gasification attachment website v2

8/3/2019 Gasification Attachment Website v2

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O. Box 217 > 7500 AE Enschede > The Netherlands > Tel +31 53 486 1186 > Fax +31 53 486 1180 > Email [email protected] > Site www.btgworld.com

Biomass consultants, researchers and engineers

BTG Biomass Technology Group BV is a private firm of consultants,

researchers and engineers, operating worldw ide in fields of

sustainable energy production from biomass and waste

Date

BTG Biomass Gasification

April 2008


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BTG Biomass Gasification

Colophon

BTG biomass technology group BV

P.O.Box 217

7500 AE Enschede

The Netherlands

Tel. +31-53-4861186

Fax +31-53-4861180

www.btgworld.com

[email protected]


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TABLE OF CONTENTS

1 INTRODUCTION ________________________________________________ 1

2 BTGS INVOLVEMENT __________________________________________ 2

2.1 Handbook Biomass Gasification ________________________________ 2

3 PROCESS ______________________________________________________ 4

3.1 Chemistry __________________________________________________4

3.2 Reactions ___________________________________________________ 4

3.3 Gasification parameters_______________________________________ 6

3.3.1 Equivalence ratio _________________________________________6

3.3.2 Superficial velocity and hearth load ___________________________ 7

3.3.3 Turn-down ratio __________________________________________7

3.3.4 Gas heating value _________________________________________ 7

3.3.5 Gas flow rate and gas production _____________________________ 7

3.3.6 Efficiency _______________________________________________ 7

3.3.7 Fuel consumption _________________________________________ 7

3.3.8 Tar and entrained particles __________________________________ 7

3.3.9 Important biomass characteristics related to gasification___________ 8

4 REACTOR DESIGNS _____________________________________________ 9

5 GAS CONDITIONING ___________________________________________ 11

5.1 Tar removal / conversion _____________________________________ 11

6

STATUS _______________________________________________________ 12

7 ECONOMICS __________________________________________________ 14


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Ir. H.A.M. Knoef Phone: +31 53 486 1190 Mobile: +31 6 52560040 [email protected]

1

1 INTRODUCTION

Biomass gasification is an endothermic thermal conversion technology where a solid fuel

is converted into a combustible gas. A limited supply of oxygen, air, steam or acombination serves as the oxidizing agent. The product gas consists of carbon monoxide,

carbon dioxide, hydrogen, methane, trace amounts of higher hydrocarbons (ethene,

ethane), water, nitrogen (with air as oxidant) and various contaminants, such as small char

particles, ash, tars, higher hydrocarbons, alkalies, ammonia, acids, alkalies, and the like.

When undertaken with air as the oxidizing agent, the produced gas has a net calorific

value (NCV) of 4 6 MJ/Nm3. The heating value of this gas makes it suitable for boiler and

engine use, and for turbine use with burner modifications (for turbine use, the gas must be

partially cooled to protect valve control materials and cleaned to protect turbine blades).

When oxygen is used, the produced gas has a NCV of 10-15 MJ/Nm3, sufficient for

limited pipeline transport and synthesis gas conversion.


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2 BTGS INVOLVEMENT

BTGs activities on the research, development, implementation and evaluation of small to

medium scale fixed bed biomass gasifiers and gasifier/generator systems started morethan 25 years ago. In the early eighties, BTG established its reputation as a world-leading

entity in the field of demonstration, dissemination and monitoring of small-to-medium

scale biomass-fuelled power and heat gasifiers. The scope of BTGs activities widened

over the years with assignments worldwide in more than 80 countries. The focus of BTG

activities gradually shifted from consulting and R&D to project engineering/

implementation and business development. AD 2007 BTG increasingly works for private

partners from the energy sector, including multi-utilities and world-wide operating

multinationals. A large share of our assignments for these companies concern the (semi)

commercial implementation of small/medium scale biomass gasification projects in

Europe and developing countries alike. A main part of BTGs activities is dedicated to

research and technology development. Innovative concepts are explored and investigated

like catalytic tar removal, supercritical gasification for hydrogen production from wet

biomass residues, and staged gas reforming for syngas production. This work is closely

linked to BTGs core business on pyrolysis technology, (see also Test Facilities on BTGs

website www.btgworld.com).

2.1 Handbook Biomass Gasification

Figure 1 Cover Handbook Biomass Gasification

The Handbook on Biomass Gasification is meant to disseminate the results of the

European Gasification Network (GasNet) to a wider audience, which started in 2001 with

funding of DG TREN. The Handbook describes specific topics discussed thoroughly


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within GasNet and additional chapters on more general aspects of biomass gasification

including gasification of pyrolysis oil, market assessments, economics, legislative

impacts, health and safety, tar standardisation and incentives for bio-energy through

gasification. The handbook is edited by Harrie Knoef (BTG), 25 authors contributed to

this highly informative book. These authors are all international biomass experts ongasification or related topics.

The handbook can be ordered on the BTG website (www.btgworld.com).


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3 PROCESS

3.1 Chemistry

The substance of a solid fuel is usually composed of the elements carbon, hydrogen and

oxygen. In the gasifiers considered, the biomass is heated by combustion. Four different

processes can be distinguished in gasification: drying, pyrolysis, oxidation and reduction.

From a chemical point of view, the process of biomass gasification is quite complex. It

includes a number of steps like

thermal decomposition to non-condensable gas, vapors and char (pyrolysis);

subsequent thermal cracking of vapors to gas and char;

gasification of char by steam or carbon dioxide;

partial oxidation of combustible gas, vapors and char.

A schematic presentation of these processes is shown below.

Figure 2 The biomass gasification process

3.2 Reactions

In complete combustion, carbon dioxide is obtained from the carbon and water from the

hydrogen. Oxygen from the fuel will be of course incorporated in the combustion

products, thereby decreasing the amount of combustion air needed.

Combustion, occurring in the oxidation zone, is described by the following chemical

formulae:

C + 02 = C02 + 401.9 kJ/mol (1)

H + 02 = H20 + 241.1 kJ/mol (2)

Biomass

Char

Permanent gases

Tar

Pyrolysis

Combustion

Gasification

Ash

Tars

CO

CO2H2

CO2

H2O

Heat

Heat Oxidant Catalyst

Oxidant


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The most important reactions that take place in the reduction zone of a gasifier between

the different gaseous and solid reactants are:

C + C02 + 164.9 kJ/mol = 2C0 (3)C + H20 + 122.6 kJ/mol = CO + H2 (4)

C02 + H2 + 42.3 kJ/mol = CO + H20 (5)

C + 2H2 = CH4 (6)

CO + 3H2 = CH4 + H20 + 205.9 kJ/mol (7)

Equation (3) and (4) are the main reactions of the reduction stage and require heat. As a

result the temperature will decrease during the reduction. Equation (5) describes the so-

called water-gas equilibrium. For each temperature, in theory, the ratio between the

product of the concentration of carbon monoxide (CO) and water vapor (H20) and the

product of the concentrations of carbon dioxide (CO,) and hydrogen (H,) is fixed by the

value of the water gas equilibrium constant (Kw). Kw is given in the next formula:

Kw = ([CO] * [H20]) / ([C02] * [H2]) (8)

In practice, the equilibrium composition of the gas will only be reached in cases where

the reaction rate and the time for reaction are sufficient.

The reaction rate decreases with failing temperature. In the case of the water-gas

equilibrium, the reaction rate becomes so low below 700 'C that the equilibrium is said to

be 'frozen'. The gas composition then remains unchanged.

Figure 3: Equilibrium of various reactions versus temperature

Introduction of the water-gas equilibrium concept provides the opportunity to calculate

the gas composition theoretically from a gasifier, which has reached equilibrium at a

given temperature.


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The next table presents typical gas composition data as obtained from wood and charcoal

co-current gasifiers operated on low to medium moisture content fuels (wood 20%,

charcoal 7%).

Table 3-1 Typical gas composition

Component Wood gas (vol%) Charcoal gas (vol%)

Nitrogen 50 54 55 65

Carbon monoxide 17 22 28 32

Carbon dioxide 9 15 1 3

Hydrogen 12 20 4 10

Methane 2 - 3 0 2

Heating value (MJ/m3) 5 5.9 4.5 5.6

3.3 Gasification parameters

3.3.1 Equivalence ratio

The water gas, water gas shift, Boudouard and methane reactions provides the

opportunity to calculate the product gas composition of a gasifier, but only in case this

equilibrium can really be reached. Models can be used to calculate the gas composition as

function of the temperature and/or the equivalence ratio (ER), which is the oxygen used

relative to the amount required for complete combustion. This dimensionless parameter

shows that curves of several parameters like chemical energy in the gas and the gas

composition change significantly at ER = 0,25.

Figure 4: Equivalence ratio

A value of zero (left side) corresponds to pyrolysis while combustion is shown at the right

hand side. At ER = 0.25 all the char is converted into gas giving the highest energy

density of the gas; at lower values char is remaining and at higher values some gas is

burned and the temperature will increase.


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3.3.2 Superficial velocity and hearth load

The superficial velocity is one of the most important parameters determining the

performance of a gasifier reactor, controlling gas production rate, gas energy content, fuel

consumption rate, power output, and tar/char production rate. The superficial velocity is

defined as the gas flow rate (m3 /s) divided by the cross sectional area (m

2). A low

superficial velocity causes relatively slow pyrolysis conditions and results in high

charcoal yields and a gas with high tar content.

3.3.3 Turn-down ratio

For every gasifier there is an optimum range of operating conditions corresponding to a

certain turn-down ratio, i.e. the ratio under which gas is produced of sufficient quality for

its application. This quality criterion is in particular related to the tar production level. Forgasifiers the turn-down ratio is typically 2-3, although some technology developers claim

higher values.

3.3.4 Gas heating value

The gas heating value is usually expressed in MJ/Nm3. A normal cubic meter is referring

to the gas volume at 1 atmosphere and 0 C.

3.3.5 Gas flow rate and gas production

The gas flow rate can be calculated from the primary air flow if the nitrogen content in

the producer gas is known, or measured by orifice plates, venturies, pitot tubes or

rotameters.

3.3.6 Efficiency

The efficiency of a gasifier reactor can be expressed on cold or hot gas basis.

3.3.7 Fuel consumption

The fuel consumption is needed to determine the gasifier and overall efficiency. The fuelconsumption can be measured by a balance or automatically by metering bins.

3.3.8 Tar and entrained particles

The amount of tar and entrained particles depends on the gasifier design and operating

conditions, in particularly the load level (actual power output to the maximum rated

power output)


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3.3.9 Important biomass characteristics related to gasification

Each type of biomass has its own specific properties, which determines its performance as

a fuel in gasification plants. The most important properties for gasification are:

moisture content

ash content and ash composition

elemental composition

heating value

bulk density and morphology

volatile matter content

other fuel related contaminants like N, S, Cl, alkalies, heavy metals, etc.

Figure 5 Relation moisture content versus heating value biomass


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4 REACTOR DESIGNS

Reactor designs have been investigated for more than a century, which resulted in theavailability of several designs at small- and large-scale. Gasifiers can be classified in

different ways.

According to:

the gasification agent o Air-blown gasifiers

o Oxygen gasifiers

o Steam gasifiers

heat for gasification o Autothermal or direct gasifiers: heat is provided by partial

combustion of the biomass

o Allothermal or indirect gasifiers: heat is supplied from an

external source through heat exchanger or indirect process,

i.e. separation of gasification and combustion zone

pressure in the gasifier o Atmospheric

o Pressurised

the design the reactor o Fixed bed

o Fluidized bed

o Entrained flow

o Twin-bed

Within each category, a further distinguish between designs can be made. This figure

shows a typical V-shaped throat design of a downdraft (co-current) fixed bed gasifier.

Figure 6 Three dimensional illustration of a typical downdraft type gasifier


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The following figure summarises the main gasifier designs and their typical operating

window.

Figure 7 Characteristics of different gasification configurations


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5 GAS CONDITIONING

Dependent on the application, type of gasifier and contaminants in the fuel, a certain levelof gas conditioning (cleaning/cooling) is required. Cleaning is in particularly needed for

combustion of producer gas in gas engines or gas turbines and synthesis gas production.

Wet and dry cleaning devices have been developed. With wet gas cleaning most of the

impurities can be removed, but in turn, a contaminated waste water is produced which

needs to be treated before disposal. With dry gas cleaning, usually only particulates are

trapped.

Heat (kilns)

Co-firing

Gas engines

Gas turbinesStirling engines

Fuel cells

Syngasppb

ppm

mg/m3Heat (kilns)

Co-firing

Gas engines

Gas turbinesStirling engines

Fuel cells

Syngasppb

ppm

mg/m3

Figure 8 Gas cleaning requirements for different application

Cooling is required for (i) combustion in gas engines, (ii) when filters are applied with a

maximum allowable temperature or (iii) when compressors are incorporated like with

atmospheric IGCC.

The most frequent impurities are hydrocarbons (tar), dust (particulates), ammonia,sulphur, chloride, alkalies, etc. which need to be removed or converted. Dust is usually

removed by cyclones and fabric filters. Ammonia, sulphur and chloride can be removed

by scrubbers or by using additives. The most critical component to be handled however is

tar.

5.1 Tar removal / conversion

Special gasifier designs have been developed to reduce the tar concentration in the

product gas. However, a tar-free gasifier does not exist. Therefore, tar removal and

conversion is required in most cases. The latter is preferred because tar has a relatively

high energy content which should preferably be utilised. Different concepts developed to

reduce the tar content include a.o.:

Specific reactor designs: downdraft gasifiers with a V-shaped throat

construction

Staged gasification were the pyrolysis, gasification and/or combustion zones are

separated

Adding catalyst in the reactor

Separate catalytic conversion of tar downstream the gasifier reactor

Physical removal by scrubbing, or absorption

Physical removal and recycling of tar into the gasifier reactor.


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6 STATUS

Gasification of biomass converts a solid renewable fuel to a gas that can be used in amodern conversion device, such as a gas turbine or engine, for electricity and heat

production. This opens the possibility of moving from the traditional, small-scale, low-

efficiency steam cycle to the efficient gas turbine. Recently, other applications became of

interest like syngas production, methane, hydrogen (for fuel cells), etc.

The following applications can be distinguished:

Gasification for heat production

Usually, the heat is used for district heating, industrial drying applications or low

pressure saturated steam for an industrial process. Both fixed bed and fluid bed

gasifiers are commercially in operation.

Fixed bed gasifiers for power or CHP production from biomass

De-central power production at a relatively small-scale is still practised in many

countries because of favourable policy measures.

Co-firing gas from biomass in existing power plants

Co-firing is seen as one of the most interesting applications because of the relatively

low investment, the available flue gas cleaning, and the available infrastructure.

Three plants are implemented so far.

Fluid bed gasification for power production (IGCC)Integrated gasification combined cycle (IGCC) technology is being demonstrated at

both atmospheric and pressurized gasifiers.

Syntheses gas production for liquid (transportation) fuels. Synthesis gas differs from

producer gas as it contains only CO and H2.

The technology is close to commercialization and therefore BTG has informed the

international community in detail about the status for many years, i.e. about the current

installations and the current manufacturers. Details can be found on www.gasifiers.org

and other websites maintained by BTG, see Links. Over 90 installations and over 60

manufacturers are listed now indicating the large interest in biomass gasification.

Despite many R,D&D efforts for the last decades, commercial status is still not achieved

for several technical and non-technical reasons. To promote the technology in general and

to contribute to the Kyoto protocol, BTG initiated in 2000 a European wide Network on

Gasification, GasNet, in which 20 members from all European countries participate.

Information on different aspects are exchanged and distributed through internet at

www.gasnet.uk.net and bi-annual newsletters. GasNet operates within the ThermalNet

network www.thermalnet.co.uk together with PyNe (Pyrolysis Network) and CombNet,

the combustion Network).


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Links are also established with the IEA Bioenergy Agreement, Task 33 on Thermal

Biomass Gasification.

Gasifier

air

H2O

biomass

N2O diluted

syngas

Separator

Fischer

Tropsch

Fischer Tropsch

Or Methanol

N2N2

liquid fuels

Gasifiersteam

biomass

Heat input from external combustion

(e.g. part of the biomass)

O2

MCV gas

Secondary

Converter syngas

Fischer

Tropschliquid fuels

liquid fuels

Gasifier

oxygen

(steam)

biomasssyngas

Fischer Tropsch

Or Methanolliquid fuels

Liquid fuels via Syngas from Biomass

Figure 9 Liquid fuels from biomass


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7 ECONOMICS

Several economic studies have been made on biomass gasification regarding the

feasibility and long-term prospects. The first demonstration projects are mostly far tooexpensive to become profitable. Investment figures can be more than 5,000 /kW electric,

which is far more than competitive technologies. However, it is expected that due to the

learning curve, the investment costs can be reduced to approximately 2,000 /kW electric

within the coming decade. Operational experience, success stories and value engineering

is needed to achieve this goal.

Another aspect is the operational costs, in particularly the price of the feedstock. These

can be expensive like short rotation coppice (SRC) or cheap (negative) like waste

residues. Transportation, fuel handling and processing adds to the cost of the feedstock.

Furthermore, labour costs must be minimised through process control and automation.

Practical experience is needed to determine the maintenance costs. Remuneration of

electricity and heat can also be decisive in the overall economics.

For the short to medium term, biomass gasification can not compete with fossil fuel

produced power. Therefore, comparison must be made to alternative renewable energy

sources. Studies showed that biomass gasification can compete with other RES when

capital costs can be reduced and favourable conditions are created. Both conditions are

likely to happen.

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