Black liquor gasification—consequences for both industry and society

Energy 29 (2004) 581–612 www.elsevier.com/locate/energy

Black liquor gasification—consequences for both industry and society H. Eriksson, S. Harvey
Heat and Power Technology Group, Department of Chemical Engineering and Environmental Science, Chalmers University of Technology, SE-412 96 Go¨teborg, Sweden Received 1 December 2001

Abstract The pulp and paper industry consumes large quantities of biofuels to satisfy process requirements. Biomass is however a limited resource, to be used as effectively as possible. Modern pulping operations have excess internal fuels compared to the amounts needed to satisfy process steam demands. The excess fuel is often used for cogeneration of electric power. If market biofuel availability at a reasonable price is limited, import/export to/from a mill however changes the amount of such biofuel available for alternative users. This work compares different mill powerhouse technologies and CHP plant configurations (including conventional recovery boiler technology and black liquor gasification technology) with respect to electric power output from a given fuel resource. Different process steam demand levels for different representative mill types are considered. The comparison accounts for decreased/increased electricity production in an alternative energy system when biofuel is imported/exported to/from the mill. The results show that black liquor gasification is in all cases considered an attractive powerhouse recovery cycle technology. For moderate values of the marginal electric power generation efficiency for biofuel exported to the reference alternative energy system, excess mill internal biofuel should be used on mill site for gas turbine based CHP power generation. The remaining excess biofuels in market pulp mills should be exported and used in the reference alternative energy system in this case. For integrated pulp and paper mills, biofuel should be imported, but only for cogeneration usage (i.e. condensing power units should be avoided). If biofuel can be used elsewhere for high efficiency CHP power generation, mill internal biofuel should 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

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Nomenclature 79 79 bar(a) 110 110 bar(a) ADt air dried tonne B bark import/biofuel D difference BB bark boiler BIG biomass fuel gasification BLG black liquor gasification CC combined cycle gas turbine CHP combined heat and power DH district heating system DS dry solids E electric power ECF elementary chlorine free F fossil fuel GT simple cycle gas turbine H heat produced HP high pressure HRSG heat recovery steam generator IP intermediate pressure ISO International Standard Organisation, Standard air conditions KAM Ecocyclic Pulp Mill L lignin import LE lignin export LHV lower heating value LLP very low pressure LP low pressure MP medium pressure NGCC natural gas combined cycle NO-GT no power production (heat only) RB recovery boiler STIG steam injected gas turbine TCF totally chlorine free g efficiency Superscript v base case (reference) value Subscripts biomass biomass fuel

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DH district heating el electricity export export tot total marginal marginal mill mill RS reference system NG natural gas NGCC natural gas combined cycle

1. Introduction Increased concern for climate change has lead to a variety of initiatives that aim to reduce the emissions of greenhouse gas emissions. Biofuels are considered CO2 neutral, and as different policy measures aiming to decrease greenhouse gas emissions are implemented, the demand for biofuels is expected to increase substantially. It is therefore important that usage of such fuels is as effective as possible. The Swedish pulp and paper industry is a substantial consumer of biomass fuels, including bark, forest logging residues and black liquor. These fuels are used to generate process heat and electric power. According to data presented in [1], the total Swedish pulp production in 2000 amounted to 11.4 million ADt, of which 7.2 million ADt were chemical pulp. 3.55 million ADt were sold as market pulp and 3.65 million ADt were used directly for paper production integrated at the mill site. Fuel and electricity consumption amounted in 2000 to 77.76 TWh, of which 21.46 TWh was electricity corresponding to 40% of the total electricity usage 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 of the 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-heat ratios can be achieved if the black liquor is instead gasified and combusted in a gas turbine CHP unit. This is particularly relevant for future pulp mills, for which available internal biomass fuels will be more than sufficient to satisfy the mill’s heat demand. The excess biofuel can therefore be used on-site to generate further amounts of electric power for export, or be exported 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 pulp producing countries. Work performed in Sweden includes amongst others studies performed by Berglin [2–4] and Maunsbach [5]. Refs. [6] and [7] illustrate the extensive work accomplished in Finland in this area during the 1990s. Refs. [8–10] illustrate the work accomplished in the area by North American researchers. All the cited studies include both market pulp mills and integrated 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

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resource. Furthermore, these studies do not compare biofuel usage within the mill powerhouse with 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 the total electric power generation from a given fixed amount of fuel. A number of studies of biofuel 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 the future. This assumptions has been made by a number of authors studying future energy system options for Sweden, see for example ref. [14]. As a result of this assumption, import of additional biofuel to the mill is assumed to correspondingly reduce the amount of biofuel available for a reference alternative biofuel user. Conversely, if excess biofuel is exported from the mill, this biofuel can be used elsewhere. The assumption of limited availability of biofuel in the long term is clearly politically challenging since this would result in significant price increases for biomass and biofuel, which could potentially lead to severe problems for the pulp and paper industry. Such macro-economic issues are not discussed further in this work. An additional assumption in this work is that changes in biofuel usage outside the mill as a result of biofuel import/export to/from the mill affect the electricity production outside the mill. The goal of this work is to identify ways in which the total electricity production can be maximised, including production at both the mill site and in the reference alternative energy system. The main focus is on electricity production based on black liquor gasification mill powerhouse recovery cycle technology. The study compares the total electricity production related to different black liquor gasification 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 increased import/export to/from the mill is assumed to correspond to decreased/increased usage elsewhere; . 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 heating), because it is a wet fuel and high total efficiency values can be achieved if a flue gas condenser unit is used. A district heating system (including CHP units) is therefore retained as the reference alternative biofuel user for this study; . The reference alternative biofuel user is assumed to have access to natural gas fuel, whereas this fuel is not assumed to be available for the mills. This assumption reflects Swedish conditions, where natural gas is only currently available on the West Coast. It is possible that this fuel will become more widely available in the future, particularly in other parts of Southern Sweden. However, it is not likely that natural gas will be available in the more remote locations typical of pulp and paper mills. . The mills are assumed to be equipped to handle large quantities of biomass. It is therefore assumed 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 to make assumptions regarding the most likely technology for new grid electric power generation capacity additions. Given that BLG technology is only likely to be commercially mature

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within 10–15 years, high performance natural gas fired combined cycle technology (60% electrical 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 pulp mills traditionally select biomass as an import fuel due to the capability to handle large quantities of biomass in the mills and the high taxes on fossil fuel alternatives. Possible import biofuels include bark, sawmill and logging waste and lignin. Biofuel can be exported from the mill as either bark or lignin. The market for primary forest fuels (i.e. bark, sawmill and logging waste) is well established. Although it is technically feasible to extract lignin from the black liquor stream [15], large-scale extraction of lignin has not been implemented so far in the pulping industry. It is however reasonable to assume that a market could be developed for lignin, since processed 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. However, given the assumptions listed above, biofuel usage is primarily focused on CHP, either at the mill site, or in the reference external energy system. Given that the investment and running costs for biofuel CHP are to a certain extent comparable for the different technologies and configurations considered, not directly considering economic aspects should therefore not severely bias 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 this study were obtained through collaboration with the ‘‘Ecocyclic Pulp Mill’’ (KAM) research program. The KAM program has so far focused on market pulp mills, and has defined a reference mill that is assumed to incorporate best available technology for all process components. The KAM reference mill has been used as a basis for a number of former studies of BLGCC systems. 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 pulp mill [1,16]. The available incoming biofuel (bark and black liquor) is sufficient to satisfy the KAM pulp mill’s heat and electricity demand. The excess biomass fuel can in this case be used to generate additional steam for expansion in a condensing steam turbine, thereby generating surplus electricity for export from the mill. Excess biofuel could also be exported and used elsewhere. Most current Scandinavian pulp mills have a net fuel deficit and have to import fuel. A similar ongoing project aims to identify the key characteristics of an integrated pulp and paper mill that uses best available technology. There is a large difference in steam demand between a pulp mill and a pulp and paper mill. Based on preliminary estimations extrapolated from the results of the KAM pulp mill project, an integrated pulp and paper mill using best available technology would have an even higher steam demand (15.8 GJ/ADt pulp) than the current average Swedish pulp mill [2,12]. An integrated pulp and paper mill has therefore an energy deficit, and is dependent upon fuel import (biomass fuel) to meet its energy demands. To more thoroughly investigate how the process steam demand affects the total electricity production, a

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hypothetical future pulp mill with a low process steam demand was also defined. Assuming a high degree of process integration, the process steam demand for this low-energy mill was set to 8 GJ/ADt, based on a study presented in [17]. Around 75% of the steam savings compared to the KAM pulp mill are achieved in the black liquor evaporation plant. Since this mill has a lower 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 cooking and 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. All available black liquor (1797 tonne DS/day) is fired in a conventional recovery boiler, producing v steam at 79 bar(a) and 480 C. 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 is fired in a bark boiler, and the rest is used as fuel in a lime-kiln. The bark has 45% dry solids content by weight and a lower heating value of 16 MJ/kg DS. A total of 210 MW of steam is produced in the recovery boiler (RB) and 12.8 MW is produced in the bark boiler (BB). The KAM’98 mill generates an excess amount of steam, even without the bark boiler. Heat and mass balances for the KAM’98 reference mill have been defined for four different operating conditions, namely winter and summer operation with both ECF and TCF bleaching. The steam demand varies between 108 and 126 MW for the different conditions. Powerhouse energy flows for 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 are as follows:

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

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. KAM’98 pulp mill. The process steam demand corresponds to summer operation with TCF bleaching: 116 MW (50 MW of 12 bar(a) MP steam and 66 MW of 4.5 bar(a) LP steam). The electricity consumption in the mill is 29 MWel. . Future process integrated pulp mills with reduced heat demand. The steam demand is assumed to be decreased through process integration measures to 8 GJ/ADt compared to 10 GJ/ADt in the reference KAM’98 mill. One way to achieve such a decrease is by using waste heat for black liquor evaporation, thus reducing the usage of live steam [17,18]. The steam demand could thereby be reduced to 92 MW (45 MW of MP steam and 47 MW of LP steam). The electricity consumption (29 MWel) is assumed not to be affected by such measures. . Integrated pulp and paper mills using best available technology. Heat and electricity requirements for an integrated mill are so far not available from the KAM program. Indicative values were estimated based on the KAM’98 reference pulp mill, and adding estimated steam and power consumption data for a modern fine paper mill. The total steam demand for an integrated pulp and paper mill was thus estimated at 15.81 GJ/ADt pulp. Further discussion may be found in [2]. It is however clear that the heat requirements for a paper mill depend very much on the paper quality being produced, and that calculations should be conducted for a variety of paper mill types before drawing general conclusions. The mill size was chosen identical 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 mill this size, the total process steam demand is 183 MW (87 MW of MP steam and 96 MW of LP steam) 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 the other is to produce process steam and possibly electricity for the mill. This can be carried out in a Tomlinson recovery boiler system or in a black liquor gasifier system. Modelling of the two systems was based upon the requirement that they be fully interchangeable with respect to the associated pulp mill process. The two mill powerhouse recovery cycle technologies considered are described below. 3.1. Conventional recovery boiler technology The KAM’98 reference pulp mill incorporates conventional Tomlinson recovery boiler technology, as depicted in Fig. 2 and described in the Swedish Ecocyclic Pulp Mill (KAM’98) project [16]. The black liquor stream is fired in a boiler, and used to generate high pressure steam at v v 79 bar(a) and 480 C from feed water at 140 C, which is then expanded in a back-pressure steam turbine to produce both process steam and electricity. The cooking chemicals are recovered in from the bottom of the boiler and mixed with weak washing liquor in the quench, producing green liquor which is then removed for further processing (Fig. 2). If the steam produced in the recovery boiler is insufficient to meet the mill’s demand, additional steam is produced in a power boiler, using bark or sawmill residuals as fuel. Recovery boiler technology with advanced v steam data (110 bar(a) and 530 C) is also considered in this study. Even the 79 bar(a) and

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

480 C steam data considered in the base case is advanced compared to standard practice in v many Swedish mills today (60 bar(a) and 440 C). 3.2. Black liquor gasification technology Black liquor gasification technology results in higher power-to-heat ratios than conventional recovery boiler technology. Furthermore, a higher degree of utilisation of the biofuel energy content is achieved since it is possible to produce steam in the gas cooler by condensing a portion 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 following operations: . . . . . . . .

v

oxygen-blown, high-temperature (ca 950 C), 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 technology together 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 power generation for a number of different conditions. Although the key components for BLGCC that require further development are the gasifier and pertaining gas clean-up equipment, the key

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

component for electric power production is the gas turbine. In this work, particular focus was thus placed on estimating gas turbine performance and how integration of fixed sized gas turbine 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 engine was chosen for this study, on the grounds of good size matching for this engine with respect to the available fuel stream if all black liquor is gasified. Furthermore, significant experience has been accumulated with this engine for low heating value fuel applications. The engine has a v design pressure ratio of 15:1, a design turbine inlet temperature of 1288 C and a power output of 70 MWel at ISO rating conditions on natural gas fuel. The engine is slightly oversized with respect to the available fuel, which implies that it should in principle operate at part-load. Performance modelling of off-design operation for the gas turbine operating on gasified black liquor fuel was based upon the following assumptions: – the gas turbine is assumed to be modified to allow air extraction for the ASU and steam injection; – gasifier air is extracted from the compressor discharge to supply approximately 1/3 of the air needed in the air separation unit; v – the combustor exit temperature is de-rated by 20 C to avoid blade cooling problems [19].

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Furthermore, part-load operation of the gas turbine was modeled assuming the following operating strategy: – the air inlet flow to the engine is first reduced using the inlet guide vanes until the air flow to the 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 configurations considered, especially those involving significant amounts of lignin export from the mill. To avoid severe part-load operation, smaller gas turbines are used instead. These include the 43 MWel Alstom GTX100, the 23 MWel GE LM2500 and the 14 MWel GE LM1600, which have similar turbine inlet temperatures and pressure ratios as the GE 6FA. All of these gas turbines 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, a fraction of the product gas is used for supplementary firing in the HRSG. In one extreme powerhouse configuration, the product gas is fired in a heat-only boiler (i.e. there is no on-site cogeneration of electric power) and large quantities of biofuel (bark and lignin) are exported from 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 electricity production; – Simple cycle gas turbine (GT) where only the gas turbine is present and producing electricity; – 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 differentiated into IP (30 bar(a)), MP (12 bar(a)) and LP (4.5 bar(a)) levels, with varying degrees of condensate return. In the market pulp mills, the condensate return temperatures were set at 124, v 156 and 188 C for LP, MP and IP condensate, respectively. For the integrated pulp and paper v mills, these temperatures were set at 116, 126 and 200 C. Steam is generated in different parts of the power island and fed into the mill steam system. Additional heat is recovered through generation 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 the technologies 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]. If a biomass gasifier is used, there is no need for a power boiler as all biomass fuel is gasified.

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. Lignin and black liquor co-gasifier: imported lignin is mixed with the available black liquor and gasified in the existing black liquor gasifier. Since the lignin is extracted from another mill’s black liquor, it is reasonable to assume that the lignin gasification can be performed in this way. Both the biomass gasifier and the power boiler use bark/sawmill residuals as fuel, while the combined black liquor gasifier and lignin gasifier uses imported lignin as additional fuel. The produced high pressure steam has the same pressure as the high pressure steam produced in the black liquor gasifier system. 3.5. Electricity consumption The electricity consumption for the pulp and paper process (powerhouse internal consumption excluded) is independent of the powerhouse configuration. The powerhouse electricity consumption will however change with the powerhouse configuration. The largest contributor to the variations in electricity consumption is the ASU compressors, since the different powerhouse configurations have different gasifier oxygen needs. The change in electricity consumption for pumping the internal streams in the powerhouse, the LLP to LP heat pump and the H2S recycle compressor 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 turbine 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 the plant. 4. System aspects of biomass fuel import and export As mentioned previously, biofuel is assumed to be a limited renewable fuel resource that should be used as efficiently as possible. It is therefore important to account for the impact on global electricity production resulting from changed biofuel availability for other potential biomass fuel users as a result of different levels of biofuels usage at the mill site. Potential competing users of biofuels include mills with the possibility to export heat to a district heating network, district heating networks, other industries, CHP plants or manufacturers of processed biomass fuels (e.g. pellets, pyrolysis oil or methanol). District heating systems are widespread in Sweden, and are particularly well suited to usage of biofuels given the possibility to achieve high efficiencies through implementation of boiler flue gas condensation. District heating systems (including CHP capability) are thus retained as the reference alternative user for biofuels in this study. It is important to note that biofuel transportation to and from the mill infers energy losses, but these losses are negligible if the biomass can be transported by ship.

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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 assessing 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. Biofuel 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.

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. B0mill is the fixed amount of internal biomass at the mill site in the form of black liquor and falling bark; . B0biomass is the fixed biofuel resource that is available primarily as fuel for the district heating network; . DBexport is the amount of biofuel that is exported from the mill site. If it is negative, it means that 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 district heating system is therefore decreased compared to B0biomass ; 0 . FNG 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; 0 þ DHmill is the mill heat consumption that is fixed for each of the three mill types, as . Hmill defined in Section 2.2. To account for the different heat demand in three mill types, the heat 0 Þ and one part that demand has been divided into two parts, one fixed reference level ðHmill varies with mill type ðDHmill Þ; 0 þ DEmill is the net electricity production at the mill powerhouse (i.e. the powerhouse . Emill internal electricity consumption is deducted). To show the different electricity productions in the different mill powerhouse configurations, the mill electricity production has been divided 0 into two parts, one fixed reference level ðEmill Þ and one part that varies with powerhouse configuration ðDEmill Þ; 0 is the fixed heat demand in the district heating system; . HDH 0 0 is . EDH þ DEDH is the electricity production from the district heating system CHP plant. EDH the reference level of electricity production when no biofuel is exported from the mill to the district heating system and DEDH accounts for the change in electricity production that relates to the exported biofuel from the mill site. When fuel is imported to the mill, the electricity production in the district heating system is reduced, and DEDH is negative; 0 0 þ DENGCC is the electricity production from the NGCC power plant. ENGCC is the ref. ENGCC 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 the exported biofuel from the mill site. Since the goal is to maximise the total electricity production from the system shown in Fig. 4 0 ), the goal function for this study is with a fixed amount of fuel (i.e. B0mill þ B0biomass þ FNG defined as: 0 0 0 þ DEmill þ EDH þ DEDH þ ENGCC þ DENGCC Goal function ¼ Emill

(1)

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

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efficiencies for the electricity and heat production in the district heating system (including CHP) and the NGCC power plant are independent of size. 0 þ DEmill is known for a given powerhouse conThe size of the mill is known, therefore Emill figuration and mill type. Since the size of the district heating system and the NGCC power plant 0 are not defined, the reference electricity production in the district heating system ðEDH Þ and the 0 reference electricity production in the NGCC power plant ðENGCC Þ are not defined. However, 0 0 knowledge of the absolute value for EDH and ENGCC is not important since these values are constant. The goal function defined in Eq. (1) is therefore maximised if the total electricity production defined in Eq. (2) is also maximised: 0 þ DEmill þ DEDH þ DENGCC Etot ¼ Emill

(2)

The change in electricity production in the district heating system and the NGCC power plant depends on the amount of biomass that is exported from or imported to the mill. The change in electricity production in the district heating system and the NGCC power plant ðDEDH þ DENGCC Þ can be expressed according to Eq. (3): DEDH þ DENGCC ¼ gel;marginal  DBexport ;

(3)

where gel;marginal ¼

DEDH þ DENGCC DBexport

(4)

The marginal electricity production efficiency in Eq. (4) thus expresses the net total combined potential for increase in electricity production in the district heating system and NGCC power plant, associated with a given export of biofuel from the mill. It is important to note that realising this potential requires that the district heating system and NGCC power plant be modified so as to maximise electric power production from the available fuel streams. Thus, gel,marginal does not refer to the potential for power increase from existing equipment, but rather the potential for power increase if the plant is rebuilt under given conditions. Different scenarios and resulting values for gel,marginal are discussed in the next section. The total electricity production in Eq. (2) can be rewritten to include the marginal electricity production efficiency, resulting in the 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 fuel resource, it is assumed that biofuel exported from the mill site to the reference alternative biofuel user is used for increasing electricity production. The potential for increased electricity production in a district heating system depends essentially on the extent to which CHP has been implemented in this system. Different degrees of CHP implementation in the district heating system and the consequences for electricity production are discussed below.

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The system outside the mill site includes both natural gas and biomass users delivering heat to a district heating system and electricity to the power grid. The available production technologies are assumed to be state-of-the-art and include the following (all efficiency values are based 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 steam turbine 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 based upon appropriate efficiency values. Detailed modelling of the different configurations considered was not performed. Depending on how the alternative energy system is configured, i.e. how this heat and electricity is produced, a marginal electricity generation efficiency can be calculated for biomass that is exported/imported from/to the mill to/from the alternative energy system. It is assumed that biofuel exported to the alternative energy system is used for biofuel based CHP in the 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 external energy system This 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 of limited access to biofuel at a reasonable price. The district heating system is thus assumed willing to purchase bark or lignin exported from the mill. Power generation in the NGCC plant is assumed 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 district heating plant.

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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 one unit 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 calculations show that balance is achieved when 3.18 units of fuel are fired in the CHP plant per unit of fuel exported from the mill. Under these conditions, the CHP plant produces 1.05 units of additional electricity. The resulting marginal electrical efficiency is therefore 105% for the biomass exported from the mill. The assumed performance data for the biomass CHP plant is representative of modern large-sized biofuel fired steam turbine CHP units. Similarly, if biomass fuel is imported to the mill, the biomass CHP will decrease in size, the biomass boiler will increase in size and the NGCC unit is unaffected. The result will be a 105% decrease in electricity production at the district heating system for each unit of biomass that is imported to the mill’s powerhouse. 4.3.2. Scenario II: Exported biofuel leads to increased power generation in a natural gas fired condensing combined cycle This 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 decrease the share of natural gas fired CHP. Assumed performance data for the CHP plant is that achieved by modern combined cycle cogeneration units, i.e. an electrical efficiency of 55% and a total efficiency of 95%. The natural gas no longer need in the CHP plant is assumed to be used for increasing electricity production in the NGCC power plant. Based on similar calculations to those presented for Scenario I, the marginal efficiency for increased electric power generation from 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 power generation in a condensing natural gas combined cycle power plant.

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biomass for this scenario, the size of the biomass CHP will decrease, the size of the natural gas CHP will increase and the size of the NGCC power plant will decrease. This leads to a decrease in 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 biomass can only be accomplished by firing the biomass in a condensing power plant. In this case, little to no increase in electric power production can be achieved compared to using the biomass fuel in a condensing power plant unit at the mill site. 4.3.3. Future biofuel gasification based CHP technology The 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 integrated gasification combined cycle (BIGCC) CHP technology was not directly investigated in this study. However, considering BIGCC CHP technology instead of steam turbine based CHP technology, the highest value for the marginal electric power generation efficiency gel,marginal would decrease from 105% to 86% based on efficiency values presented in [22] for mature BIGCC CHP technology ðgel ¼ 45%; gtot ¼ 95%Þ. This is because the BIGCC CHP unit cannot achieve the high total efficiencies achieved by the biofuel steam turbine unit. Therefore, the range of marginal electricity generation efficiencies considered in this study covers the case where 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 mill powerhouse configurations and technologies. In this section, only the most relevant results for the different mill systems are presented. The different cases investigated together with the pertaining notations were as follows: . For the KAM pulp mill: – – – – –

v

Recovery boiler with standard steam data (79 bar(a) and 485 C) (RB 79) v Recovery boiler with advanced steam data (110 bar(a) and 530 C) (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: – – – –

v

Recovery boiler with standard steam data (79 bar(a) and 485 C) (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)

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. 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 integrated pulp and paper mill. For both market pulp mill options, on-site biofuel usage is limited to a bark boiler, as in the KAM reference mill configuration. Since fuel export is seen as the most attractive option for market pulp mills, the amounts of biofuel combusted in the bark boiler are relatively small. Gasification of relatively small amounts of biofuel would therefore not 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 considered. Gas turbine load is defined as the amount of fuel supplied to the engine compared to the amount necessary to run the engine at full-load. The supplementary firing fraction is defined as 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 electricity to 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 those based upon back liquor gasification) have been modelled in detail, as opposed to the reference system that was modelled more simply based upon representative values for heat, electrical and total 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 more effective alternative usage for part of the fuel elsewhere. The RB 79 and RB 110 cases therefore include condensing steam turbine units for the market pulp mill cases. Import/export of biofuel to/from the mill was not considered in these cases, which corresponds to current practise in the pulp 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 different from the mill site production Emill. As discussed previously, off-site usage of biofuel is characterised by the marginal electricity generation efficiency gel,marginal of the reference alternative energy system. With the exception of the BLG CC case, all BLG CHP configurations are sized so as to exactly match the mill process steam demand. Excess fuel is assumed to be exported, or the necessary amount of fuel is imported, as appropriate. Results for the BLG CC case are presented, for which all mill internal fuel is used on-site. This result can therefore be directly compared to those of other authors who have studied electric power generation using black liquor gasification technology without considering system consequences due to limited availability of biofuel. Finally, results are presented for mill powerhouse configurations that produce heat only, so as to maximise fuel export.

Mill site steam and power production Net process steam production (Hmill) Electricity production Gas turbine Steam turbine Auxilliaries Net production (Emill)

267.28

267.28

0.00 55.82 3.70 52.12

0.00 0.00

0.00 0.00

MW MW MW MW

267.28

267.28

0.00 59.79 4.09 55.70

115.56

15.80 18.11

15.80 18.11

115.56

251.68 33.71 15.80 301.19

RB 110 yes no no yes no no N/A N/A

RB110

251.68 33.71 15.80 301.19

MW

Results for mill site only Available biofuel at mill site BL MW Bark MW Tall oil MW Total MW Biofuel not used for CHP Tall oil, sold MW Bark to lime kiln MW Available biofuel for CHP at mill site Total (Bmill) MW Exported biofuel Bark MW Lignin MW Total biofuel consumption for CHP production at mill site Total ðBmill  DBexport Þ MW

Mill powerhouse configuration characteristics Chemical recovery technology RB HP steam pressure 79 Steam turbine yes Gas turbine no Bark gasification no Bark boiler yes Bark import/export no Lignin import/export no Gas turbine load N/A Supplementary firing fraction N/A

RB79

60.49 24.15 10.18 74.46

115.56

267.50

0.00 0.00

267.50

15.80 17.68

251.68 33.69 15.80 301.17

BLG 79 yes 6FA no yes no no 93% 0%

BLG CC

36.82 9.30 9.96 36.16

115.56

188.85

17.25 62.83

268.93

15.80 16.44

251.68 33.69 15.80 301.17

BLG 79 yes GTX 100 no no export export 100% 10%

BLG CC LE

Table 1 Powerhouse characteristics and energy system performance results for the KAM pulp mill (heat demand: 10 GJ/ADt)

37.41 0.00 9.73 27.68 (continued on next page)

115.56

176.17

17.57 75.51

269.25

15.80 16.13

251.68 33.69 15.80 301.17

BLG 30 no GTX 100 no no export export 100% 10%

BLG GT LE

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Total electricity production Marginal electrical efficiency, ref. sys Biofuel usage Available at mill (Bmill) Exported biofuel from mill (DBexport) Biofuel used at mill for CHP ðBmill  DBexport Þ Electricity production At Mill (Emill) Contribution from off mill site (gel,marginal
DBexport) Total (Etot)

Table 1 (continued )

267.28 0.00 267.28

52.12 0.00 52.12

MW MW MW

MW MW MW

49%

RB79

52.12

52.12 0.00

267.28

267.28 0.00

105%

55.70

55.70 0.00

267.28

267.28 0.00

49%

RB110

55.70

55.70 0.00

267.28

267.28 0.00

105%

74.46

74.46 0.00

267.50

267.50 0.00

49%

BLG CC

74.46

74.46 0.00

267.50

267.50 0.00

105%

75.40

36.16 39.24

188.85

268.93 80.08

49%

BLG CC LE

120.25

36.16 84.09

188.85

268.93 80.08

105%

73.29

27.68 45.61

176.17

269.25 93.08

49%

BLG GT LE

125.41

27.68 97.73

176.17

269.25 93.08

105%

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

Mill site steam and power production Net process steam production (Hmill) Electricity production Gas turbine Steam turbine Auxilliaries Net production (Emill) 92.41 0.00 61.37 3.70 57.67

MW MW MW MW

60.49 29.69 10.18 80.00

92.41

21.12 4.97 9.59 16.50

92.41

135.65

267.50

270.31 18.63 116.03

267.50

267.28

15.80 15.06

251.68 33.69 15.80 301.17

BLG 79 yes LM2500 no no export export 100% 7%

BLG CC LE

0.00 0.00

15.80 17.88

15.80 18.11

0.00 0.00 at mill site 267.28

251.68 33.69 15.80 301.17

BLG 79 yes 6FA no yes no no 93% 0%

BLG CC

251.68 33.71 15.80 301.19

RB 79 yes no no yes no no N/A N/A

MW

Results for mill site only Available biofuel at mill site BL MW Bark MW Tall oil MW Total MW Biofuel not used for CHP Tall oil, sold MW Bark to lime kiln MW Available biofuel for CHP at mill site Total (Bmill) MW Exported biofuel Bark MW Lignin MW Total biofuel consumption for CHP production Total ðBmill  DBexport Þ MW

Mill powerhouse configuration characteristics Chemical recovery technology HP steam pressure Steam turbine Gas turbine Bark gasification Bark boiler Bark import/export Lignin import/export Gas turbine load Supplementary firing fraction

RB79

0.00 0.00 13.93 13.93 (continued on next page)

92.41

93.15

19.88 158.53

271.56

15.80 13.81

251.68 33.69 15.80 301.17

BLG 30 no no no no export export N/A 100%

BLG NO-GT LE

Table 2 Powerhouse characteristics and energy system performance results for a future process integrated pulp mill with reduced heat demand (8 GJ/ ADt)

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Biofuel usage Available at mill (Bmill) Exported biofuel from mill (DBexport) Biofuel used at mill for CHP ðBmill  DBexport Þ Electricity production At Mill (Emill) Contribution from off mill site (gel,marginal
DBexport) Total (Etot)

Total electricity production Marginal electrical efficiency, ref. sys

Table 2 (continued )

267.28 0.00 267.28

57.67 0.00 57.67

MW MW MW

MW MW MW

49%

RB79

57.67

5.67 0.00

267.28 0.00 267.28

105%

80.00

80.00 0.00

267.50 0.00 267.50

49%

BLG CC

80.00

80.00 0.00

267.50 0.00 267.50

105%

82.49

16.50 65.98

270.31 134.66 135.65

49%

BLG CC LE

157.89

16.50 141.39

270.31 134.66 135.65

105%

173.40

13.93 187.33

13.93 87.42 73.49

271.56 178.41 93.15

105%

271.56 178.41 93.15

49%

BLG NO-GT LE

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no yes import no N/A N/A

Bark gasification Bark boiler Bark import/export Lignin import/export Gas turbine load Supplementary firing fraction

Results for mill site only Available biofuel at mill site BL MW 251.68 Bark MW 33.71 Tall oil MW 15.80 Total MW 301.19 Biofuel not used for CHP Tall oil, sold MW 15.80 Bark to lime kiln MW 18.11 Available biofuel for CHP at mill site Total (Bmill) MW 267.28 Exported biofuel Bark MW 14.39 Lignin MW 0.00 Total biofuel consumption for CHP production at mill site Total ðBmill  DBexport Þ MW 281.66

RB 79 yes no

Mill powerhouse configuration characteristics Chemical recovery technology HP steam pressure Steam turbine Gas turbine

RB 79 B

266.52 0.00 -46.54

267.50 83.33 0.00 350.83

267.50 -54.45 0.00 321.94

313.06

15.80 18.85

15.80 17.88

15.80 17.88

251.68 33.69 15.80 301.17

no yes no import 100% 14%

BLG 79 yes 6FA

BLG CC L

251.68 33.69 15.80 301.17

BLG 79 yes 6FA, LM1600 yes no import no 100% 4%

BLG/BIG CC B

251.68 33.69 15.80 301.17

no yes import no 92% N/A

BLG 79 yes 6FA

BLG CC B

(continued on next page)

181.70

0.00 87.86

269.56

15.80 15.81

251.68 33.69 15.80 301.17

no yes no export N/A 100%

BLG 30 no NO

BLG NOGT LE

Table 3 Powerhouse characteristics and energy system performance results for a reference integrated pulp and paper mill (heat demand: 15.8 GJ/ADt)

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Biofuel usage Available at mill (Bmill) Exported biofuel from mill (DBexport) Biofuel used at mill for CHP ðBmill  DBexport Þ Electricity production At Mill (Emill) Contribution from off mill site (gel,marginal
DBexport) Total (Etot)

Total electricity production Marginal electrical efficiency, ref. sys

Mill site steam and power production Net process steam production (Hmill) Electricity production Gas turbine Steam turbine Auxilliaries Net production (Emill)

Table 3 (continued )

105%

39.47 70.90 15.11 26.68 24.37

MW 39.47 MW 7.05 MW 32.42

44.23

267.28 267.50 14.39 54.45 281.66 321.94

MW 267.28 MW 14.39 MW 281.66

49%

49%

60.26 21.00 10.36 70.90

MW MW MW MW

0.00 43.22 3.74 39.47

182.97

49%

91.77 20.08 15.90 95.95

182.97

13.74

55.12

70.90 95.95 57.17 40.83

49%

65.47 21.16 10.26 76.38

182.97

BLG CC L

8.45

53.57

95.95 76.38 87.50 22.80

267.50 266.52 83.33 46.54 350.83 313.06

105%

BLG/BIG CC B

267.50 267.50 54.45 83.33 321.94 350.83

105%

BLG CC B

MW 182.96

RB 79 B

27.51

29.26

78.46

13.79 92.25

76.38 13.79 48.87 43.05

105% 269.56 87.86 181.70

49%

266.52 269.65 46.54 87.86 313.06 181.70

105%

0.00 0.00 13.79 13.79

182.97

BLG NOGT LE

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605

The KAM pulp mill and the future pulp mill with a reduced heat demand (process integrated mill) have a net surplus of biomass fuel at the mill site. If this surplus fuel is used on-site, there is no import or export of biomass. If the biomass fuel export figures in Tables 1 and 2 are negative, biomass fuel is imported to the mill. As discussed previously, the GE 6FA gas turbine is too big for some of the powerhouse configurations considered, especially those involving significant amounts of lignin export from the mill. To avoid severe part-load operation, smaller gas turbines are used instead. These include the 43 MWel Alstom GTX100, the 23 MWel GE LM2500 and the 14 MWel GE LM1600, which have similar turbine inlet temperatures and pressure ratios to the GE 6FA. For example, the smaller Alstom GTX100 engine was selected for the BLG CC LE and BLG GT LE cases in the KAM pulp mill to avoid severe part-load operation (a GE 6FA engine would have operated at around 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 the BLG NO-GT LE case considered for the process integrated pulp mill with reduced heat demand would require extracting approximately 65% of the lignin content of the black liquor. The remaining black liquor has a low heating value, which may lead to combustion problems in a conventional recovery boiler. The gasification reactor is however oxygen-blown, resulting in a syngas 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. This can 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 plant and the NGCC power plant In this section, the total electricity production (Etot) results presented in Tables 1–3 and in Figs. 7–9 are discussed. 5.1.1. Results for market pulp mills The results show that if the marginal electrical efficiency is high (105%) for the reference alternative biofuel user, the highest total electricity production is achieved by powerhouse configurations that export large amounts of biofuel from the mill. For example, for the process integrated pulp mill, the BLG CC LE configuration achieves a total electricity production of 158 MWel assuming a marginal electricity efficiency of 105% for the 135 MW of exported biofuel. This can be compared to the BLG CC configuration for the same mill-type, that does not export any fuel at all, and achieves a total electricity production of 80 MWel (including power generated in a condensing steam turbine unit). Therefore, when the 135 MW of excess biofuel that 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 power generation 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

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

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

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

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607

production of mill process steam, and the remaining biofuel (178 MW) is exported to the alternative biofuel energy system, where it can be used to achieve a total electric power production of up to 173 MWel when the marginal electrical efficiency is 105%. It should be noted that the BLG NO-GT LE configuration would also result in the highest total electricity production for the KAM pulp mill. This case was however not included in this study and specific results are therefore not available. Conversely, if the marginal electricity generation efficiency for exported biofuel is low, powerhouse technologies that consume larger quantities of fuel at the mill site become more favourable. This can be seen for example by comparing the BLG CC LE configuration with the BLG CC configuration for the process integrated pulp mill. If the marginal electrical efficiency for exported biofuel is 49%, the BLG CC LE has a total electricity production of 82.5 MWel compared to 80 MWel for the BLG CC. In this case, the on-site electricity production for the BLG CC LE amounts to only 16.5 MW. The remaining 66 MW of electricity are produced in the reference alternative energy system from the 135 MW of exported biofuel. In this case, it is clear that the gain in total electricity production is essentially insignificant in comparison to the effort required 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 be used at the mill site for electricity production or exported and used elsewhere. First, the marginal electricity generation efficiency from excess biofuel is computed for the mill site. This marginal efficiency is then compared with the marginal electrical efficiency for exported fuel in the alternative energy system. Defining an appropriate value for the marginal electrical efficiency for usage of excess biofuel at the mill site is not simple. This is because the fuel is in many cases used in a number of different operations at the mill site (e.g. bark boiler and black liquor gasification gas turbine CHP plant). The marginal efficiency can in theory only be defined for a given usage of a given fuel. An indicative value can however often be obtained by comparing the millsite electric power outputs and fuel consumption of two different powerhouse configurations. When the indicative mill-site marginal electric power generation efficiency for excess biofuel has been determined, this can be compared to the marginal efficiency value for off-site power generation. Conclusions can then be drawn as to which usage of excess biofuel leads to the highest total 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 in order 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 case compared 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 the excess fuel is exported. This higher marginal electrical efficiency is achieved because the electricity is produced using the mill’s heat load for efficient CHP production. If more electricity is generated at the mill, it is necessary to partially implement condensing power generation operation, leading to lower efficiency. So by comparing the BLG CC case with the BLG NO-GT LE case, it is possible to produce 94 MWel (30.5 MWel from CHP and 63.5 MWel from condensing power) using 178 MW of extra biofuel resulting in a marginal electrical efficiency of 54%. However, the 63.5 MWel of condensing power produced using an additional 135 MW of biofuel compared to the BLG CC LE case will only result in a marginal electrical efficiency of 47%

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which is lower than the marginal electrical efficiency for exported fuel in the alternative energy system. Hence it is theoretically better to export 135 MW of biofuel than to use it at the mill site for electricity production in order to achieve a higher total electricity production (82.5 MWel). The results presented in Tables 1 and 2 can also be used to analyse the implications for electricity 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 produced. For example, the BLG CC powerhouse configuration (that does not export biofuel) produces 74.5 MWel in the KAM pulp mill, compared to 80 MWel in the process integrated pulp mill, i.e. a gain of 5.5 MWel. If excess biofuel is exported, a reduced mill heat demand increases the amount of excess fuel available for export. Considering the BLG CC LE configuration, the gain in total electricity production is shown to be 7.1 MWel for gel,marginal equal to 49% and 37.6 MWel 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 to production of mill process steam, and the remaining biofuel (178 MW) is exported to the alternative biofuel energy system, where it can be used to produce up to 173 MWel when the marginal electrical efficiency is 105%. It should be noted that the BLG NO-GT LE configuration would also result in the highest total electricity production for the KAM pulp mill. This case was 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 the mill to the alternative biofuel user when gel,marginal is equal to 105% and biofuel usage at the mill site is restricted to production of process steam. When gel,marginal is close to 50% or lower, biofuel export is not advantageous, and high total electricity production figures are achieved by a black liquor gasification combined cycle configuration, with or without a condensing steam turbine unit. 5.1.2. Results for integrated pulp and paper mills For 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 CHP should be as low as possible. Cogeneration of electric power at the mill site should in fact be avoided, as for the market pulp mills, and fuel usage should be restricted to production of mill process steam. In this case ðgel;marginal ¼ 105%Þ, the BLG NO-GT LE powerhouse configuration that exports 88 MW of biofuel and achieves a total electricity production of 78.5 MWel clearly outperforms the other cases by 200%. If the powerhouse at the mill has to produce both electricity and heat, then the BLG CHP configurations with biofuel import will not perform better than 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 better than most of the BLG systems with imported biomass. Only the BLG CC L system performs better (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 electricity production).

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When the marginal electric power generation efficiency for alternative off-site usage of biofuel is low, it is possible to determine the mill-site marginal electrical efficiency for the integrated pulp 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 the import of biofuel. For example, the BLG CC L case produces 90 MWel more electricity than the BLG NO-GT LE case using 131 MW more biofuel ðgel;marginal ¼ 69%Þ not accounting for the 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 electricity production will decrease from by 23 to 67 MWel. It is nevertheless favourable to import the 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 biofuel 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 however be mentioned that compared to the BLG NO-GT LE case, 110 MWe more electricity is produced 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 conclusion that no electricity should be produced at the mill (69% < 105%) and that only heat should 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. Current availability of biofuel at a reasonable price is essentially unlimited in most Swedish biofuel markets. In the future, when black liquor gasification technology is likely to become mature, it is reasonable to assume that demand for biofuels will be significantly stronger than today, and that availability of biofuel at a reasonable price will be limited. Despite this outlook, many studies presenting electric power generation potential in the pulp and paper industry with black liquor gasification do not account for alternative uses of biofuels. In this section, the electric power 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, corresponding to a low efficiency usage of the biofuel, which was not the aim of this study. For integrated pulp and paper mills, it is necessary to import biofuel to operate a CHP plant that is sized to cover the full mill heat demand. Different mill powerhouse configurations furthermore require different amounts of imported biofuel, which makes comparisons difficult. High on-site electric power output can be achieved by importing large amounts of biofuel and running condensing steam turbine units. In this section, the discussion is therefore limited to market pulp mills, for which the amount of mill internal biofuel is constant, and for which it is assumed that additional 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

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potential for increasing on-site electric power generation by implementing black liquor gasification technology instead of advanced recovery boiler technology is large. For the process integrated pulp mill, the BLG CC powerhouse configuration is shown to produce 22 MWel more electricity than a conventional RB powerhouse. If alternative usage for biofuel is now considered, very different conclusions may be drawn. In the most extreme case (i.e. BLG NO-GT LE configuration with a marginal efficiency for alternative biofuel usage of 105%), the increase in total electric power production compared to a conventional recovery boiler configuration where all mill internal fuel is used at the mill site is 116 MWel, i.e. 94 MWel more than when alternative 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 production at the mill site only for the integrated pulp and paper mill, the highest on-site electricity production is achieved by the BLG/BIG CC B configuration (96 MWel) not taking into account the decrease in electricity production outside the mill because of the imported biofuel. The lowest on-site power production is achieved by the RB 79 B configuration (39.5 MWel), as seen in Table 3. This is expected, since this BLG powerhouse configuration has the highest fuel consumption and only the mill site is included for energy performance assessment. These results should be compared to 8 MWel for the BLG/BIG CC B configuration and 24 MWel for the RB 79 B configuration, if the decrease in total electricity production is included with gel;marginal ¼ 105%. 6. Conclusions In this study, the performance of different black liquor recovery powerhouse systems were examined. The objective of the study was to identify mill powerhouse configurations that maximise the total electric power production whilst satisfying the mill’s process heat demand and taking into account the effects that export or import of biofuel has on the electricity production from alternative biofuel users outside the mill. In this study, we assumed that exported biofuel could be used in an alternative energy system consisting of a district heating network and a natural gas combined cycle power plant. Different marginal electric power generation efficiencies for exported biofuel were considered, namely 49% and 105%. Similarly, when biofuel was imported to the mill, the decrease in off-site electric power generation potential due to reduced availability of biofuel was considered. The results can be summarised as follows: . Black liquor gasification technology performs better than conventional and advanced recovery boiler technology in all cases considered; . For both market pulp mills and integrated pulp and paper mills, excess biomass fuel export is the best way to achieve high total electricity production if the marginal electricity generation efficiency for the exported fuel is higher than that for on site usage (~50% for condensing power generation). This is the case if biofuel exported from the mill can be used to increase the degree of CHP in a district heating system. Biofuel usage at the mill site should then be restricted to the amount necessary to satisfy the mill’s heat load, i.e. CHP should be avoided.

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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 gasification 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 policies, 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 district 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 technology 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 investments 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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