black liquor gasification comsequences for both industry and society

Energy 29 (2004) 581–612

www.elsevier.com/locate/energy

Black liquor gasification—consequences for both industryand society

H. Eriksson, S. Harvey �

Heat and Power Technology Group, Department of Chemical Engineering and Environmental Science,Chalmers University of Technology, SE-412 96 Goteborg, Sweden

Received 1 December 2001

Abstract

The pulp and paper industry consumes large quantities of biofuels to satisfy process requirements. Bio-mass is however a limited resource, to be used as effectively as possible. Modern pulping operations haveexcess internal fuels compared to the amounts needed to satisfy process steam demands. The excess fuelis often used for cogeneration of electric power. If market biofuel availability at a reasonable price is lim-ited, import/export to/from a mill however changes the amount of such biofuel available for alternativeusers. This work compares different mill powerhouse technologies and CHP plant configurations (includ-ing conventional recovery boiler technology and black liquor gasification technology) with respect toelectric power output from a given fuel resource. Different process steam demand levels for different rep-resentative mill types are considered. The comparison accounts for decreased/increased electricity pro-duction in an alternative energy system when biofuel is imported/exported to/from the mill. The resultsshow that black liquor gasification is in all cases considered an attractive powerhouse recovery cycle tech-nology. For moderate values of the marginal electric power generation efficiency for biofuel exported tothe reference alternative energy system, excess mill internal biofuel should be used on mill site for gas tur-bine based CHP power generation. The remaining excess biofuels in market pulp mills should beexported and used in the reference alternative energy system in this case. For integrated pulp and papermills, biofuel should be imported, but only for cogeneration usage (i.e. condensing power units should beavoided). If biofuel can be used elsewhere for high efficiency CHP power generation, mill internal biofuelshould be used exclusively for process heating, and the remainder should be exported.# 2003 Elsevier Ltd. All rights reserved.

� Corresponding author. Tel.: +46–31-7728531; fax: +46-31-821928.E-mail address: [email protected] (S. Harvey).

0360-5442/$ - see front matter # 2003 Elsevier Ltd. All rights reserved.doi:10.1016/j.energy.2003.09.005

Nomenclature

79 79 bar(a)110 110 bar(a)ADt air dried tonneB bark import/biofuelD differenceBB bark boilerBIG biomass fuel gasificationBLG black liquor gasificationCC combined cycle gas turbineCHP combined heat and powerDH district heating systemDS dry solidsE electric powerECF elementary chlorine freeF fossil fuelGT simple cycle gas turbineH heat producedHP high pressureHRSG heat recovery steam generatorIP intermediate pressureISO International Standard Organisation, Standard air conditionsKAM Ecocyclic Pulp MillL lignin importLE lignin exportLHV lower heating valueLLP very low pressureLP low pressureMP medium pressureNGCC natural gas combined cycleNO-GT no power production (heat only)RB recovery boilerSTIG steam injected gas turbineTCF totally chlorine freeg efficiency

Superscriptv

base case (reference) value

Subscripts

biomass biomass fuel

H. Eriksson, S. Harvey / Energy 29 (2004) 581–612582

1. Introduction

Increased concern for climate change has lead to a variety of initiatives that aim to reduce theemissions of greenhouse gas emissions. Biofuels are considered CO2 neutral, and as differentpolicy measures aiming to decrease greenhouse gas emissions are implemented, the demand forbiofuels is expected to increase substantially. It is therefore important that usage of such fuels isas effective as possible. The Swedish pulp and paper industry is a substantial consumer of bio-mass fuels, including bark, forest logging residues and black liquor. These fuels are used to gen-erate process heat and electric power. According to data presented in [1], the total Swedish pulpproduction in 2000 amounted to 11.4 million ADt, of which 7.2 million ADt were chemicalpulp. 3.55 million ADt were sold as market pulp and 3.65 million ADt were used directly forpaper production integrated at the mill site. Fuel and electricity consumption amounted in 2000to 77.76 TWh, of which 21.46 TWh was electricity corresponding to 40% of the total electricityusage in the industrial sector. 41.7 TWh of mill fuel usage was based on internal biomass fuels(i.e. black liquor and falling bark), mainly used for process steam and electricity production.On-site generation of electric power (3.96 TWh in 2000) accounts for only a small fraction ofthe total usage (21.46 TWh).In a conventional mill powerhouse, black liquor is fired in a recovery boiler to produce high

pressure steam which is thereafter expanded in a steam turbine CHP unit. Higher power-to-heatratios can be achieved if the black liquor is instead gasified and combusted in a gas turbineCHP unit. This is particularly relevant for future pulp mills, for which available internal bio-mass fuels will be more than sufficient to satisfy the mill’s heat demand. The excess biofuel cantherefore be used on-site to generate further amounts of electric power for export, or beexported directly and used for other applications elsewhere.Previous system-oriented studies of opportunities for increased electricity production with

black liquor gasification CHP systems have been performed by researchers in most major pulpproducing countries. Work performed in Sweden includes amongst others studies performed byBerglin [2–4] and Maunsbach [5]. Refs. [6] and [7] illustrate the extensive work accomplished inFinland in this area during the 1990s. Refs. [8–10] illustrate the work accomplished in the areaby North American researchers. All the cited studies include both market pulp mills and inte-grated pulp and paper mills. The configurations considered often have a low total efficiency (e.g.systems including condensing steam turbines) corresponding to a poor usage of the biofuel

DH district heatingel electricityexport exporttot totalmarginal marginalmill millRS reference systemNG natural gasNGCC natural gas combined cycle

583H. Eriksson, S. Harvey / Energy 29 (2004) 581–612

resource. Furthermore, these studies do not compare biofuel usage within the mill powerhousewith other potential uses (e.g. in district heating systems).This study is a continuation of previous work by the authors [11,12]. The goal is to identify

mill powerhouse recovery cycle technology and CHP plant configurations that maximise thetotal electric power generation from a given fixed amount of fuel. A number of studies of bio-fuel availability in Sweden as a function of cost have been published recently; see e.g. ref. [13].In this work, the availability of biofuel at reasonable costs is assumed to be limited in thefuture. This assumptions has been made by a number of authors studying future energy systemoptions for Sweden, see for example ref. [14]. As a result of this assumption, import ofadditional biofuel to the mill is assumed to correspondingly reduce the amount of biofuel avail-able for a reference alternative biofuel user. Conversely, if excess biofuel is exported from themill, this biofuel can be used elsewhere. The assumption of limited availability of biofuel in thelong term is clearly politically challenging since this would result in significant price increasesfor biomass and biofuel, which could potentially lead to severe problems for the pulp and paperindustry. Such macro-economic issues are not discussed further in this work. An additionalassumption in this work is that changes in biofuel usage outside the mill as a result of biofuelimport/export to/from the mill affect the electricity production outside the mill. The goal of thiswork is to identify ways in which the total electricity production can be maximised, includingproduction at both the mill site and in the reference alternative energy system. The main focusis on electricity production based on black liquor gasification mill powerhouse recovery cycletechnology. The study compares the total electricity production related to different black liquorgasification gas turbine CHP configurations with conventional recovery boiler configurations.Some of the important assumptions made for this study are summarised below:

. Availability of biofuel at a reasonable price is assumed to be limited, therefore increasedimport/export to/from the mill is assumed to correspond to decreased/increased usage else-where;

. Biofuel usage elsewhere is assumed to follow the same target as biofuel usage at the mill site,namely high electricity production with high total efficiency;

. Biofuel is particularly attractive for low temperature heat load applications (e.g. district heat-ing), because it is a wet fuel and high total efficiency values can be achieved if a flue gas con-denser unit is used. A district heating system (including CHP units) is therefore retained asthe reference alternative biofuel user for this study;

. The reference alternative biofuel user is assumed to have access to natural gas fuel, whereasthis fuel is not assumed to be available for the mills. This assumption reflects Swedish con-ditions, where natural gas is only currently available on the West Coast. It is possible thatthis fuel will become more widely available in the future, particularly in other parts of South-ern Sweden. However, it is not likely that natural gas will be available in the more remotelocations typical of pulp and paper mills.

. The mills are assumed to be equipped to handle large quantities of biomass. It is thereforeassumed that they will cover their energy demands exclusively with biofuel;

. Since the focus of the study is on high performance electricity production, it is important tomake assumptions regarding the most likely technology for new grid electric power gener-ation capacity additions. Given that BLG technology is only likely to be commercially mature


within 10–15 years, high performance natural gas fired combined cycle technology (60% elec-trical efficiency) is retained as the reference technology for added grid capacity.

Import and export of fuels to and from mills can occur in many different ways. Swedish pulpmills traditionally select biomass as an import fuel due to the capability to handle large quanti-ties of biomass in the mills and the high taxes on fossil fuel alternatives. Possible import bio-fuels include bark, sawmill and logging waste and lignin. Biofuel can be exported from the millas either bark or lignin. The market for primary forest fuels (i.e. bark, sawmill and loggingwaste) is well established. Although it is technically feasible to extract lignin from the black liq-uor stream [15], large-scale extraction of lignin has not been implemented so far in the pulpingindustry. It is however reasonable to assume that a market could be developed for lignin, sinceprocessed lignin has similar properties to bark.Finally, it should be noted that this study focuses on biomass energy conversion potential,

thus investment costs and other economic aspects are not explicitly taken into account. How-ever, given the assumptions listed above, biofuel usage is primarily focused on CHP, either atthe mill site, or in the reference external energy system. Given that the investment and runningcosts for biofuel CHP are to a certain extent comparable for the different technologies and con-figurations considered, not directly considering economic aspects should therefore not severelybias the results in favour of certain configurations in relation to others.

2. Mill steam and power balances

The steam and power balances for the mill powerhouse configurations considered in thisstudy were obtained through collaboration with the ‘‘Ecocyclic Pulp Mill’’ (KAM) research pro-gram. The KAM program has so far focused on market pulp mills, and has defined a referencemill that is assumed to incorporate best available technology for all process components. TheKAM reference mill has been used as a basis for a number of former studies of BLGCCsystems.The energy consumption for the KAM reference market pulp mill (KAM pulp mill) is about

10 GJ/ADt compared to an average of 15.5 GJ/ADt for today’s typical Swedish market pulpmill [1,16]. The available incoming biofuel (bark and black liquor) is sufficient to satisfy theKAM pulp mill’s heat and electricity demand. The excess biomass fuel can in this case be usedto generate additional steam for expansion in a condensing steam turbine, thereby generatingsurplus electricity for export from the mill. Excess biofuel could also be exported and used else-where. Most current Scandinavian pulp mills have a net fuel deficit and have to import fuel. Asimilar ongoing project aims to identify the key characteristics of an integrated pulp and papermill that uses best available technology. There is a large difference in steam demand between apulp mill and a pulp and paper mill. Based on preliminary estimations extrapolated from theresults of the KAM pulp mill project, an integrated pulp and paper mill using best availabletechnology would have an even higher steam demand (15.8 GJ/ADt pulp) than the currentaverage Swedish pulp mill [2,12]. An integrated pulp and paper mill has therefore an energydeficit, and is dependent upon fuel import (biomass fuel) to meet its energy demands. To morethoroughly investigate how the process steam demand affects the total electricity production, a


hypothetical future pulp mill with a low process steam demand was also defined. Assuming ahigh degree of process integration, the process steam demand for this low-energy mill was set to8 GJ/ADt, based on a study presented in [17]. Around 75% of the steam savings compared tothe KAM pulp mill are achieved in the black liquor evaporation plant. Since this mill has alower process steam demand, more fuel is available for electricity production.

2.1. Steam and power balances for the KAM’98 reference pulp mill

The KAM’98 reference pulp mill (see ref. [16]) is a sulphate (kraft) pulp mill that uses 70%round wood and 30% chips from saw mills. The process includes continuous isothermal cookingand delignification with two oxygen steps. The bleaching process is either totally chlorine free(TCF) or elementary chlorine free (ECF). The mill produces 1000 ADt of pulp per day. Allavailable black liquor (1797 tonne DS/day) is fired in a conventional recovery boiler, producingsteam at 79 bar(a) and 480

vC. The black liquor has 80% dry solids content by weight and a

lower heating value of 12.1 MJ/kg DS. In addition, about half of the available falling bark isfired in a bark boiler, and the rest is used as fuel in a lime-kiln. The bark has 45% dry solidscontent by weight and a lower heating value of 16 MJ/kg DS. A total of 210 MW of steam isproduced in the recovery boiler (RB) and 12.8 MW is produced in the bark boiler (BB). TheKAM’98 mill generates an excess amount of steam, even without the bark boiler. Heat andmass balances for the KAM’98 reference mill have been defined for four different operating con-ditions, namely winter and summer operation with both ECF and TCF bleaching. The steamdemand varies between 108 and 126 MW for the different conditions. Powerhouse energy flowsfor TCF summer operation are shown in Fig. 1.

2.2. Steam and power balances for the pulp mill configurations considered in this study

The steam and power balances for the different mill configurations retained for this study areas follows:

Fig. 1. Heat and electricity production in the KAM’98 reference pulp mill powerhouse (summer operation, TCFbleaching).


. KAM’98 pulp mill. The process steam demand corresponds to summer operation with TCFbleaching: 116 MW (50 MW of 12 bar(a) MP steam and 66 MW of 4.5 bar(a) LP steam). Theelectricity consumption in the mill is 29 MWel.

. Future process integrated pulp mills with reduced heat demand. The steam demand is assumedto be decreased through process integration measures to 8 GJ/ADt compared to 10 GJ/ADtin the reference KAM’98 mill. One way to achieve such a decrease is by using waste heat forblack liquor evaporation, thus reducing the usage of live steam [17,18]. The steam demandcould thereby be reduced to 92 MW (45 MW of MP steam and 47 MW of LP steam). Theelectricity consumption (29 MWel) is assumed not to be affected by such measures.

. Integrated pulp and paper mills using best available technology. Heat and electricity require-ments for an integrated mill are so far not available from the KAM program. Indicative val-ues were estimated based on the KAM’98 reference pulp mill, and adding estimated steamand power consumption data for a modern fine paper mill. The total steam demand for anintegrated pulp and paper mill was thus estimated at 15.81 GJ/ADt pulp. Further discussionmay be found in [2]. It is however clear that the heat requirements for a paper mill dependvery much on the paper quality being produced, and that calculations should be conductedfor a variety of paper mill types before drawing general conclusions. The mill size was chosenidentical to the KAM’98 reference mill size, i.e. a kraft pulp production of 1000 ADt per day,which corresponds to a paper production of approximately 1300 tonnes per day. For a millthis size, the total process steam demand is 183 MW (87 MW of MP steam and 96 MW of LPsteam) and the electricity demand is 56 MWel.

3. Pulp mill powerhouse technologies and modelling assumptions

The powerhouse has two tasks to perform, one is to regenerate the cooking chemicals and theother is to produce process steam and possibly electricity for the mill. This can be carried out ina Tomlinson recovery boiler system or in a black liquor gasifier system. Modelling of the twosystems was based upon the requirement that they be fully interchangeable with respect to theassociated pulp mill process. The two mill powerhouse recovery cycle technologies consideredare described below.

3.1. Conventional recovery boiler technology

The KAM’98 reference pulp mill incorporates conventional Tomlinson recovery boiler tech-nology, as depicted in Fig. 2 and described in the Swedish Ecocyclic Pulp Mill (KAM’98) pro-ject [16]. The black liquor stream is fired in a boiler, and used to generate high pressure steam at79 bar(a) and 480

vC from feed water at 140

vC, which is then expanded in a back-pressure

steam turbine to produce both process steam and electricity. The cooking chemicals are recov-ered in from the bottom of the boiler and mixed with weak washing liquor in the quench, pro-ducing green liquor which is then removed for further processing (Fig. 2). If the steam producedin the recovery boiler is insufficient to meet the mill’s demand, additional steam is produced in apower boiler, using bark or sawmill residuals as fuel. Recovery boiler technology with advancedsteam data (110 bar(a) and 530

vC) is also considered in this study. Even the 79 bar(a) and


480vC steam data considered in the base case is advanced compared to standard practice in

many Swedish mills today (60 bar(a) and 440vC).

3.2. Black liquor gasification technology

Black liquor gasification technology results in higher power-to-heat ratios than conventionalrecovery boiler technology. Furthermore, a higher degree of utilisation of the biofuel energycontent is achieved since it is possible to produce steam in the gas cooler by condensing a por-tion of the steam produced by gasification and in the gasifier quench zone.Fig. 3 shows the process layout of a black liquor gasification system, which includes the fol-

lowing operations:

. oxygen-blown, high-temperature (ca 950vC), pressurised (ca 25 bar(a)) entrained flow gasifier;

. rapid cooling of the product gas flow in a quench vessel;

. further cooling of the gas in a waste heat boiler;

. cryogenic air separation unit, partly integrated with the gas turbine;

. acid gas removal system, with H2S recycle to the gasifier;

. combustion of the gas in an ‘‘F’’ class gas turbine, modified for air extraction;

. cooling of the gas turbine exhaust in a three-pressure heat recovery steam generator

. steam distribution system (including a back-pressure steam turbine).

A more detailed description of the black liquor gasification recovery cycle technologytogether with the pertaining modelling assumptions may be found in refs. [4] and [12].

3.3. Gas turbines for black liquor gasification applications

The key focus of this study is on system aspects related to the potential for electric powergeneration for a number of different conditions. Although the key components for BLGCC thatrequire further development are the gasifier and pertaining gas clean-up equipment, the key

Fig. 2. Flowsheet schematic of a conventional Tomlinson recovery boiler system.


component for electric power production is the gas turbine. In this work, particular focus wasthus placed on estimating gas turbine performance and how integration of fixed sized gas tur-bine engines with a fixed sized mill can affect system performance.For optimum system performance, it is important to select a gas turbine that is appropriately

sized with respect to the available gasified black liquor stream. The General Electric 6FA enginewas chosen for this study, on the grounds of good size matching for this engine with respect tothe available fuel stream if all black liquor is gasified. Furthermore, significant experience hasbeen accumulated with this engine for low heating value fuel applications. The engine has adesign pressure ratio of 15:1, a design turbine inlet temperature of 1288

vC and a power output

of 70 MWel at ISO rating conditions on natural gas fuel. The engine is slightly oversized withrespect to the available fuel, which implies that it should in principle operate at part-load. Per-formance modelling of off-design operation for the gas turbine operating on gasified black liq-uor fuel was based upon the following assumptions:

– the gas turbine is assumed to be modified to allow air extraction for the ASU and steaminjection;

– gasifier air is extracted from the compressor discharge to supply approximately 1/3 of the airneeded in the air separation unit;

– the combustor exit temperature is de-rated by 20vC to avoid blade cooling problems [19].

Fig. 3. Flowsheet schematic of a black liquor gasification combined cycle powerhouse system.


Furthermore, part-load operation of the gas turbine was modeled assuming the followingoperating strategy:

– the air inlet flow to the engine is first reduced using the inlet guide vanes until the air flow tothe compressor is reduced by 20% while maintaining the design firing temperature;

– further power reduction is achieved by reducing the engine firing temperature.

The GE 6FA gas turbine discussed previously is too big for some of the powerhouse config-urations considered, especially those involving significant amounts of lignin export from themill. To avoid severe part-load operation, smaller gas turbines are used instead. These includethe 43 MWel Alstom GTX100, the 23 MWel GE LM2500 and the 14 MWel GE LM1600, whichhave similar turbine inlet temperatures and pressure ratios as the GE 6FA. All of these gas tur-bines engines are assumed to be suitable for both combined cycle and simple cycle operation.For the cases where the gas turbine that best matches the available fuel stream is too small, afraction of the product gas is used for supplementary firing in the HRSG. In one extremepowerhouse configuration, the product gas is fired in a heat-only boiler (i.e. there is no on-sitecogeneration of electric power) and large quantities of biofuel (bark and lignin) are exportedfrom the mill.The gas turbine cycle configurations considered are as follows (all configurations include a

heat recovery steam generator):

– Combined cycle (CC), which consists of a gas turbine and a steam turbine for electricityproduction;

– Simple cycle gas turbine (GT) where only the gas turbine is present and producingelectricity;

– Heat-only configuration (NO-GT) without on-site cogeneration of electric power.

3.4. Mill steam system characteristics

The other parts of the mill are modeled as set steam demands. The steam demand is differ-entiated into IP (30 bar(a)), MP (12 bar(a)) and LP (4.5 bar(a)) levels, with varying degrees ofcondensate return. In the market pulp mills, the condensate return temperatures were set at 124,156 and 188

vC for LP, MP and IP condensate, respectively. For the integrated pulp and paper

mills, these temperatures were set at 116, 126 and 200vC. Steam is generated in different parts

of the power island and fed into the mill steam system. Additional heat is recovered throughgeneration of very low pressure steam (LLP) at 1.5 bar(a) in the product gas cooling process.This LLP steam is then mechanically recompressed to LP steam.If the black liquor recovery boiler or gasification system cannot produce enough process

steam to meet the mill’s demand, additional process steam must be generated using one of thetechnologies described below.

. Bark power boiler: high efficiency bubbling fluidised bed boiler, as described in [16].

. Biomass gasifier: high pressure, air-blown gasifier with hot gas clean-up as described in [5]. Ifa biomass gasifier is used, there is no need for a power boiler as all biomass fuel is gasified.


. Lignin and black liquor co-gasifier: imported lignin is mixed with the available black liquorand gasified in the existing black liquor gasifier. Since the lignin is extracted from anothermill’s black liquor, it is reasonable to assume that the lignin gasification can be performed inthis way.

Both the biomass gasifier and the power boiler use bark/sawmill residuals as fuel, while thecombined black liquor gasifier and lignin gasifier uses imported lignin as additional fuel. Theproduced high pressure steam has the same pressure as the high pressure steam produced inthe black liquor gasifier system.

3.5. Electricity consumption

The electricity consumption for the pulp and paper process (powerhouse internal consump-tion excluded) is independent of the powerhouse configuration. The powerhouse electricity con-sumption will however change with the powerhouse configuration. The largest contributor tothe variations in electricity consumption is the ASU compressors, since the different powerhouseconfigurations have different gasifier oxygen needs. The change in electricity consumption forpumping the internal streams in the powerhouse, the LLP to LP heat pump and the H2S recyclecompressor are also accounted for.

3.6. Powerhouse performance estimation tools

The mill powerhouse performance was estimated using the following simulation tools:

. an in-house model for the gasifier (see ref. [4]);

. a specialised power cycle simulation software package (GateCycle [20]) that includes gas tur-bine part-load and off-design simulation capability for gas turbine performance calculations;

. a general chemical process simulation program, HYSYS [21], for all other components of theplant.

4. System aspects of biomass fuel import and export

As mentioned previously, biofuel is assumed to be a limited renewable fuel resource thatshould be used as efficiently as possible. It is therefore important to account for the impact onglobal electricity production resulting from changed biofuel availability for other potential bio-mass fuel users as a result of different levels of biofuels usage at the mill site. Potential compet-ing users of biofuels include mills with the possibility to export heat to a district heatingnetwork, district heating networks, other industries, CHP plants or manufacturers of processedbiomass fuels (e.g. pellets, pyrolysis oil or methanol). District heating systems are widespread inSweden, and are particularly well suited to usage of biofuels given the possibility to achieve highefficiencies through implementation of boiler flue gas condensation. District heating systems(including CHP capability) are thus retained as the reference alternative user for biofuels in thisstudy. It is important to note that biofuel transportation to and from the mill infers energylosses, but these losses are negligible if the biomass can be transported by ship.


The focus of this study is to maximize high efficiency electricity production from available

biofuel, accounting for both on-site and off-site production. Important assumptions for assess-

ing system aspects are discussed below.

4.1. Availability of biomass and natural gas fuels

Biofuels are available at the mill in the form of bark and black liquor fuels. Excess biofuel

can be exported from the mill to the district heating system in the form of bark or lignin. Bio-

fuel can also be imported to the mill but this will result in a corresponding decrease in biofuel

usage in the district heating system.Natural gas is currently only available on the west coast of Sweden. In this study, it is

assumed that the natural gas grid is extended and is thus an available fuel option for most

major district heating systems (mostly located in the more densely populated southern part of

Sweden). As discussed previously, it is assumed that natural gas is not an available option for

most pulp mill locations. Natural gas can also be used for electric power generation in high

efficiency combined cycle (NGCC) condensing power plants.

4.2. Heat and power production and fuel consumption: assumptions and definitions

The fuel and energy flows for the mill and reference energy systems considered in this study

are shown in Fig. 4. The different flows and related assumptions are discussed below:

Fig. 4. Overview of fuel and energy flows for the mill and reference energy systems.


. B0mill is the fixed amount of internal biomass at the mill site in the form of black liquor and

falling bark;. B0

biomass is the fixed biofuel resource that is available primarily as fuel for the district heatingnetwork;

. DBexport is the amount of biofuel that is exported from the mill site. If it is negative, it meansthat biofuel is imported to the mill;

. B0biomass þ DBexport is the total flow of biofuel to the district heating system. For the case where

biofuel is imported to the mill ðDBexport < 0Þ, the amount of biofuel available for the districtheating system is therefore decreased compared to B0

biomass;

. F0NG is the fixed amount of natural gas that can be used in the district heating system for

CHP or in the NGCC power plant for electricity production;. H0

mill þ DHmill is the mill heat consumption that is fixed for each of the three mill types, asdefined in Section 2.2. To account for the different heat demand in three mill types, the heat

demand has been divided into two parts, one fixed reference level ðH0millÞ and one part that

varies with mill type ðDHmillÞ;. E0

mill þ DEmill is the net electricity production at the mill powerhouse (i.e. the powerhouseinternal electricity consumption is deducted). To show the different electricity productions inthe different mill powerhouse configurations, the mill electricity production has been divided

into two parts, one fixed reference level ðE0millÞ and one part that varies with powerhouse con-

figuration ðDEmillÞ;. H0

DH is the fixed heat demand in the district heating system;. E0

DH þ DEDH is the electricity production from the district heating system CHP plant. E0DH is

the reference level of electricity production when no biofuel is exported from the mill to thedistrict heating system and DEDH accounts for the change in electricity production that relatesto the exported biofuel from the mill site. When fuel is imported to the mill, the electricityproduction in the district heating system is reduced, and DEDH is negative;

. E0NGCC þ DENGCC is the electricity production from the NGCC power plant. E0

NGCC is the ref-erence level of electricity production when no biofuel is exported from the mill and DENGCC

accounts for the change in electricity production in the NGCC power plant that relates to theexported biofuel from the mill site.

Since the goal is to maximise the total electricity production from the system shown in Fig. 4with a fixed amount of fuel (i.e. B0

mill þ B0biomass þ F0

NG), the goal function for this study isdefined as:

Goal function ¼ E0mill þ DEmill þ E0

DH þ DEDH þ E0NGCC þ DENGCC (1)

The base size of the district heating system and the NGCC power plant are not defined. Thedistrict heating system and the NGCC power plant are however assumed to be large enoughthat they can accommodate all the assumed changes in fuel flows. The district heating systemand the NGCC power plant may consist of one physical system or several systems withinreasonable transportation distance for the fuel considered. The mill, the DH system and theNGCC do not necessarily need to be at the same site, since fuel and especially natural gasare assumed to be easily shifted between different users. It is furthermore assumed that the


efficiencies for the electricity and heat production in the district heating system (including CHP)and the NGCC power plant are independent of size.The size of the mill is known, therefore E0

mill þ DEmill is known for a given powerhouse con-figuration and mill type. Since the size of the district heating system and the NGCC power plant

are not defined, the reference electricity production in the district heating system ðE0DHÞ and the

reference electricity production in the NGCC power plant ðE0NGCCÞ are not defined. However,

knowledge of the absolute value for E0DH and E0

NGCC is not important since these values areconstant. The goal function defined in Eq. (1) is therefore maximised if the total electricity pro-duction defined in Eq. (2) is also maximised:

Etot ¼ E0mill þ DEmill þ DEDH þ DENGCC (2)

The change in electricity production in the district heating system and the NGCC power plantdepends on the amount of biomass that is exported from or imported to the mill. The change inelectricity production in the district heating system and the NGCC power plant ðDEDH þDENGCCÞ can be expressed according to Eq. (3):

DEDH þ DENGCC ¼ gel;marginal � DBexport; (3)

where

gel;marginal ¼DEDH þ DENGCC

DBexport(4)

The marginal electricity production efficiency in Eq. (4) thus expresses the net total combinedpotential for increase in electricity production in the district heating system and NGCC powerplant, associated with a given export of biofuel from the mill. It is important to note that realis-ing this potential requires that the district heating system and NGCC power plant be modifiedso as to maximise electric power production from the available fuel streams. Thus, gel,marginaldoes not refer to the potential for power increase from existing equipment, but rather the poten-tial for power increase if the plant is rebuilt under given conditions. Different scenarios andresulting values for gel,marginal are discussed in the next section. The total electricity productionin Eq. (2) can be rewritten to include the marginal electricity production efficiency, resulting inthe following goal function for this study:

Etot ¼ Emill þ DEDH þ DENGCC ¼ Emill þ gel;marginal � DBexport ðLHVÞ (5)

4.3. Detailed discussion of the systems outside the mill that are affected by biofuel export

Since the goal of this study is to maximise the total electricity production from a given fuelresource, it is assumed that biofuel exported from the mill site to the reference alternative bio-fuel user is used for increasing electricity production.The potential for increased electricity production in a district heating system depends essen-

tially on the extent to which CHP has been implemented in this system. Different degrees ofCHP implementation in the district heating system and the consequences for electricity pro-duction are discussed below.


The system outside the mill site includes both natural gas and biomass users delivering heatto a district heating system and electricity to the power grid. The available production tech-nologies are assumed to be state-of-the-art and include the following (all efficiency values arebased upon the fuel lower heating value):

. Biofuel heat-only boilers with flue gas condensation units, gboiler ¼ 105%;

. Biofuel CHP based on fluidised bed boiler technology with flue gas condensation, and steamturbine CHP, gel ¼ 33%, gtot ¼ 105%;

. Natural gas combined cycle CHP, gel ¼ 55%, gtot ¼ 95%;

. Natural gas combined cycle (NGCC) power plant, gel ¼ 60%.

It should be noted that the performance of the alternative energy system is computed basedupon appropriate efficiency values. Detailed modelling of the different configurations consideredwas not performed. Depending on how the alternative energy system is configured, i.e. how thisheat and electricity is produced, a marginal electricity generation efficiency can be calculated forbiomass that is exported/imported from/to the mill to/from the alternative energy system. It isassumed that biofuel exported to the alternative energy system is used for biofuel based CHP inthe district heating plant. Two scenarios are considered, as discussed below.

4.3.1. Scenario I: exported biofuel is used to increase the share of CHP in the reference externalenergy systemThis scenario is depicted in Fig. 5. The reference system initially includes a biomass heat-only

boiler and a NGCC condensing power plant. CHP is not implemented initially as a result oflimited access to biofuel at a reasonable price. The district heating system is thus assumed will-ing to purchase bark or lignin exported from the mill. Power generation in the NGCC plant isassumed to be unaffected by the biofuel export from the mill. The district heating plant is

Fig. 5. Fuel flow diagram for the scenario where exported biomass leads to increased biofuel fired CHP in a districtheating plant.


assumed to be modified to use the biomass in a CHP unit with an electrical efficiency of 33%and a total efficiency of 105%. The existing biomass boiler has a boiler efficiency of 105%. If oneunit of biofuel is exported from the mill (i.e. DBexport ¼ 1) and fired in the CHP unit, gel,CHP(0.33) units of electricity are produced together with gtot;CHP � gel;CHP (0.72) units of heat. 0.72

units less heat need to be produced in the heat-only boiler, corresponding to 0.72/gtot,blr (0.686)units of biofuel. This biofuel is therefore also available for firing in the CHP plant. Further cal-culations show that balance is achieved when 3.18 units of fuel are fired in the CHP plant perunit of fuel exported from the mill. Under these conditions, the CHP plant produces 1.05 unitsof additional electricity. The resulting marginal electrical efficiency is therefore 105% for the bio-mass exported from the mill. The assumed performance data for the biomass CHP plant is rep-resentative of modern large-sized biofuel fired steam turbine CHP units. Similarly, if biomassfuel is imported to the mill, the biomass CHP will decrease in size, the biomass boiler willincrease in size and the NGCC unit is unaffected. The result will be a 105% decrease in elec-tricity production at the district heating system for each unit of biomass that is imported to themill’s powerhouse.

4.3.2. Scenario II: Exported biofuel leads to increased power generation in a natural gas firedcondensing combined cycleThis scenario is depicted in Fig. 6. In this case, it is assumed that CHP capacity is fully

developed in the reference district heating plant, using natural gas fired CHP technology.Increased availability of biofuel can be used to increase the share of biofuel CHP and decreasethe share of natural gas fired CHP. Assumed performance data for the CHP plant is that ach-ieved by modern combined cycle cogeneration units, i.e. an electrical efficiency of 55% and atotal efficiency of 95%. The natural gas no longer need in the CHP plant is assumed to be usedfor increasing electricity production in the NGCC power plant. Based on similar calculations tothose presented for Scenario I, the marginal efficiency for increased electric power generationfrom exported biomass is shown to be 49% in this case. Similarly, if the mill has to import

Fig. 6. Fuel flow diagram for the scenario where exported biomass creates the potential to increase electric powergeneration in a condensing natural gas combined cycle power plant.


biomass for this scenario, the size of the biomass CHP will decrease, the size of the natural gasCHP will increase and the size of the NGCC power plant will decrease. This leads to a decreasein electricity production of 49% for each unit of biomass that is imported to the mill.It should finally be noted that if all heat production in the reference district heating network

is accomplished in a biofuel CHP unit, increased electricity production from exported biomasscan only be accomplished by firing the biomass in a condensing power plant. In this case, littleto no increase in electric power production can be achieved compared to using the biomass fuelin a condensing power plant unit at the mill site.

4.3.3. Future biofuel gasification based CHP technologyThe biomass CHP technology considered in the reference district heating system is a fluidised

bed boiler combined with a steam turbine unit and a flue gas condenser unit. Biomass inte-grated gasification combined cycle (BIGCC) CHP technology was not directly investigated inthis study. However, considering BIGCC CHP technology instead of steam turbine based CHPtechnology, the highest value for the marginal electric power generation efficiency gel,marginalwould decrease from 105% to 86% based on efficiency values presented in [22] for matureBIGCC CHP technology ðgel ¼ 45%; gtot ¼ 95%Þ. This is because the BIGCC CHP unit cannotachieve the high total efficiencies achieved by the biofuel steam turbine unit. Therefore, therange of marginal electricity generation efficiencies considered in this study covers the casewhere BIGCC CHP is available for CHP power generation in the district heating system.

5. Results and discussion

The electric power generation potential was investigated for a large number of possible millpowerhouse configurations and technologies. In this section, only the most relevant results forthe different mill systems are presented. The different cases investigated together with the per-taining notations were as follows:

. For the KAM pulp mill:

– Recovery boiler with standard steam data (79 bar(a) and 485vC) (RB 79)

– Recovery boiler with advanced steam data (110 bar(a) and 530vC) (RB 110)

– Black liquor gasifier/combined cycle, with no fuel import or export (BLG CC)– Black liquor gasifier/combined cycle, with lignin export (BLG CC LE)– Black liquor gasifier/gas turbine simple cycle, with lignin export (BLG GT LE)

. For the future process integrated pulp mill with reduced heat demand:

– Recovery boiler with standard steam data (79 bar(a) and 485vC) (RB 79)

– Black liquor gasifier/combined cycle, with no fuel import or export (BLG CC)– Black liquor gasifier/combined cycle, with lignin export (BLG CC LE)– Black liquor gasifier/heat-only boiler, with lignin export (BLG NO-GT LE)


. For the integrated pulp and paper mill:

– Recovery boiler (standard steam data), bark fuel import (RB 79 B)– Black liquor gasifier/combined cycle, bark import (BLG CC B)– Black liquor gasifier/biofuel gasifier/combined cycle, bark import (BLG/BIG CC B)– Black liquor gasifier/combined cycle, lignin import (BLG CC L)– Black liquor gasifier/heat-only boiler, with lignin export (BLG NO-GT LE)

It should be noted that gasification of biofuel is only considered as an option for the inte-grated pulp and paper mill. For both market pulp mill options, on-site biofuel usage is limitedto a bark boiler, as in the KAM reference mill configuration. Since fuel export is seen as themost attractive option for market pulp mills, the amounts of biofuel combusted in the barkboiler are relatively small. Gasification of relatively small amounts of biofuel would thereforenot affect the results of this study to any significant extent.The results for the different mill powerhouse configurations studied are listed in Tables 1–3.

The tables also list the main characteristics of the different mill powerhouse configurations con-sidered. Gas turbine load is defined as the amount of fuel supplied to the engine compared tothe amount necessary to run the engine at full-load. The supplementary firing fraction is definedas the fraction of clean syngas produced that is used for supplementary firing in the HRSG.Since the BLG NO-GT LE configurations produce only process steam and consume electricityto run powerhouse auxiliaries, electricity production at the mill site is negative for these cases.Finally, it is important to point out that the mill powerhouse configurations (especially thosebased upon back liquor gasification) have been modelled in detail, as opposed to the referencesystem that was modelled more simply based upon representative values for heat, electrical andtotal efficiencies.For the mill powerhouse configurations with conventional recovery boiler technology, it is

assumed that all internal biofuel Bmill is used at the mill site, regardless of the potential for moreeffective alternative usage for part of the fuel elsewhere. The RB 79 and RB 110 cases thereforeinclude condensing steam turbine units for the market pulp mill cases. Import/export of biofuelto/from the mill was not considered in these cases, which corresponds to current practise in thepulp and paper industry. For most black liquor gasification configurations considered, import/export of biofuel to/from the mill is considered, leading to a total electricity production Etot dif-ferent from the mill site production Emill. As discussed previously, off-site usage of biofuel ischaracterised by the marginal electricity generation efficiency gel,marginal of the reference alterna-tive energy system. With the exception of the BLG CC case, all BLG CHP configurations aresized so as to exactly match the mill process steam demand. Excess fuel is assumed to beexported, or the necessary amount of fuel is imported, as appropriate. Results for the BLG CCcase are presented, for which all mill internal fuel is used on-site. This result can therefore bedirectly compared to those of other authors who have studied electric power generation usingblack liquor gasification technology without considering system consequences due to limitedavailability of biofuel. Finally, results are presented for mill powerhouse configurations thatproduce heat only, so as to maximise fuel export.


Table1

Powerhouse

characteristics

andenergysystem

perform

ance

resultsfortheKAM

pulp

mill(heatdem

and:10GJ/ADt)

RB79

RB110

BLGCC

BLG

CCLE

BLG

GTLE

Millpowerhouse

configurationcharacteristics

Chem

icalrecoverytechnology

RB

RB

BLG

BLG

BLG

HPsteam

pressure

79

110

79

79

30

Steam

turbine

yes

yes

yes

yes

no

Gasturbine

no

no

6FA

GTX100

GTX100

Bark

gasification

no

no

no

no

no

Bark

boiler

yes

yes

yes

no

no

Bark

import/export

no

no

no

export

export

Lignin

import/export

no

no

no

export

export

Gasturbineload

N/A

N/A

93%

100%

100%

Supplementary

firingfraction

N/A

N/A

0%

10%

10%

Resultsformillsite

only

Availablebiofuelatmillsite

BL

MW

251.68

251.68

251.68

251.68

251.68

Bark

MW

33.71

33.71

33.69

33.69

33.69

Talloil

MW

15.80

15.80

15.80

15.80

15.80

Total

MW

301.19

301.19

301.17

301.17

301.17

BiofuelnotusedforCHP

Talloil,sold

MW

15.80

15.80

15.80

15.80

15.80

Bark

tolimekiln

MW

18.11

18.11

17.68

16.44

16.13

AvailablebiofuelforCHPatmillsite

Total(B

mill)

MW

267.28

267.28

267.50

268.93

269.25

Exported

biofuel

Bark

MW

0.00

0.00

0.00

17.25

17.57

Lignin

MW

0.00

0.00

0.00

62.83

75.51

Totalbiofuelconsumptionfor

CHPproductionatmillsite

TotalðB

mill�

DBexportÞ

MW

267.28

267.28

267.50

188.85

176.17

Millsite

steam

andpower

production

Netprocesssteamproduction(H

mill)MW

115.56

115.56

115.56

115.56

115.56

Electricity

production

Gasturbine

MW

0.00

0.00

60.49

36.82

37.41

Steam

turbine

MW

55.82

59.79

24.15

9.30

0.00

Auxilliaries

MW

3.70

4.09

10.18

9.96

9.73

Netproduction(E

mill)

MW

52.12

55.70

74.46

36.16

27.68

(continued

onnextpage)


Table1(continued)

RB79

RB110

BLGCC

BLG

CCLE

BLG

GTLE

Totalelectricityproduction

Marginalelectricaleffi

ciency,ref.sys

49%

105%

49%

105%

49%

105%

49%

105%

49%

105%

Biofuelusage

Availableatmill(B

mill)

MW

267.28

267.28

267.28

267.28

267.50

267.50

268.93

268.93

269.25

269.25

Exported

biofuelfrom

mill

(DBexport)

MW

0.00

0.00

0.00

0.00

0.00

0.00

80.08

80.08

93.08

93.08

BiofuelusedatmillforCHP

ðBmill�

DBexportÞ

MW

267.28

267.28

267.28

267.28

267.50

267.50

188.85

188.85

176.17

176.17

Electricity

production

AtMill(E

mill)

MW

52.12

52.12

55.70

55.70

74.46

74.46

36.16

36.16

27.68

27.68

Contributionfrom

offmillsite

(gel,marginal�D

Bexport)

MW

0.00

0.00

0.00

0.00

0.00

0.00

39.24

84.09

45.61

97.73

Total(E

tot)

MW

52.12

52.12

55.70

55.70

74.46

74.46

75.40

120.25

73.29

125.41


Table2

Powerhouse

characteristics

andenergysystem

perform

ance

resultsforafuture

process

integratedpulp

millwithreducedheatdem

and(8

GJ/

ADt)

RB79

BLG

CC

BLG

CCLE

BLG

NO-G

TLE

Millpowerhouse


Chem


RB

BLG

BLG

BLG

HPsteam

pressure

79

79

79

30

Steam

turbine

yes

yes

yes

no

Gasturbine

no

6FA

LM2500

no

Bark

gasification

no

no

no

no

Bark

boiler

yes

yes

no

no

Bark

import/export

no

no

export

export

Lignin

import/export

no

no

export

export

Gasturbineload

N/A

93%

100%

N/A

Supplementary

firingfraction

N/A

0%

7%

100%

Resultsformillsite

only


BL

MW

251.68

251.68

251.68

251.68

Bark

MW

33.71

33.69

33.69

33.69

Talloil

MW

15.80

15.80

15.80

15.80

Total

MW

301.19

301.17

301.17

301.17


Talloil,sold

MW

15.80

15.80

15.80

15.80

Bark

tolimekiln

MW

18.11

17.88

15.06

13.81


Total(B

mill)

MW

267.28

267.50

270.31

271.56

Exported

biofuel

Bark

MW

0.00

0.00

18.63

19.88

Lignin

MW

0.00

0.00

116.03

158.53

TotalbiofuelconsumptionforCHPproductionatmillsite

TotalðB

mill�

DBexportÞ

MW

267.28

267.50

135.65

93.15

Millsite

steam

andpower

production

Netprocesssteam

production(H

mill)

MW

92.41

92.41

92.41

92.41

Electricity

production

Gasturbine

MW

0.00

60.49

21.12

0.00

Steam

turbine

MW

61.37

29.69

4.97

0.00

Auxilliaries

MW

3.70

10.18

9.59

13.93

Netproduction(E

mill)

MW

57.67

80.00

16.50

�13.93

(continued

onnextpage)


Table2(continued)

RB79

BLG

CC

BLG

CCLE

BLG

NO-G

TLE



ciency,ref.sys

49%

105%

49%

105%

49%

105%

49%

105%

Biofuelusage

Availableatmill(B

mill)

MW

267.28

267.28

267.50

267.50

270.31

270.31

271.56

271.56

Exported

biofuelfrom

mill(D

Bexport)

MW

0.00

0.00

0.00

0.00

134.66

134.66

178.41

178.41


ðBmill�

DBexportÞ

MW

267.28

267.28

267.50

267.50

135.65

135.65

93.15

93.15

Electricity

production

AtMill(E

mill)

MW

57.67

5.67

80.00

80.00

16.50

16.50

�13.93

�13.93

Contributionfrom

offmillsite

(gel,marginal�

DBexport)

MW

0.00

0.00

0.00

0.00

65.98

141.39

87.42

187.33

Total(E

tot)

MW

57.67

57.67

80.00

80.00

82.49

157.89

73.49

173.40


Table3

Powerhouse

characteristics

andenergysystem

perform

ance

resultsforareference

integratedpulp

andpaper

mill(heatdem

and:15.8GJ/ADt)

RB79B

BLG

CCB

BLG/BIG

CCB

BLG

CCL

BLG

NO-

GTLE

Millpowerhouse


Chem


RB

BLG

BLG

BLG

BLG

HPsteam

pressure

79

79

79

79

30

Steam

turbine

yes

yes

yes

yes

no

Gasturbine

no

6FA

6FA,

LM1600

6FA

NO

Bark

gasification

no

no

yes

no

no

Bark

boiler

yes

yes

no

yes

yes

Bark

import/export

import

import

import

no

no

Lignin

import/export

no

no

no

import

export

Gasturbineload

N/A

92%

100%

100%

N/A

Supplementary

firingfraction

N/A

N/A

4%

14%

100%

Resultsformillsite

only


BL

MW

251.68

251.68

251.68

251.68

251.68

Bark

MW

33.71

33.69

33.69

33.69

33.69

Talloil

MW

15.80

15.80

15.80

15.80

15.80

Total

MW

301.19

301.17

301.17

301.17

301.17


Talloil,sold

MW

15.80

15.80

15.80

15.80

15.80

Bark

tolimekiln

MW

18.11

17.88

17.88

18.85

15.81


Total(B

mill)

MW

267.28

267.50

267.50

266.52

269.56

Exported

biofuel

Bark

MW

�14.39

-54.45

�83.33

0.00

0.00

Lignin

MW

0.00

0.00

0.00

-46.54

87.86

TotalbiofuelconsumptionforCHPproductionatmillsite

TotalðB

mill�

DBexportÞ

MW

281.66

321.94

350.83

313.06

181.70

(continued

onnextpage)


Table3(continued)

RB79B

BLG

CCB

BLG/BIG

CCB

BLG

CCL

BLG

NO-

GTLE

Millsite

steam

andpower

production

Netprocesssteam

production(H

mill)

MW

182.96

182.97

182.97

182.97

182.97

Electricity

production

Gasturbine

MW

0.00

60.26

91.77

65.47

0.00

Steam

turbine

MW

43.22

21.00

20.08

21.16

0.00

Auxilliaries

MW

3.74

10.36

15.90

10.26

13.79

Netproduction(E

mill)

MW

39.47

70.90

95.95

76.38

�13.79



ciency,ref.sys

49%

105%

49%

105%

49%

105%

49%

105%

49%

105%

Biofuelusage

Availableatmill(B

mill)

MW

267.28

267.28

267.50

267.50

267.50

267.50

266.52

266.52

269.65

269.56

Exported

biofuelfrom

mill(D

Bexport)

MW

�14.39

�14.39

�54.45

�54.45

�83.33

�83.33

�46.54

�46.54

87.86

87.86


ðBmill�

DBexportÞ

MW

281.66

281.66

321.94

321.94

350.83

350.83

313.06

313.06

181.70

181.70

Electricity

production

AtMill(E

mill)

MW

39.47

39.47

70.90

70.90

95.95

95.95

76.38

76.38

�13.79

�13.79

Contributionfrom

offmillsite

(gel,marginal�

DBexport)

MW

�7.05

�15.11

�26.68

�57.17

�40.83

�87.50

�22.80

�48.87

43.05

92.25

Total(E

tot)

MW

32.42

24.37

44.23

13.74

55.12

8.45

53.57

27.51

29.26

78.46


The KAM pulp mill and the future pulp mill with a reduced heat demand (process integratedmill) have a net surplus of biomass fuel at the mill site. If this surplus fuel is used on-site, thereis no import or export of biomass. If the biomass fuel export figures in Tables 1 and 2 are nega-tive, biomass fuel is imported to the mill.As discussed previously, the GE 6FA gas turbine is too big for some of the powerhouse con-

figurations considered, especially those involving significant amounts of lignin export from themill. To avoid severe part-load operation, smaller gas turbines are used instead. These includethe 43 MWel Alstom GTX100, the 23 MWel GE LM2500 and the 14 MWel GE LM1600, whichhave similar turbine inlet temperatures and pressure ratios to the GE 6FA. For example, thesmaller Alstom GTX100 engine was selected for the BLG CC LE and BLG GT LE cases in theKAM pulp mill to avoid severe part-load operation (a GE 6FA engine would have operated ataround 55% load for these cases).If large amounts of biofuel are to be exported from the mill, significant quantities of lignin

must be extracted from the black liquor. For example, exporting 178 MW of biofuel for theBLG NO-GT LE case considered for the process integrated pulp mill with reduced heat demandwould require extracting approximately 65% of the lignin content of the black liquor. Theremaining black liquor has a low heating value, which may lead to combustion problems in aconventional recovery boiler. The gasification reactor is however oxygen-blown, resulting in asyngas that has a heating value that is sufficiently high for usage as boiler fuel (3.7 MJ/kg(LHV)).As discussed previously, an integrated pulp and paper mill using best available technology

must import fuel in order to cogenerate both heat and electricity in the mill powerhouse. Thiscan be seen in the negative biomass fuel export figures in Table 3.

5.1. Total combined electricity production at the mill site, the reference district heating CHP plantand the NGCC power plant

In this section, the total electricity production (Etot) results presented in Tables 1–3 and inFigs. 7–9 are discussed.

5.1.1. Results for market pulp millsThe results show that if the marginal electrical efficiency is high (105%) for the reference alter-

native biofuel user, the highest total electricity production is achieved by powerhouse config-urations that export large amounts of biofuel from the mill. For example, for the processintegrated pulp mill, the BLG CC LE configuration achieves a total electricity production of158 MWel assuming a marginal electricity efficiency of 105% for the 135 MW of exported bio-fuel. This can be compared to the BLG CC configuration for the same mill-type, that does notexport any fuel at all, and achieves a total electricity production of 80 MWel (including powergenerated in a condensing steam turbine unit). Therefore, when the 135 MW of excess biofuelthat could potentially be exported are used instead on site to increase electric power generation,63.5 MWel of additional power are produced, corresponding to a marginal mill electric powergeneration of 47% for this case.The powerhouse configuration that has the highest total electricity production for the process

integrated pulp mill is the BLG NO-GT LE configuration. On-site biofuel usage is restricted to


Fig. 7. Total electricity production potential for different pulp mill powerhouse configurations (results presented forthe KAM pulp). The two different colours on the bars represent different marginal efficiencies for the biofuel used offmill site.

Fig. 8. Total electricity production potential for different pulp mill powerhouse configurations (results presented for afuture process integrated pulp mill with reduced heat demand). The two different colours on the bars represent differ-ent marginal efficiencies for the biofuel used off mill site.

Fig. 9. Total electricity production potential for different mill powerhouse configurations for a reference integratedpulp and paper mill. The two different colours on the bars represent different marginal efficiencies for the biofuel usedoff mill site.


production of mill process steam, and the remaining biofuel (178 MW) is exported to the alter-native biofuel energy system, where it can be used to achieve a total electric power productionof up to 173 MWel when the marginal electrical efficiency is 105%. It should be noted that theBLG NO-GT LE configuration would also result in the highest total electricity production forthe KAM pulp mill. This case was however not included in this study and specific results aretherefore not available.Conversely, if the marginal electricity generation efficiency for exported biofuel is low, power-

house technologies that consume larger quantities of fuel at the mill site become more favour-able. This can be seen for example by comparing the BLG CC LE configuration with the BLGCC configuration for the process integrated pulp mill. If the marginal electrical efficiency forexported biofuel is 49%, the BLG CC LE has a total electricity production of 82.5 MWel com-pared to 80 MWel for the BLG CC. In this case, the on-site electricity production for the BLGCC LE amounts to only 16.5 MW. The remaining 66 MW of electricity are produced in the ref-erence alternative energy system from the 135 MW of exported biofuel. In this case, it is clearthat the gain in total electricity production is essentially insignificant in comparison to the effortrequired to extract and transport the excess biofuel.When gel,marginal for electric power generation from exported biofuel to the alternative energy

system is low, the following analysis can be performed to determine if excess biofuel should beused at the mill site for electricity production or exported and used elsewhere. First, the mar-ginal electricity generation efficiency from excess biofuel is computed for the mill site. This mar-ginal efficiency is then compared with the marginal electrical efficiency for exported fuel in thealternative energy system. Defining an appropriate value for the marginal electrical efficiency forusage of excess biofuel at the mill site is not simple. This is because the fuel is in many casesused in a number of different operations at the mill site (e.g. bark boiler and black liquor gasifi-cation gas turbine CHP plant). The marginal efficiency can in theory only be defined for a givenusage of a given fuel. An indicative value can however often be obtained by comparing the mill-site electric power outputs and fuel consumption of two different powerhouse configurations.When the indicative mill-site marginal electric power generation efficiency for excess biofuel hasbeen determined, this can be compared to the marginal efficiency value for off-site power gener-ation. Conclusions can then be drawn as to which usage of excess biofuel leads to the highesttotal electricity production. This can be illustrated by the following example assuming a 49%marginal efficiency for exported biofuel. The process integrated pulp mill uses 93 MW of fuel inorder to cover its heat demand only (BLG NO-GT LE case). To cogenerate 30.5 MWel

(16.5�(�14)) of electricity, an additional 42.5 MW of biofuel is needed (the BLG CC LE casecompared to the BLG NO-GT LE case), resulting in a marginal electrical efficiency of 72%.This can be compared with the 49% marginal efficiency that can be achieved if this part of theexcess fuel is exported. This higher marginal electrical efficiency is achieved because the elec-tricity is produced using the mill’s heat load for efficient CHP production. If more electricity isgenerated at the mill, it is necessary to partially implement condensing power generation oper-ation, leading to lower efficiency. So by comparing the BLG CC case with the BLG NO-GT LEcase, it is possible to produce 94 MWel (30.5 MWel from CHP and 63.5 MWel from condensingpower) using 178 MW of extra biofuel resulting in a marginal electrical efficiency of 54%. How-ever, the 63.5 MWel of condensing power produced using an additional 135 MW of biofuelcompared to the BLG CC LE case will only result in a marginal electrical efficiency of 47%


which is lower than the marginal electrical efficiency for exported fuel in the alternative energysystem. Hence it is theoretically better to export 135 MW of biofuel than to use it at the millsite for electricity production in order to achieve a higher total electricity production (82.5MWel).The results presented in Tables 1 and 2 can also be used to analyse the implications for elec-

tricity production potential when the process steam demand at the mill is reduced. As expected,if all mill internal biofuel is used for on-site electricity generation, more electricity can be pro-duced. For example, the BLG CC powerhouse configuration (that does not export biofuel) pro-duces 74.5 MWel in the KAM pulp mill, compared to 80 MWel in the process integrated pulpmill, i.e. a gain of 5.5 MWel. If excess biofuel is exported, a reduced mill heat demand increasesthe amount of excess fuel available for export. Considering the BLG CC LE configuration, thegain in total electricity production is shown to be 7.1 MWel for gel,marginal equal to 49% and 37.6MWel for gel,marginal equal to 105%.The powerhouse configuration that has the highest total electricity production for the process

integrated pulp mill is the BLG NO-GT LE configuration. On-site biofuel usage is restricted toproduction of mill process steam, and the remaining biofuel (178 MW) is exported to the alter-native biofuel energy system, where it can be used to produce up to 173 MWel when the mar-ginal electrical efficiency is 105%. It should be noted that the BLG NO-GT LE configurationwould also result in the highest total electricity production for the KAM pulp mill. This casewas however not included in this study and specific results are therefore not available.To summarise the results for the market pulp mills with a net excess of mill internal biofuel,

total electricity production can substantially be increased by exporting excess biofuel from themill to the alternative biofuel user when gel,marginal is equal to 105% and biofuel usage at the millsite is restricted to production of process steam. When gel,marginal is close to 50% or lower, bio-fuel export is not advantageous, and high total electricity production figures are achieved by ablack liquor gasification combined cycle configuration, with or without a condensing steam tur-bine unit.

5.1.2. Results for integrated pulp and paper millsFor integrated pulp and paper mills (results shown in Table 3 and Fig. 9), the results show

that if gel,marginal is high for alternative usage of biofuel, import of biofuel to the mill for CHPshould be as low as possible. Cogeneration of electric power at the mill site should in fact beavoided, as for the market pulp mills, and fuel usage should be restricted to production of millprocess steam. In this case ðgel;marginal ¼ 105%Þ, the BLG NO-GT LE powerhouse configurationthat exports 88 MW of biofuel and achieves a total electricity production of 78.5 MWel clearlyoutperforms the other cases by 200%. If the powerhouse at the mill has to produce both elec-tricity and heat, then the BLG CHP configurations with biofuel import will not perform betterthan the RB configurations. This can be seen for the conventional recovery boiler configuration(24.4 MWel total electricity production with 14 MW of imported biofuel) that performs betterthan most of the BLG systems with imported biomass. Only the BLG CC L system performsbetter (27.5 MWel total electricity production with 46.5 MW imported biofuel). The BLG/BIG CC B configuration performs least well of all (83 MW biofuel import, 8 MWel electricityproduction).


When the marginal electric power generation efficiency for alternative off-site usage of biofuelis low, it is possible to determine the mill-site marginal electrical efficiency for the integratedpulp and paper mills, as performed previously for the process integrated pulp mill. In this case,it is important to account for the decrease in electricity production outside the mill due to theimport of biofuel. For example, the BLG CC L case produces 90 MWel more electricity thanthe BLG NO-GT LE case using 131 MW more biofuel ðgel;marginal ¼ 69%Þ not accounting forthe decrease in electricity production outside the mill because of the 46.5 MW of imported fuel.When a marginal electrical efficiency of 49% is assumed for the imported biofuel, the total elec-tricity production will decrease from by 23 to 67 MWel. It is nevertheless favourable to importthe 46.5 MW of biofuel to the mill, since it is used with a marginal electrical efficiency (69%)that is higher than 49%. Assume that the BLG/BIG CC B technology which imports more bio-fuel than the BLG CC L technology was chosen instead. Using 38 MW more biofuel, 20 MWel

more electricity can be produced, leading to gel;marginal ¼ 52% compared to the BLG CC L case,

which is higher than 49% and hence it is favourable to use that technology. It should howeverbe mentioned that compared to the BLG NO-GT LE case, 110 MWe more electricity is pro-duced with gel;marginal ¼ 65% which is lower than the 69% achieved for the BLG CC L case.

Using the same reasoning for an off-site marginal electrical efficiency of 105% leads to the con-clusion that no electricity should be produced at the mill (69% < 105%) and that only heatshould be produced at the mill site, exporting 88 MW of biofuel.

5.2. Electricity production at the mill site

Black liquor gasification is only likely to be commercially mature within 10–15 years. Currentavailability of biofuel at a reasonable price is essentially unlimited in most Swedish biofuel mar-kets. In the future, when black liquor gasification technology is likely to become mature, it isreasonable to assume that demand for biofuels will be significantly stronger than today, andthat availability of biofuel at a reasonable price will be limited. Despite this outlook, many stud-ies presenting electric power generation potential in the pulp and paper industry with black liq-uor gasification do not account for alternative uses of biofuels. In this section, the electricpower production at the mill site (Emill) is compared with the total electricity production (Etot)with a view to illustrating the major differences that can result.If alternative biofuel usage is not considered, excess biofuel in market pulp mills is clearly

used for increased on-site power generation involving condensing steam turbine units, corre-sponding to a low efficiency usage of the biofuel, which was not the aim of this study. For inte-grated pulp and paper mills, it is necessary to import biofuel to operate a CHP plant that issized to cover the full mill heat demand. Different mill powerhouse configurations furthermorerequire different amounts of imported biofuel, which makes comparisons difficult. High on-siteelectric power output can be achieved by importing large amounts of biofuel and running con-densing steam turbine units. In this section, the discussion is therefore limited to market pulpmills, for which the amount of mill internal biofuel is constant, and for which it is assumed thatadditional biofuel is not imported to the mill.For market pulp mills, excess internal mill fuel can only be used for on-site power generation

if alternative usage is not considered. In this case, the results show for example that the


potential for increasing on-site electric power generation by implementing black liquor gasifi-cation technology instead of advanced recovery boiler technology is large. For the process inte-grated pulp mill, the BLG CC powerhouse configuration is shown to produce 22 MWel moreelectricity than a conventional RB powerhouse. If alternative usage for biofuel is now con-sidered, very different conclusions may be drawn. In the most extreme case (i.e. BLG NO-GTLE configuration with a marginal efficiency for alternative biofuel usage of 105%), the increasein total electric power production compared to a conventional recovery boiler configurationwhere all mill internal fuel is used at the mill site is 116 MWel, i.e. 94 MWel more than whenalternative biofuel usage is not considered!This section is included to show what happens when different amounts of fuel are used, and a

comparison is made of the electricity production. If the analysis is restricted to electricity pro-duction at the mill site only for the integrated pulp and paper mill, the highest on-site electricityproduction is achieved by the BLG/BIG CC B configuration (96 MWel) not taking into accountthe decrease in electricity production outside the mill because of the imported biofuel. The low-est on-site power production is achieved by the RB 79 B configuration (39.5 MWel), as seen inTable 3. This is expected, since this BLG powerhouse configuration has the highest fuel con-sumption and only the mill site is included for energy performance assessment. These resultsshould be compared to 8 MWel for the BLG/BIG CC B configuration and 24 MWel for theRB 79 B configuration, if the decrease in total electricity production is included withgel;marginal ¼ 105%.

6. Conclusions

In this study, the performance of different black liquor recovery powerhouse systems wereexamined. The objective of the study was to identify mill powerhouse configurations that maxi-mise the total electric power production whilst satisfying the mill’s process heat demand andtaking into account the effects that export or import of biofuel has on the electricity productionfrom alternative biofuel users outside the mill. In this study, we assumed that exported biofuelcould be used in an alternative energy system consisting of a district heating network and anatural gas combined cycle power plant. Different marginal electric power generation efficienciesfor exported biofuel were considered, namely 49% and 105%. Similarly, when biofuel wasimported to the mill, the decrease in off-site electric power generation potential due to reducedavailability of biofuel was considered.The results can be summarised as follows:

. Black liquor gasification technology performs better than conventional and advanced recov-ery boiler technology in all cases considered;

. For both market pulp mills and integrated pulp and paper mills, excess biomass fuel export isthe best way to achieve high total electricity production if the marginal electricity generationefficiency for the exported fuel is higher than that for on site usage (~50% for condensingpower generation). This is the case if biofuel exported from the mill can be used to increasethe degree of CHP in a district heating system. Biofuel usage at the mill site should then berestricted to the amount necessary to satisfy the mill’s heat load, i.e. CHP should be avoided.


For the process integrated market pulp mill for example, 178.4 MW of biofuel can be

exported and used to cogenerate 173 MWel off-site in district heating CHP plant, assuming a

marginal electrical efficiency value of 105% for biofuel usage in the district heating plant. If

all fuel is instead used on-site (which would include condensing power generation) at most 80

MWel can be produced with the BLG CC technology.. For market pulp mills, all internal mill biofuel should be used to fuel a black liquor gasifi-

cation combined cycle if the marginal electricity generation efficiency for exported biofuel is

lower than that for on-site usage (~50%). This is the case for example if CHP is fully built out

in the reference district heating system, based on natural gas fuel. Exported biofuel can be

used to partly replace natural gas fired CHP with biofuel CHP, and the natural gas no longer

needed for CHP in the district heating plant can be used for electric power generation in a

natural gas combined cycle condensing power plant instead.. For the integrated pulp and paper mills, fuel can be imported to the mill for use in a black

liquor gasification CHP powerhouse configuration, provided that the incremental marginal

electric power generation efficiency for each additional unit of imported fuel is higher than

that achieved by the alternative energy system. Since the marginal electrical efficiency is

between 60% and 70% for the black liquor gasification based CHP powerhouses compared to

heat-only powerhouse configurations, biofuel should be imported if gel;marginal ¼ 49% for the

alternative energy system (the highest total electricity production that can be achieved is in

this case equal to 55 MWel for the BLG/BIG CC B case).

These conclusions are clearly only valid for the conditions assumed in this study. The district

heating system chosen in this study for alternative usage of biofuel was chosen as relevant for a

future situation in Sweden, but in other countries with other resources and different energy poli-

cies, a totally different alternative biofuel user may well be more relevant. It is important to

point out for example that the results of this study are in part due to the assumption that an

alternative use for biomass is combustion in a district heating system with a very high efficiency

(due mainly to flue gas condensation that is possible given the low temperature level of the dis-

trict heating heat demand).Finally, it is important to note that this study was restricted to energy aspects of fuel usage.

In practise, economic aspects will play an important role in determining the choice of tech-

nology configuration. New energy policies (e.g. joint implementation measures as provided for

in the Kyoto protocol or trading of renewable electricity certificates [23]) can also encourage

interaction between different biofuel users with an aim to share costs and profits from invest-

ments made to increase the total electricity production from available biomass fuel resources.

Acknowledgements

This research was funded by the Swedish National Energy Administration as a part of the

‘‘Bioenergy Systems’’ research program.


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