warming or heating reduces the viscosity exponentially (the opposite is true for gases!) and eases pumping as well as pneumatic atomization in the gasifier nozzles. The first steps of biofeed preparation are performed in distributed regional pyrolysis plants. They produce preforms of bioslurries and take special care of storage and transport, but not yet to the final gasifier feed properties. Stepwise feed preparation in the sequence of operations seems to be a superior concept. Heating with process waste heat makes sense only at the end of the process at the gasifier site and includes all subsequent operations which become easier with the lower viscosity of a hot feed, like pumping, mixing, atomizing etc. Gasifier feed heating: Preheating and pressurizing the feed are stepwise operations done just before feeding. Some finishing steps for phase-separated feeds like additional mixing, liquid milling, slurry particle deagglomeration, better homogenization etc. can be integrated in-line into the successive feeding steps. Feed heating in a single step is probably not the best choice. Efficient feed heating to higher temperatures without loss of volatile constituents is possible only after pressurization. In view of the large amount of water in the feed, heating to > 100 °C under pressure is highly desirable e.g. to improve pumping and atomization. The thermal stability of pressurized biooils or bioslurries has not yet been investigated, but is urgently needed. In gasification campaigns of KIT at Freiberg (10 years ago) heating a ca.2 bar pressurized bioslurry was achieved by immersing a coil of the feed transfer pipe into a 90 °C hot water bath; this avoids slurry decomposition at hot spots.


If there is a decomposition risk for a hot feed stream during a longer transfer stop, the pipe must be flushed immediately and automatically with an inert liquid or gas. The same action is also needed after a stop of a pneumatic particle or a slurry transport to prevent pipe plugging by sedimentation. A washout of the sensitive pipe section to the final spray nozzle is a reasonable general measure after any feed stop to avoid trouble later on.

Heating the O2-stream is also desirable, not only to increase the gasification efficiency, but also the injection velocity (kinetic energy) in the gasifier nozzles for better fuel atomization.

2.2.4 BioslurryfinishingwithcolloidmixersColloid mixing improves the homogeneity – even for two-phase biooils and slurries – and reduces the viscosity of biooil/char slurries by de-agglomeration of larger char particle aggregates with a high shear stress in the order of dv/dx ≈ 10-4 s-1. Colloid mixers are applied in the construction industry for the generation of very homogeneous cement lime. The robust mixer design e.g. of MAT (Mischanlagentechnik GmbH, Augsburg, Germany) is a cylindrical vessel with a strong axial stirrer with some rectangular perforated blades extending over the total vessel diameter. The fast rotation is generated by a powerful motor and heats the viscous slurry gradually by several 10 K. At still faster rotations, the stage of colloid milling would be reached, but this is not recommended because of the extremely high power consumption.

The optimum place of colloid mixing in the sequence of bioslurry preparation steps is probably the last operation at atmospheric pressure at the central gasifier site, e.g. not at the pyrolysis plants. This includes the possibility to combine the mixing step with further heating as well as char particle addition to increase the particle load. Since shortly before gasifier injection, a long term stability of slurry properties is not needed, there are large and yet uninvestigated possibilities of feed treatment. A conclusion from our practical experience on bench and pilot scale is, that colloid mixing – or an equivalent operation – is an indispensable step for a smooth operation of the feeding line.

The high power consumption is not used for particle de-agglomeration but is dissipated into heat. The very poor energy efficiency below 1 % is typical in general for all diminution processes. The energy consumption can be estimated from the temperature increase ΔT, assuming a specific slurry heat capacity of ca. 2 kJ/kg and a warming-up of 25 K. A 50 % motor efficiency results in 100 kJ/kg electricity consumption. Electricity generation via conventional combustion requires ca. 3*100 = 300 kJ/kg of the initial bioenergy (ca. 18 MJ/kg). Assuming a slurry yield of 80 mass% based on the initial biomass, this amounts to 240 kJ/kg biomass or ~ 1 % of the bioenergy. Even warming up by 50 K with a 2 times higher energy consumption could be acceptable, since a large part of the electric energy is not lost but used for feed heating. [17],[18],[19]

2.2.5 DesignofthegasifierburnerThe autothermal gasification reactions in a PEF gasifier proceed in an exothermal gasifier jet flame, similar to combustion, but usually with pure O2 and only 30±10 % of combustion stoichiometry. It is a turbulent diffusion flame for safety reasons, not a premixed one. Fuel and gasification agent O2 are mixed directly at the exit of the burner nozzle in the gasification chamber.

Mixing of fuel and oxygen can be achieved pneumatically or mechanically. We have no experience with mechanical mixing and the following info is from hear-saying: Thin ca. 10-4 m thick sleeves are peeled off mechanically from soft feed solids (hot vacuum residues) by rapidly rotating “knives”; the sheets are then disrupted into small pieces and distributed equally within the O2-stream. In this script the discussion is restricted to pneumatic


fuel atomization and mixing by turbulence generation, which requires a liquid feed with a viscosity < 0.3 Pa·s (comparable to edible oil). It was found, that warming up a bioslurry by 50 K reduces the viscosity by an order of magnitude [18]; experimental results are shown in Fig. 5. Warming up from 20 °C to ca. 120 °C under pressure would thus reduce an initial 10 Pa·s bioslurry at ambient temperature to 0.1 Pa·s in the hot burner nozzle, which is suited for efficient atomization.

Fig. 5: Decrease of slurry viscosity with increasing temperature [18]

2.2.6 LargestcredibleaccidentinaPEFgasifierLargest credible accident is an unexpected sudden stop of the fuel inflow and a continuation of the O2 -flow into the large inventory of hot, high pressure raw syngas with the result of an explosive liberation of considerable combustion heat. Even a drastic drop in the fuel heating value can cause such an accident, since the slightly exothermal autothermal gasification reaction is replaced by a strongly exothermal combustion. An undetected phase separation in biooil or bio-slurries can create such a situation with an undetected, longer plug of a low-HHV aqueous biooil phase section in the feed line. Such accidents must be prevented by an immediate stop of the O2 flow within 1 s. A serious emergency situation must be reliably and timely detected within 1s, not only by a single measurement but redundantly by several diverse methods. The accidental explosion risks are extreme in small pilot gasifiers with only a single feed line. Large technical PEF gasifiers in the 100-1000 MW range have several independent feed lines; a breakdown of a single one will be less serious and can probably be compensated by the others. Reports on the management of such problems in operating commercial gasifiers have not been found.

2.3 InteractionsinthetotalPEFgasification/synthesisprocessnetworkEvaluation of feed preparation and properties, handling and feeding systems for PEF gasifiers requires a thorough knowledge of all process steps and their mutual interactions in the total PEF gasification/synthesis process network.

2.8 2.9 3.0 3.1 3.2 3.3 3.4 3.5 3.6-5

-4

-3

-2

-1

0

1

2

3

0 %

20 %

26 %

30 %33 %

Feststoffgehalt:

Loga

rith

mus

der

Vis

kosi

tät[

ln(

)]

reziproke Temperatur [1000/Kelvin]

70°C 50°C 30°C 10°C

reciprocal temperature [1000/Kelvin]

loga

rithm

of v

isco

sity

[ln

(/P

as)]

solid content


The process can be divided into four successive parts; the front end parts 1 and 2 deal directly with feed operations, but operations in the tail end part 3 and 4 can have an influence on the selection of reasonable operations in the front end. Mutual interactions in the network are complex.

1. Feed preparation from fossil fuels or biomass feedstocks

2. Gasifier feeding and gasification reactions

3. Syngas cleaning and conditioning

4. Synthesis and product work-up

Such processes cannot be considered as a simple sequence of steps in a one-way street, but are a mutually interacting network with recycle of materials and energy. An extensive systematic analysis of all the complex potential interactions is outside of our capabilities. With a few examples it will be shown, how the technology of an individual step can be affected or modified by the choice of conditions or equipment, in other steps of the interacting network.

2.3.1 AdditionalfeedfromrecyclestreamsWhen looking to the feed and the plant capacity of PEF gasifiers, one should not forget, that a significant increase of the feed flow can be caused by recycle streams from the process. Three examples are mentioned:

Unconverted feed from chemical cooling: In two-stage gasifiers a “chemical cooling” step improves the gasifier efficiency. In most cases this is an endothermal gasification of pulverized coal or char with H2O or CO2 in the hot syngas. It is process inherent that during efficient “chemical cooling” the fuel cannot be converted completely, since the gasification temperature goes down until at around 1000 °C the gasification rate becomes too low. The unconverted fuel fraction is then recovered and recycled.

Soot gasification: Undesired solid carbon can be generated from gaseous precursors in form of soot. Thermodynamic soot formation limits can be estimated from the equilibrium of the Boudouard reaction 2 CO ⇄ C + CO2 ; and methane formation CH4 ⇄ C + H2 ; [33]. The very high PEF gasification temperatures are above the soot formation limits, but during cooling of the hot raw syngas the temperature limits are crossed. During a water quench the limit is crossed so quickly, that there is not sufficient time for much soot formation and at the rather low final quench temperature the soot formation rate is already too slow.

Excessive soot formation at slow cooling rates, e.g. in heat exchangers, especially in presence of soot formation catalysts, should be avoided. Separation of colloid-like soot suspensions from the wash water circuit is a nasty operation. Separated ash and soot solids can be gasified and recycled to the main stream or combusted with heavy oil to control build-up. Development work to this topic is still continuing.

Side-product recycle: A complete syngas conversion is unlikely. The few percent of unconverted syngas are usually diluted with N2 and complete recycle without bleed stream would result in an unacceptable N2 accumulation. Unconverted syngas is therefore usually combusted. An accumulation risk does not exist for by- and couple products which are separated from or together with the main product; they can be recycled completely.

Side-product recycle to the main gasifier feed can significantly increase the process yield. This should be mentioned in comparisons to guarantee a fair evaluation of different process versions, especially if additional energy import is the consequence of by-product recycle.


Recycle of liquid by-products can be used for example to suspend pulverized pyrolysis chars if the amount of pyrolysis liquids is not sufficient. Recycle of hot gases or vapours can be used as a carrier stream for pulverized fuels. Such options can even affect the design of a pyrolysis process.

Less valuable by- or side-products can be combusted to contribute to the energy supply of the process either in form of high pressure steam or electricity. A surplus of electricity can easily be exported and thus contribute as a credit to the main product.

2.3.2 HeatingoffuelandoxygenHeating the PEF gasifier fuel and also the O2-stream improves the energy efficiency of the process and does not require valuable high temperature heat. There are many heat recovery opportunities in the process which can provide sufficient energy for that purpose, e.g. product cooling or heat recovery from the cooling screen (both at 200+ °C). Such options must be weighed against the expenses for heat exchange installations and their operation. Heating up the feed e.g. by T ≈ 120°C would reduce the sensible heat requirement for a raw syngas at 1200 °C by ca. 6 %, due to the increase of the specific heat capacity with temperature. Heating of biooil or bioslurries is limited due to their thermal sensitivity. Much higher preheating temperatures are possible for coal/ water slurries.

2.3.3 CO2fortransferofpulverizedfuelA CO-shift is usually necessary to adjust the desired H2/CO-ratio for synthesis. The considerable volume of pure CO2 is separated downstream and can be used e.g. for the transfer of pulverized fuel into the PEF gasifier.

2.3.4 GasificationtemperatureandashmeltingAll PEF gasifiers are slagging gasifiers and operate with molten slag at temperatures above the ash melting point (m.p.). The minimum gasification temperature is therefore given by the ash melting behaviour. A free draining down of molten slag at the inner gasifier wall into a water bath at the bottom requires a lower slag viscosity of about 1 Pa·s (edible oil). Thus the ash composition in the feed, eventually modified by flux addition, is a feed property of eminent importance. Volatility of certain constituent like KCl in biomass, which reduces the melting point, must be considered in addition.

2.3.5 RawsyngascompositionThe raw syngas composition in a PEF gasifier is easily calculated from the feed composition (CHO, ash and moisture) and the gasification temperature, because thermodynamic equilibrium is rapidly attained. Temperatures > 1200 °C are above the soot formation limit and the homogeneous shift reaction CO + H2O ⇄ CO2 + H2 is the only key reaction and sufficient for the estimate: equilibrium constant K (at 1500 K) ca. 0.3.

2.3.6 CatalystpoisonsThere are many catalyst poisons – S, N, Cl,…– which must be removed to the ppb level. An exclusion of feedstocks with a high poison content could make sense. Biomass e.g. cereal straw contains much KCl, partly from fertilizers. The KCl from fresh “yellow” straw is washed out in the field by rain and the K is recycled directly to the soil. The remaining “grey” straw contains only little KCl and is a better gasifier feed.

The high N-content of protein rich biomass (16% N in proteins) is party converted to HCN and NH3 in gasifiers, which are catalyst poisons. Thermochemical conversion of protein rich biomass with > ca. 2 mass% nitrogen is less desirable because N-fertilizer production in the


Haber-Bosch process requires much energy. In biochemical fermentation processes however, the N can act as a fertilizer for the microorganisms.

3 Suitablefeedpreparation,feedpropertiesandfeedingsystemsforPEFgasifiers

All gases and liquids either without or with entrained or suspended pulverized fuel are suited as PEF gasifier feed, if their heating value is high enough to attain the desired high gasification temperatures with an acceptably low O2-fraction compared to combustion stoichiometry. Complete feed homogeneity is not required, two-phase emulsions or suspensions (slurries) of powders are acceptable, if composition fluctuations at the gasifier inlet are smoothed out in less than few percent of the gasifier flame residence time (ca. 1 s). The majority of PEF fuels are solids, above all pulverized hard coal.

3.1 GaseousfuelGas compression for transfer is standard technology for gaseous fuels as well as for O2 or air as gasifying agents. Pipeline supply of pressurized natural gas is desirable as auxiliary fuel.

3.2 Liquidfuel,transferandatomizationLiquids with a viscosity up to ca. 0.3 Pa·s (like edible oil) can be pumped with standard equipment and can be atomized pneumatically. Warming and heating reduces the liquid fuel viscosity exponentially and must be done anyway for energy efficiency reasons. For biooils from FP an order of magnitude viscosity decrease was found for each 50 K temperature increase. This is also reasonable assumption for other liquid fuels. Heating should be done in the gasifier feeding line immediately prior to gasifier injection, to reduce heat losses and to limit thermal decomposition during longer delays.

Heating in two steps is reasonable; the first step up to ca. 60-80 °C at atmospheric pressure, eventually in combination with colloid mixing, in closed vessels to prevent the release of toxic (e.g. formaldehyde) volatiles with pungent, acrid smell. In a second step the heating can be continued under pressure to temperatures above 100 °C and as high as reasonably possible without feed decomposition. A suitable final temperature depends on the feed type and has not yet been investigated, 150 °C and a sufficiently short residence time seems to be a maximum for biofuels. Thermal stability of fossil fuels is higher.

Two different homogeneous liquids or phase-separated liquids like biotar and water-rich biooils (“pyrolysis waters”) can be fed as separate homogeneous phases or in form of an single emulsion prepared shortly prior to gasifier feeding, either with or without a stabilizing agent. Some experience with pumping and handling of two-phase condensates has been obtained with pyrolysis liquids in the KIT pyrolysis pilot facilities [34].

3.3 Pulverisedfuel,handlingandtransferThe reason for the large feedstock flexibility of PEF gasifiers is the conversion of all types of solid fuel into a fine powder with a particle size usually below 10-4 m, as is practiced in pulverized coal power stations. The optimum diminution methods for the different feedstocks are different. Roll crushing mills flushed with hot flue gases to prevent dust explosions are used for simultaneous drying and grinding of hard coal down to a mean size of ca. 50 µm. The risk of spontaneous ignition and dust explosion must be carefully controlled. The abundant lignocellulosic biomass like wood, straw or other herbaceous biomaterials cannot be milled directly, because about 50 % of the biomass constituents are cellulose fibers


and produce a fibrous product muddle like cotton-wool. A suitable brittle char material for easy milling is obtained by pyrolysis (see next chapters 5.2 and 5.3).

There are two already commercially practiced ways to feed pulverized fuels into a PEF gasifier:

(1) Pneumatic dense phase particle transport with inert gas (“dry” transfer)

(2) Particle transport as a liquid suspension (“wet” transfer)

Pneumatic fuel particle transfer: Pneumatic or dry transfer requires a free flowing fuel powder with a typical size of 50 µm and a lock hopper system as shown in Fig. 6. The particles are fluidized with a compressed inert gas like N2 (from an on-site ASU), CO2 (from the CO-shift during syngas conditioning) or recycled syngas. A dense particle stream is entrained in the flowing gas via short pipe connections from the fluidized bed into the gasifier. The specific surface of the small particles with <10-4 m size is large enough for complete conversion in ca. 1 s at the high temperatures and pressures. After mixing with O2, the reaction proceeds in the film diffusion regime at the particle surface and is above ca. 1200 °C therefore almost independent from the temperature.

Fig. 6: Lock hopper system [35]

Slurry transfer: Transfer of a fuel particle suspension in a liquid, either water, a liquid fuel, or a combustible liquid waste is technically simpler and cheaper than lock hopper systems. Many commercial PEF gasifiers are fed with pulverized coal/water slurries. Pressures as high as 80 bar and coal/water slurries with only 30 mass% water and 70 % coal dust have been gasified by Texaco already in the 80ies [3]. Feeding of coal/water slurries to a PEF gasifier with extruders has also been investigated on a pilot plant scale [36],[37]. The required water fraction could be reduced to ca. 15%, thus improving the energy efficiency. In a free flowing slurry, the concentration of regular formed (aspect ratio ca. 1) particles can be slightly above 50 % by volume. This corresponds to slurries with ca. 70 mass% non-porous hard coal and 30 mass% of water. A comparable volume ratio with the highly porous biochars from pyrolysis results in only 30 mass% char and 70 mass% liquid.


In the range of the sedimentation density with a particle volume fraction of about half, the viscosity rises steeply; the particles are so densely packed, that free movement of individual particles in the suspending liquid is hindered. With smaller particles and a broader size distribution, the sedimentation limit can be shifted to slightly higher values of ca. 40 % by volume. The slurry viscosity below sedimentation density is determined mainly by the suspending liquid and decreases exponentially with the temperature. In multicomponent liquids like biooil, this can be amplified by melting or better solubility of some constituents, but has not yet been investigated in detail. Feed injection, atomization and mixing with O2: These operations are usually combined and realized simultaneously in special nozzles at the gasifier inlet. Design details and operating experience are in most cases kept secret and are not reported in the accessible literature. The turbulence energy for rapid mixing and atomization is obtained pneumatically by a large velocity difference > 100 m/s between the adjacent co-current fuel and O2-streams with a small inclination angle between each other. Liquids or slurries are injected with few m/s and the O2 in a fast jet with v = 100 m/s or more. The kinetic energy v2 supplies the

energy for efficient atomization and is proportional to v2. A possible nozzle geometry is a circular hole surrounded by an annular slit as is shown in Fig. 7.

Fig. 7: Nozzle geometry for injection and mixing of liquids or slurries and O2 (adapted from [14])

The rough fuel/O2 mass ratio is in a range of about 2:1. Liquid fuel densities are ca. 1000 kg/m3 and preheated O2 at 1 bar has about 1 kg/m3. At atmospheric pressure the O2-gas volume is about 1000 times larger than the liquid fuel volume, but at 100 bar only a factor of ten. At higher pressures the nozzle orifice area for the O2-jet becomes very small and sensitive since the high velocity must be maintained. Further options are two opposite fuel jets hitting each other with high velocity; the possibility of mechanical atomization was already mentioned.

gasification

agent

(O2, air, …)

liquid or

slurry

fuel

pipe wall

Feed orifice


4 ExperiencefromcommercialPEFgasifiers

4.1 Characteristicsofcommercialgasifiers Fig. 8 outlines the geometry of burner installation in some gasifiers. Characteristics of important commercial PEF gasifiers are put together in Tab. 2. Other PEF gasifiers not included in the table do either not show new technical characteristics or have less importance in the market; companies are Mitsubishi Heavy Industries, MHI-gasifier; Electric Power Development Company (Japan), EAGLE-gasifier; Pratt & Whitney Rocketdyne, PWR-gasifier. (see [14])

Fig. 8: Geometry of burner installation in some gasifiers

A swirling flow of burner flames and the complex mixing patterns in the gasification chamber are not described. The main conversion takes place in the turbulent gasifier flame in the course of 1s or less. The volume of the total gasification chamber is probably well mixed by the jet flame and the total gas residence time amounts to few seconds.


Tab. 2: Characteristics of important commercial PEF gasifiers

company Lulea university

Shell Krupp-Uhde

General Electric

Siemens

(former company) Chemrec (Texaco) (Texaco) (DBI, Noell,FE) name abreviation SCGP Prenflo GEE SFGT temperature °C ca.1000°C ≥ 1400 1350+100°C 1200-1600 pressure bar 32 30-40 ~80

reactor wall type membrane membrane refractory membrane gasification stages 1 stage 1 stage 1 stage 1 stage 1 stage syngas flow downflow upflow upflow downflow downflow syngas cooling water gas quench

+cooler gas quench +cooler

water or syngas quench

ash removal slagging slagging slagging slagging slagging discharge feed type slurry dry dry slurry dry, slurry gasifying agent O2 O2 (steam) O2 O2 O2 feed burner axial horizontal axial commercial plants MW

Buggenum Puertollano many Several modular 500 MW coal gasifiers in China, new 850 MW modul size

Status pilot commercial commercial commercial commercial company ConocoPhillips Inst. Clean Coal Tech. (former company) (Dow) Shanghai name abreviation E-Gas ICCT OMB temperature °C ≥ 1200 ≥ 1200 pressure bar 65 reactor wall refractory refractory (membrane) gas stages 2 stages syngas flow upflow downflow syngas cooling ash removal slagging slagging discharge lock hoppers feed type slurry slurry, (dry) gasifying agent O2 feed burner opposed side commercial plants MW

Wabash River 3 plants 2005 ca. 1000 tpd

Status commercial development started 1995


4.2 DescriptionoftypicalfuelfeedingtechniquesExample 1: Shell Coal Gasification Process (SCGP) [3], see Fig. 9

Hard coal is milled in roller mills and simultaneously dried with hot flue gases, similar to conventional technology in coal-fired power stations. After pressurization in lock hoppers a dense-phase stream of pulverized coal is transferred to the PEF gasifier burners. In IGCC power plants the N2 transport gas is preheated; for chemical applications with downstream synthesis some syngas is recycled to avoid syngas dilution with N2. To reduce the O2-consumption and to improve the gasification efficiency, also the stream of compressed O2 as the gasification agent is preheated and usually mixed with hot steam as a moderator.

The high gasification temperature of ≥ 1400 °C suppresses hydrocarbon (tar and methane) production thermodynamically. The ash melts to a slag and drains down the refractory-lined wall into a water bath at the bottom. The ca. cm-thick slag layer protects the inner reactor wall from erosion and corrosion. The slag cools and solidifies partly in direct contact at the wall. The liquid slag drops into the water bath below and is crushed into gravels; a slag gravel/water slurry is removed at the bottom via a lock hopper. The hot raw syngas is quenched with cooled recycled syngas to below the ash softening point. Almost half of the ash, some unreacted carbonaceous coke material and newly formed soot are recovered and recycled to the gasifier burner.

The SCGP was first applied commercially in the IGCC plant in Buggenum, The Netherlands, in 1993.

Example 2: General Electric Energy (GEE), formerly Texaco, see Fig. 10

The refractory lined gasifier in the IGCC plant operates at 1350 ±100 °C and 30 bar, but for chemical applications with downstream synthesis up to 80 bar pressure. An axial downflow burner at the top is fed with a pulverized coal/water slurry and O2 (steam). The molten ash melt flows down the walls by gravity and is quenched in a water bath at the bottom and removed via a lock hopper system. The hot raw syngas is either cooled to 700 °C by a water quench or in a radiant cooler for high pressure steam and power generation. The quench version is preferred, because it efficiently removes ash/slag, soot and corrosive impurities.

An application is the IGCC Polk power station, Tampa Electric Corp., which uses a high ash, high sulfur coal. Several gasifiers have also been built in chemical plants, e.g. US. Eastman Chemicals (Kingsport, TN), e.g. for acetic acid and acetic anhydride production. Newer developments focus on the cogeneration of chemicals and electricity.


Fig. 9: Shell coal gasifier (from [14])

Fig. 10: General Electric Energy (GEE) gasifier (from [14])

Example 3: Siemens Fuel Gasification Technology (SFGT) (previously Deutsches Brennstoff-Institut (DBI), Babcock Borsig Power (BBP), Noell and Future Energy (FE)), [25],[27], see Fig. 11

The technology was developed since 1973 by the DBI, Freiberg, East Germany, under the name GSP (Gaskombinat Schwarze Pumpe)-process for the salt (NaCl) containing brown coal in central Germany. The technology has been selected for the KIT bioslurry gasification process bioliq®, because many biomass feedstocks e.g. cereal straw, contain KCI in the 1 % range and cause similar corrosion problems. Coals contain usually much less chlorine but more sulphur than biomass.


Fig. 11: Siemens Fuel Gasification Technology (SFGT) gasifiers; different setups: with cooling screen (left) and with cooling wall (right); from [14]

The SFGT gasifier burner is mounted in the axis at the top of a cylindrical gasification vessel and is fed either pneumatically with pulverized coal in a dense-phase flow with an inert N2, CO2 or raw syngas carrier stream or with a fuel powder slurry. The slurry is pneumatically atomized with a high velocity (>100 m/s) oxygen jet.

To keep the pressure resistant outer steel cylinder at lower temperatures 230 °C, the gasifier chamber has either a thick refractory liner suited for low ash materials or a cooled membrane wall with a castable inner SiC liner for high ash materials. The membrane wall is a gastight cylinder containment of pipes, cooled with pressurized water at ca. 250 °C. An amount of at least 1 % ash with a suitable melting point is required in the feed to produce a protective ca. 1 cm thick slag layer at the inner wall, to achieve sufficient thermal insulation and protection against corrosion. An acceptable heat loss through the membrane wall is between 0.1-0.2 MW/m2 and depends on the gasification temperature and the ash/slag composition and viscosity characteristics. This heat loss is the reason why only large gasifiers with 100-1000 MW capacity have a sufficiently low specific cooling screen surface area to keep the heat loss in a reasonable range of few percent down to ca. 1%.

The first commercial 130 MW GSP gasifier has been operated since 1978 in the Schwarze Pumpe facilities, East Germany, for almost 20 years without serious trouble and with many different feeds, mainly pulverized fuels fed pneumatically via lock hoppers.

For a few years a number of 500 MW Siemens PEF gasifiers are now in operation in China for different applications, mainly power and chemicals generation from coal. Development work for a huge almost ca. 1 GW PEF gasifier with ca. 4+ m diameter is almost ready.


Example 4: Mitsubishi Heavy Industry (MHI) [24]:

The MHI process is a 2-stage, air-blown (slightly O2 enriched) PEF gasifier, divided into a lower combustion section and an upper reducing section without O2. About half of the pulverized coal and all O2 are fed to the combustor section. At a very high temperature of ca. 1700 °C the ash melts without any flux addition and a slag/water slurry is removed at the gasifier bottom with a lock hopper system.

The other half of the coal powder is added without oxygen into the reducing gasifier section; the majority is gasified in endothermal reactions; the upflowing syngas leaves the 2nd stage at 700 °C with non-sticky ash particles. Unconverted coal dust is recovered in a cyclone and a successive candle filter and recycled to the first high temperature stage; a carbon conversion > 99.8 % has been achieved. The two-stage concept with chemical quenching to a low syngas exit temperature improves the gasification efficiency at the expense of some technical complexity.

Example 5: ConocoPhillips E-Gas process (developed by Dow, formerly Destec)

This is also a two-stage, slagging PEF coal gasifier with syngas upflow similar to the MHI-type but fed with a “wet” slurry. The coal-slurry feed contains 30-50 mass% water. About 80 % of the slurry is injected with 95 % O2 into the lower stage operating at ca. 1400°C and 30 bar. Molten slag flows down through a hole at the bottom into the water quench bath and is removed without a lock hopper system. The upflowing syngas reacts in the upper stage with the 20 % rest of coal-slurry and leaves with about 1050 °C. After cooling to ca. 370 °C, the crude syngas is cleaned and the recovered char and ash particles are recycled pneumatically in a separate feed line to stage 1. The only operating E-Gas gasifier is the IGCC repowering project at Wabash River (West Terre Haute, IN, USA).

Example 6: Shanghai University, ICCT OMB process [32], see Fig. 12

The ICCT (Institute of Clean Coal Technology) at Shanghai University started with the development of their OMB (Opposed Multiple Burners) gasifier in 1995. The standard version is a refractory lined cylinder vessel with four opposed side burners at the top (“bottom-up, top-down” of the Shell type). The coal/water slurry feed is produced in a horizontal ball mill and conveyed with membrane piston pumps. The slurry technology is conventional, the opposed side burners at the top are unique.

Meanwhile several commercial plants with capacities > 10³ t/d are in operation at pressures up to 65 bar and carbon conversion rates > 98% [38]. The hot raw syngas is quenched with water; the slag is cooled and removed with a water bath at the bottom via a lock hopper system. The ICCT has now extended its development work also to membrane walls with cooled pressurized water and pulverized coal feed with N2 or CO2 as carrier gas. The activities cover nearly all PEF technologies, those from western countries and also their developments.


Fig. 12: Shanghai University (China), ICCT OMB process (from [14])

Examples for PEF gasification of biomass e.g. the Chemrec (now university Lulea, Sweden) “black liquor” gasification or the Choren CarboV process (bankrupt in 2011) and the KIT concept are treated in chapter 6.

4.3 StatusoftheworldgasificationtechnologyWorld syngas capacity: The 2010 status was taken from the internet (www.netl.doe.gov). The worldwide syngas production capacity in 2010 from different feedstock is shown in Tab. 3 together with a prediction for 2016.

Tab. 3: Status of the world gasification technology; capacities in MW (thermal) and numbers of gasifiers in brackets

Feedstock (gasifier number) capacity expected in 2016 coal (gasifiers) 36,3 (201) 75,5 (276) petroleum (gasifiers) 17,9 (138) 17,9 (138) gas (gasifiers) 15,3 (59) 15,3 (59) petcoke (gasifiers) 0,9 (5) 12,9 (21) biomass/waste (gasifiers) 0,37 (9) 0,4 (11) total capacity (gasifiers) 70,8 (412) 122 (505) The already dominant role of coal as feedstock will be further amplified within the next years. The main capacities are located in Asia, especially China, also in the near future.

Products: The main products from syngas are chemicals (45 %) and liquid motor fuels 38 %; power and gaseous fuels comprise 11 and 6 %. The sequence will not change within the next years.

China is today the leading country for the development and application of all foreign and own types of coal gasification technologies for the production of chemicals and synfuels beyond the traditional products [39]. All western-type PEF gasifiers are competitors for the best technology in the actual development mix.


5 BiomassconversiontoPEFgasifierfeed

5.1 FocusonlignocellulosicfeedstocksLignocellulose (LC) is the structural material of the cell walls and with almost 90% the most abundant terrestrial biomass. All large-scale technical biomass applications must therefore rely on this material, which is not in direct competition to food supply [7],[8]. The most important feedstock sources are low quality wood residues from the round wood harvest in forestry and herbaceous agricultural residues like cereal straw from wheat, maize, rice and barley or bagasse. The latter contain much ash with 5-10 % or more, contrary to wood with only 0.5-2 %. A simplified averaged elemental composition of a moisture and ash free (maf) lignocellulosics is C1 H1,44 O0,66 or in an oversimplified formula C3H4O2, which represents formally a 50 %/50 % mass mix of carbon and “water” (in form of –OH and –H groups). This is a helpful simplification for rough estimates of the chemical behaviour (except HHV). In reality, the LC contains 10-50 % water and between 1 -10 % ash, woody biomass around 1 % and herbaceous biomass 5-10 % or more (e.g. rice straw) mineral ash constituents in addition.

Production of a fine powder suited for PEF gasification by milling of biomass, did not succeed; the result was a wadding-type cellulose fiber muddle, which could not be correctly conveyed. Cellulose fibers are therefore first decomposed by depolymerization by pyrolysis in an oxygen–free atmosphere. Two promising pyrolyzing modes are being investigated: Torrefaction or low temperature pyrolysis at 270-300 °C and fast pyrolysis (FP) at 500±30 °C.

5.2 TorrefactionTorrefaction takes ca 103 s-depending on the material size – and generates ca. 80+ mass% of a highly porous, hydrophobic char material (“roasted wood”) and ca. 20 mass% of combustible gases, which contain ca. 10+ % of the bioenergy. Their combustion with air should supply sufficient energy for a self-sustained torrefaction process. Many torrefaction chars from herbaceous biomass have small particles and are highly reactive and tend to self-ignition. For safe handling expensive technical efforts like inertization or char pelletization is required to control the ignition risk during handling. The fine porous pyrolysis char powders required for dense phase pneumatic transport into the gasifier create additional dust explosion or inhalation risks. Torrefaction chars are directly suited for co-firing with pulverized coal into commercial coal power stations. [2]

5.3 FastpyrolysisA simplified flow sheet for fast pyrolysis (FP) of biomass is outlined in Fig. 13. FP of dry biomass, ground to < 3mm size and mixed with a large surplus of a hot heat carrier like 1 mm quartz sand or small stainless steel spheres of few mm takes only 1 or few s and generates 65±15 % mass% of condensable pyrolysis liquids as the main product, which contain ca 50-70 % of the initial bioenergy. Pyrolysis char and also pyrolysis gas have mass yields in the range of only ca. 20±10 %. The combustion energy of the pyrolysis gases are sufficient to supply the energy for a self-sustained pyrolysis process. The fine porous char powder carries usually 20-35 % of the bioenergy and can be suspended in the pyrolysis condensates to a bioslurry. This eliminates the self-ignition risk of the pyrolysis chars, reduces the product volume and increases the energy content of the transport product by a drastic percentage in the range of about 50 % [16],[13]. Free floating and pumpable bioslurries can accommodate up to ca. 35±10 mass% char particles depending on the liquid properties and the form, size and size distribution of the char particles. The high slurry density is ca. 1200+ kg/m³ and the energy density is in the range of about 20±4 MJ/m³depending mainly on the water content. This volumetric energy density is ca 2/3 of that of heating oil and about 6 (wood) to 12 times


(straw bales) higher than the original biomass feedstocks. The favorable storage and transport characteristics of a bioslurry allow an economic transport over several hundred km, e.g. via electric rail from many regional pyrolysis plants to a large and more economic central gasification and synthesis plant. A bioslurry is not only a suitable feed for a PEF gasifier, but simultaneously also a suitable dense storage and transport form. The combination of good transport and feeding properties is an essential feature of the bioslurry gasification concept [10],[11],[12].

Fig. 13: Simplified flowsheet for fast pyrolysis of biomass (part of the bioliq pilot plant at KIT); from [8]

The bioslurry properties depend on the properties of the individual constituents. Sedimentation and phase separation are undesired phenomena. Small char particles combined with a high viscosity of the liquid can sufficiently delay or prevent sedimentation for months or even years; viscosity increase by addition of small amounts of surfactants or gelling agents are other possibilities for future R+D. Another rather simple option is resuspension in the feeding line sequence, immediately prior to feeding, e.g. by colloid mixing.

Spontaneous phase separation of homogeneous biooils (=pyrolysis liquids) or bioslurries occurs if the water content of the biooils exceeds ca. 35 mass%. A light aqueous phase with ca. 50 mass% water and soluble organics can separate as an upper phase with about 20±5 % of the slurry volume. Preparation of a sufficiently stable emulsion of the two phases immediately prior to feeding could be a simple measure to prepare a suitable PEF gasifier feed. Up to now there has been little effort to qualify a broad spectrum of slurry types as suitable gasifier feeds, to enhance the gasifier flexibility. [4],[5],[6],[11],[16]

6 Pilotprojectsforlarge‐scalePEFgasificationofbiomass

6.1 Luleauniversity,Sweden(formerlyChemrec):BlackliquorfeedThis is the most advanced biomass gasification concept and will most likely be the first commercial BTL (biomass-to-liquid) application. Black liquor is the waste stream in pulp mills of the Kraft-process and contains more than half of the energy of the wood feedstock. Ca. 10 % of the black aqueous solution/suspension are lignin residues and some other


organics and ca. 5 % are inorganic cooking chemicals, which must be recovered and regenerated. In conventional pulp mills a black liquor concentrate is combusted in so-called Tomlinson boilers to recover the energy and the Na-hydroxide and sulfide.

A better energy recovery is offered by black liquor PEF gasification with O2 at ca 30 bar. Replacing an old Tomlinson boiler by a PEF gasifier, a co-production of biosynfuel, DME or methanol becomes attractive also for economy and sustainability reasons. Chemrec company (see EU-project RENEW final report 2008 [40]), now Lulea university, Sweden, is developing the gasifier technology. Biosynfuel production from black liquor has a potential to provide over 45 million m3/a motor fuel. In presence of the catalytically active inorganic Na-salt melt with a low melting point, the refractory lined gasifier can operate at only ca. 1000 °C with a low tar production and almost 99,9 % carbon conversion. The melt is quenched with water and recovered as a “green liquor” solution.

The Chemrec and the Siemens gasifier have the same development origin from a former cooperation of DBI Freiberg, Germany and Kvaerner, Sweden. Feeding systems and burner technologies are therefore rather similar. The burner is mounted at the top in the axis of cylindrical gasifier chamber. Pneumatic atomization of the feed with O2 generates a downflowing gasifier flame with about 1 s residence time. Spray nozzle design details are not available.

Among the many BTL process developments worldwide, black liquor gasification has a high state of development and belongs to the economically attractive options, especially for pulp mills which have to replace their old Tomlinson boilers. A consortium of Chemrec, Haldor Topsoe, Volvo, Prem, Total, Delphi and ETC has performed a successful demo plant operation [41].

6.2 FeedingconceptoftheChorenCarbo‐VprocessThe complex feeding system consists of 3 coupled stages. Stage 1 is a biomass pyrolysis in a stirred pressurized vessel (ca. 5 bar) with a hot char bed as heat carrier. The hot combustible gases and vapours are routed directly to the top burner of a ca 5 bar PEF gasifier as stage 2. The design corresponds to the Siemens gasifier with a top burner and a downflow gasifier flame with a very high temperature ca 1500 °C. The hot char powder from the pyrolyser bottom is pneumatically transferred with recycled syngas to the exit of the gasifier chamber and mixed with side burners into the very hot raw syngas stream. Steam and CO2 in the syngas convert a part of the char powder in endothermal gasification reactions. This chemical cooling or quenching in stage 3 proceeds down to ca 1000 °C until the gasification rate becomes too slow. About half of the char can be converted and the remainder is recovered downstream and pneumatically recycled to the top burner. There have been a number of test campaigns in the CarboV pilot facilities at Freiberg, Germany, but little design details and operating experience have been reported. In 2012 Choren company went bankrupt and the equipment has been dismantled. [1],[23]

6.3 KIT“bioliq”pilotfacilitiesThe KIT bioslurry gasification process “bioliq” has been described in sufficient details in [7],[8]. Here, only the essential characteristics of the unique KIT feeding concept are briefly repeated. The concept was designed to allow the use of large and more economic central gasification/synthesis plants also for fuel sources which are distributed over large areas, especially for biomass feedstocks.

The first fuel preparation step is the suspension of a pulverized fuel in a combustible liquid or even water, which has already been practiced for coal. The pumpable suspension, slurry,


paste, sludge or mud has a high energy density and the small specific volume is well suited for compact storage or transport. Economic transport by electric rail over several 100 km to a large central facility is possible.

For biomass, the concept was proposed in 2002 with fast pyrolysis as a first de-central treatment step. Since then, it was developed with focus on the frontend steps from the laboratory, via bench to the present pilot plant scale with 6-12 tpd capacity.

Fast pyrolysis (FP) converts dry, ground lignocellulosic biomass like wood and straw, mainly to a liquid condensate and little char and pyrolysis gas. The gas is sufficient to supply the energy of the FP process. The brittle and porous biochar powder is suspended in the pyrolysis condensate (“biooil”), which contributes about 65±10 % to the bioslurry energy; the char contributes about 35±10 %. Without the char, the energy efficiency of the thermochemical bioslurry concept would be drastically lower.

After delivery of bioslurries from many distributed sites to the central gasifier, there are still some finishing steps needed until the bioslurry is ready as a PEF gasifier feed: heating, colloid mixing, possibly flux addition etc. In the long bioslurry production chain there is a large number of possible processing variations which are still under investigation.

Fig. 14: Concept of the KIT bioslurry gasification concept [8]

7 RecentprocessdevelopmentsThe longest tradition in the production of synfuels and chemicals can be found in coal conversion technologies. However, coal was not cost-competitive for long times compared to crude oil refining. Recently, new production facilities have been erected e.g. in China with large production capacities to produce methanol, propene and SNG from coal. Also, natural gas is utilized for Fischer-Tropsch synthesis e.g. in oil and gas producing countries like Qatar, Nigeria or Malaysia. Increasing interest is also devoted to biomass utilization in the context with renewable energy and climate change, say CO2 emission reduction targets. However, the value of a bio-fuel has to be scored in terms of its CO2 reduction potential. In EU, the greenhouse gas emission saving from the use of biofuels and bio-liquids shall be at least 50% and 60% in 2018 for those installations in which production started on or after 1 January 2017.


Biomass is the only renewable carbon resource and should preferentially be used for the production of carbon containing products, when heat and electrical power can be efficiently be provided by other renewable energy sources. However, the annual upgrowth and sustainably available amount of biomass are limited and are not sufficient to cover the huge demand for all transportation fuels. Yet the much smaller carbon demand for organic chemistry and hydrocarbon aviations fuels could be supplied via biomass. Therefore, syngas is a valuable switch between energy and chemistry as well as between fossil feedstocks (coal and natural gas) and biomass. In Tab. 4, a selection of pilot and demo projects is compiled, giving evidence to the range of technologies, conversion capacities, and state of development. Due to the importance of Scandinavians wood industries the main efforts can be observed in this region. Projects A-C are pilot plants in operation today, pilot plants D and E have been commissioned recently. F-H are first of its kind demo plants planned or in design phase. Tab. 4: Current synfuel pilot or demo projects using biomass-based feedstock

No. Project Feedstock Pre‐treatment+gasificationfeatures

Synthesis,products

A BioDME, S Black liquor Chemrec EF, 3 MW, 30 bar DME, methanol B Güssing, A Wood Repotecdual fluid bed,

8 MW, atm. CHP, SNG (1 MW) and FT-laboratory plant

C NSE Biofuels Varkaus, S

Forest biomass

CFB, 12 MW (5 MW for synfuel application)

Heat for a lime kiln, FT-products

D bioliq/KIT Lignocellulose Fast pyrolysis 2 MW+ Lurgi EF, 5 MW, 80 bar

DME, gasoline

E GoBiGas, S Forest biomass

metso/repotec Dual bed, 20 MW

bio-methane

F BioTfuel, F Forest biomass

Torrefaction + UhdePrenflow EF, 15 MW

FT-products

G Värmlandsmetanol, S

Forest biomass

Uhde-HTW gasifier , 111 MW

Methanol

H ForestBtL, F Wood, tall oil LindeCarbo-V EF 2x160 MW

FT-products

8 ExperiencefromslurrygasificationOn site of KIT, a 2-5 MWth pilot plant has been erected for process demonstration and as a platform for further research and development in regard to process improvement and optimization. The separated process steps have been commissioned in 2013, joint operation has been achieved in 2014 for the first time. To date, experiments are conducted on the KIT bioliq entrained flow gasifier (see Fig. 15) in order to generate comprehensive operating experience and knowledge for scale-up and for future plants.

However, up to now, most data are confidential and only little data are accessible for protection of know-how. Prior to designing the gasifier facilities at Karlsruhe, gasification experiments were carried out in the years 2002–2005 in cooperation with Future Energy


(today Siemens Fuel Gasification Technologies) in Freiberg, Germany. For details on the specific technology see chapter 4.2, example 2.

Fig. 15: Construction scheme of the 5 MWth gasifier at KIT

Results of the Freiberg gasification campaigns, using a 3-5 MWth pilot gasifier are reported in [42]. For production of the diverse feed materials, colloidal mixers have been used; processing condensates and chars from wood pyrolysis and char from straw prepared by intermediate pyrolysis. To simulate the slag behavior of straw ash, about 3 mass% straw ash from straw combustion as well as some additional KCl have been added.

The composition of the dry syngas varied in the ranges of 13–31 % CO2, 21–32 % H2, 27–47 % CO, 9–17 % N2, and 0–0.5 % CH4. For slurry heating values between 19 and 24 MJ/kg, gasification efficiencies ranged between 50 and 71 % as can be seen in Fig. 16. This will be improved drastically in a large > 1 GW gasifier, because the heat loss via the radiation screen will be decreased as will the O2 consumption. Furthermore, the inert gas flows (9–17 % N2 in the experiments) can also be reduced to a few percents.

Fig. 16: Cold gas efficiencies for different slurry heating values

Because of the stoichiometry of CO and H2 formation, the theoretically lowest -value for complete gasification of lignocellulosic biomass is 0.16 (λ is defined as the ratio between actual oxygen consumption and theoretical oxygen consumption for stochiometric

0

10

20

30

40

50

60

70

80

5 10 15 20 25 30

Co

ld g

as

eff

icie

nc

y / %

BioSyncrude heating value (MJ/kg)

Typical syngas composition


combustion). In reality, more oxygen than λ = 0.16 is needed, because the reaction temperature is achieved by partial combustion. The heat loss in a >3 GW gasifier is much lower than in the pilot-scale gasifier of 3 MW. Therefore, the λ-values measured on the pilot scale will be significantly higher than on the large scale. Furthermore, the water–gas shift reaction lowers CO and increases H2 by forming CO2. All in all, λ ≈ 0.33 can be expected for a large-scale entrained-flow gasification of biomass [42].

Slurry gasification stoichiometry: For a theoretical consideration of gasification of energy carriers from the BioBoost project, a slurry from the pyrolysis of wheat straw shall be considered. In the BioBoost Deliverable 2.3 Fast pyrolysis tests, chapter 6, table 5, properties of slurries obtained with the process demonstration plant (PDU) are given. From the known elemental compositions, the simplified stoichiometric chemical formula of all slurries can be expressed as C3H5O2. This corresponds to a molecular weight of ca. 73 g/mol. In the case of the wheat straw - slurry (from dried biomass) with a higher heating value of ca. 16.44 MJ/kg, this corresponds to a heat of combustion of ca. 1200 kJ/mol.

The mass and energy balance for autothermal gasification of a slurry of products from pyrolysis of wheat straw in PEF gasifiers is outlined in Tab. 5 with slightly simplified but still typical realistic stoichiometric reaction equations.

Tab. 5: Stoichiometry of autothermal gasification of a slurry produced by pyrolysis of wheat straw in PEF gasifiers

(Eq.1): Stoichiometric combustion of a slurry of pyrolysis products from wheat staw:

C3H5O2 + 3.25 O2 3 CO2 + 0 H2 + 0 CO + 2.5 H2O

-1200 0 - 0 (kJ/mol)

i cHi (kJ) heat of combustion = rH heat of reaction;

(Eq.2): Autothermal gasification (assuming zero water):

C3H5O2 + 1 O2 1 CO2 + 2.5 H2 + 2 CO + 0! H2O

-1200 0 2.5*286 2*283 0 (kJ/mol)

i cHi = rH = -81 kJ/mol

(Eq.3): Autothermal gasification (corrected with WGS equilibrium):

C3H5O2 + 1 O2 0.3 CO2 + 1.8 H2 + 2.7 CO + 0.7 H2O

-1200 0 1.8*286 2.7*283 0 (kJ/mol)

i cHi = rH = -79 kJ/mol

Vol. % composition: 5 CO2 33 H2 49 CO 13 H2O

WGS: CO + H2Ogas ⇄ CO2 + H2 ; rH = -41 kJ/mol

WGS-equilibrium: ∗

∗ = K = exp ( – 4.33) , T in K; K ~ 0.3 at T = 1500 K


Stoichiometric combustion requires 3.25 molecules of O2 per formula unit (see Eq.1 in the table above). From experience we know that about 1/3 or only one O2 per formula is sufficient for gasification and to heat-up the products to ca. 1200 °C gasification temperature. Eq.2 in Tab. 5 is just a formal equation assuming no water formation that helps in the calculation procedure according to chapter 2.1.4.

The required amount of oxygen for a selected gasification temperature can be determined by trial and error in few iterative steps. Sufficient reaction enthalpy cH must be released to heat the syngas products – together with some ash and moisture – to the desired gasification temperature, taking into account some insulation losses. As shown above, about 7 % (rH = -79 kJ/mol) of the initial energy of the bioslurry (cH ca. -1200 kJ/mol) is converted to high temperature heat and later on partly converted to steam and electricity for a self-sustained process. For slurries derived from pyrolysis of other feedstocks – e.g. those investigated within BioBoost (miscanthus and scrap wood) – the above equations change only slightly.

9 SummaryandconclusionsPEF gasifiers operate at high temperatures > 1000°C and pressures up to 100 bar or more. A tar-free, low-methane raw syngas is produced and fuel conversion usually exceeds 99%. Gasification proceeds to equilibrium in one or few seconds. Pure oxygen is the preferred gasification agent for processes with downstream synthesis, not air. Ash is removed as molten slag. A slag layer is frequently necessary to protect the inner gasifier wall from corrosion. The ash melting point determines the minimum gasification temperature with the minimum O2 consumption. An ash composition with low melting point and feed preheating above 100 °C as far as compatible with thermal feed stability, improves the energy efficiency of gasification.

Feeds for a PEF gasifier can be prepared from a multitude of different carbon feedstocks and in many different ways, contributing to a high process flexibility. The few essential preconditions for a suitable PEF gasifier feed do not justify the frequent rush and unnecessary requests for a rigid standardization of feed properties and restrictions to the already well known feeding systems. Statements should be replaced by experimental work to qualify different feed types to extend the flexibility of PEF gasifiers.

Minimum preconditions for PEF gasifier feeds are:

A minimum heating value > ca. 10 MJ/kg

A transfer possibility into a high pressure gasifier vessel

A feed atomization possibility for special liquid or slurry feeds (pneumatic; information about mechanical atomization is poor)

Suited feed forms are gases either without or with entrained pulverized fuel; or liquids either without or with an immiscible second liquid emulsified in form of small droplets or a pulverized and suspended solid fuel. All these possibilities are already practiced commercially, but essential details are frequently kept secret and are not available in the open literature. Pneumatic atomization of liquid or slurry feed combined with mixing can be achieved in special nozzles by rapid injection of O2 with a jet velocity of > 100 m/s.

Extreme stability towards phase separation of unstable liquids or sedimentation of slurries is not required, since re-suspension or re-mixing is possible at the gasifier site in a short time directly before feeding. A stepwise feed preparation can be reasonable for pulverized fuel,


liquid mixtures, emulsions or slurries, because certain preparations, e.g. heating, make sense only on-site near the gasifier shortly before feeding.

Large technical gasifiers with a capacity of > 100 MW have usually several independent feed lines – last not least also for safety reasons, to control a sudden feed stop of a single line. Also simultaneous feeding of different feed forms is possible.

Large-scale biomass gasification must rely on lignocellulose like wood, straw and other straw-like and herbaceous biomass. Due to the presence of much cellulose fibers direct milling produces an unsuited fibrous cotton pad-like material. After pyrolysis a brittle porous char is obtained, which is easily pulverized.

Torrefaction or low temperature pyrolysis of lignocellulosics at < 300°C on the one hand produces little liquid and gas but a porous torrefaction char, which must be pelletized for safe handling to prevent self-ignition. Fast pyrolysis at 500 °C on the other hand produces mainly pumpable pyrolysis liquids (biooil) and little char powder, which can be suspended in the liquor to a pumpable slurry, sludge or mud with a high energy density around ca. 20+ MJ/kg for economic storage and transport from many regional pyrolysis plants to a large central gasifier station. Biooil without the ash-containing char contains only 60-70% of the slurry energy and is therefore less efficient. Slurry feeding plus atomization at higher gasification pressures is more favorable than dense phase transfer of torrefaction char powders with a compressed inert gas stream, like N2, CO2 or recycled syngas.


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