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S balances
Applied Thermal Engineering 43 (2012) 42e50
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Process integration of near-neutral hemicellulose extraction in a Scandinavian kraft pulp mill e Consequences for the steam and Na/S balances Valeria Lundberg*, Erik Axelsson, Maryam Mahmoudkhani, Thore Berntsson Chalmers University of Technology, Department of Energy and Environment/Heat and Power Technology, Kemivägen 4, SE-412 96 Göteborg, Sweden
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 August 2011 Accepted 5 March 2012 Available online 29 March 2012
While in a conventional kraft pulp mill, most of the hemicellulose and lignin fraction of the wood is burned in the recovery boiler to produce steam, in a biorefinery it can partially be used to produce added-value products. In this paper, the most important consequences of integrating a bioethanol production plant with a model pulp mill are presented in terms of steam and Na/S balances. The model mill represents an average Scandinavian hardwood kraft pulp mill, and the bioethanol plant is based on the “near-neutral” hemicellulose pre-extraction method. Regarding the steam balance, a comprehensive heat integration study is performed. Implementing hemicellulose extraction increases the net steam demand by 48 MW. However, process integration at the mill and the bioethanol plant individually leads to significant steam savings, and a corresponding net increase of steam by only 3 MW. Additional steam savings can be achieved if the total integration of the two processes (between the pulp mill and the bioethanol plant) is considered (3 MW), resulting in a biorefinery with no increase of steam demand. As regards the Na/S balance, it is shown that green liquor export from the mill to the bioethanol plant results in severe disruptions in the sodium and sulphur balance of the mill. Different attempts to solve this problem are discussed, but are very costly and/or negatively affect the water and steam balance of the mill. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Pulp mill Process integration Near-neutral Hemicellulose extraction Pinch analysis Hardwood Biorefinery Bioethanol
1. Introduction The Nordic pulp and paper industry is currently facing several challenges such as rising energy costs, strong competition from countries with significantly lower feedstock and production costs, and decreasing demand for some paper grades. Consequently, efforts have been made to increase the energy efficiency of mills and to diversify the mix of products. Previous studies have shown that it should be possible to reduce the steam demand and/or to create a steam surplus in both chemical and mechanical pulp mills via process integration [1e6]. Excess heat at the mill facilitates the integration of biorefinery concepts, thus enabling additional products which can increase both the profitability and sustainability of mills. An overview of several biorefinery concepts in pulp mills has been presented previously [7,8], for example hemicellulose extraction to produce ethanol, lignin extraction to produce new chemicals or materials, and biomass gasification to produce transportation fuels. However, integration of biorefinery concepts is challenging since the steam
* Corresponding author. Tel.: þ46 31 7723011; fax: þ46 31 821928. E-mail address: [email protected] (V. Lundberg). 1359-4311/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2012.03.037
production capacity at the mill is diminished as wood components are withdrawn from the pulp line. Moshkelani et al. [9] have developed a methodology to perform high levels of process integration and intensive energy efficiency, and concluded that the sustainability of biorefinery concepts depends upon the successful implementation of intensive energy integration and optimization measures. As previously mentioned, large potentials for process integration in kraft pulp mills have been identified in earlier studies. The importance of process integration for different biorefineries has also been discussed by several authors; for example, Jönsson et al. [10] compared the profitability and carbon dioxide emissions of different technology pathways for using excess steam in a typical Scandinavian pulp mill. Garcia et al. [11] studied the consequences of integrating the organosolv process (for fractionating wood components) into a mill. Fornell et al. [12,13] studied the total conversion of a pulp mill into a bioethanol plant as well as the production of dimethyl ether (DME) by gasification of black liquor. A biorefinery concept that continues to gain attention is lignocellulosic bioethanol production, as it may replace fossil fuels in the transport sector. According to Hämäläinen et al. [14], the increasing price of oil is the most important incentive for forest biorefineries in Scandinavia, North America and South America.
V. Lundberg et al. / Applied Thermal Engineering 43 (2012) 42e50
The simultaneous production of pulp and bioethanol is advantageous since pulp mills generally have existing infrastructure, logistics and know-how about processing large amounts of lignocellulosic materials. Nevertheless, production of bioethanol is a complex and energyintensive process. Hence, several researchers have investigated different routes for ethanol production as well as how to increase the energy efficiency of the process. Bioethanol can be produced both in thermomechanical pulp mills (TMP) and in chemical pulp mills. In the case of kraft mills, bioethanol can be produced by extracting carbohydrates prior to the digestion and subsequent fermentation into ethanol. Hemicellulose can be extracted by different methods, such as alkaline extraction [15], acid extraction [16], water extraction [17] and green liquor extraction, so-called “near-neutral” pre-extraction [18]. Hemicellulose extraction will affect the mill in various ways; for example, the severe conditions in water and dilute acid extraction can have a negative impact on the fibre properties. On the other hand, it has also been shown that, when using milder conditions, as in the near-neutral extraction method, hemicellulose can be extracted while maintaining the quantity (i.e. pulp yield) and quality (e.g. tear index) of the pulp [19]. In addition to possible changes in pulp quality and quantity, extraction of hemicellulose affects the operation and performance of process equipment at the mill. In the case of near-neutral extraction, for example, the continuous export of green liquor from the mill to the bioethanol plant could potentially destabilize the Na/S balance of the mill, if the sodium and sulphur ions present in green liquor are not reincorporated back in the recovery cycle of the mill (see Section 7). Moreover, the water use at the mill is significantly increased due to the washing of the extracted wood chips. It has previously been shown that the potential for energy savings at a mill, as well as the best approach for steam savings, is highly dependent on the water consumption [5,20]. Another effect of extracting hemicellulose from the wood chips is a reduction in the organic material burned in the recovery boiler and consequently a decrease in the steam production at the mill. This, in combination with the relatively high steam demand of the bioethanol plant, results in a net steam deficit, assuming that the wood input is kept constant. The deficit could in principle be compensated for by burning extra fuel at the mill, but this could jeopardize the economic and environmental performance of the biorefinery. Alternatively, the steam demand can be reduced by increasing the energy efficiency via process integration [21e23]. However, process integration studies focusing on hemicellulose extraction, and particularly on the near-neutral method, have seldom been presented. Van Heiningen et al. [24] studied hemicellulose extraction based on the near-neutral method in kraft pulp mills, but no heat integration studies were performed. Marinova et al. [25] identified energy efficiency measures to reduce the steam demand of a Canadian kraft mill with near-neutral hemicellulose extraction and bioethanol production, but the consequences for the steam demand of different parts of the mill were not taken into account (except for the change of steam production in recovery boiler). In addition, the consequences for the Na/S balance of extraction using the near-neutral method have not been thoroughly presented in the literature. The authors have therefore evaluated the heat integration potential for integration of an average Scandinavian hardwood model mill with near-neutral pre-extraction in a previous article [26]. In the present paper, the consequences of such integration for the steam balance are presented in a more comprehensive way. In addition, the scope of the study is extended to include the consequences for the Na/S balance, as this also may have a high impact on the energy and/or materials consumption.
43
2. Aims One aim of this paper is to analyse the consequences of integrating a bioethanol production plant (based on green liquor extraction, also called near-neutral hemicellulose extraction) with a kraft pulp mill, in terms of the steam (i.e. steam demand and production) and Na/S balances. Another aim is to identify the potential for steam savings within the mill, as well as within the bioethanol plant, and whether it is advantageous to export heat from the mill to the biorefinery or vice versa, i.e. to have total integration. Furthermore, the paper aims at discussing different practical retrofits that could result in steam savings. Concerning the Na/S balance aspects, the intention is to investigate the resulting Na/S deficit at the mill upon extraction of hemicellulose with green liquor, as well as to discuss the drawbacks of different attempts to compensate for this deficit (e.g. in the water balance of the mill or the profitability of the project). 3. Method In this study, relevant process data have been gathered from simulation models of the mill and bioethanol plant. In the case of the kraft mill, process data were available from a WinGEMS simulation of a hardwood Scandinavian mill [27]. The model represents a typical Scandinavia mill as regards equipment and level of resource utilization. For the bioethanol plant, mass and energy balances for different process units were generated by the authors with the help of data available in literature [19,28]. In order to study the consequences of integrating a bioethanol plant in a kraft mill, a base case has been defined to allow further comparisons. The base case represents a mill in which changes in process streams that result from implementing hemicellulose preextraction (i.e. changes in composition, mass or enthalpy flow rate) have been taken into account, but no attempt has been made to improve the system by process integration. Thereafter, two cases with different levels of process integration were studied. In the simplest case, the possibility to save steam and release excess heat on a stand-alone basis (i.e. separately, within each of the processes) was investigated. In the second case, the benefits of total integration were studied; i.e. heat/steam may also be transferred between the mill and the bioethanol plant. The process integration potential at the mill and bioethanol plant was analysed via pinch analysis [21e23]. Since the introduction of pinch analysis, i.e. heat integration, a number of different industries including the pulp and paper industry have benefited from it [29]. Moreover, the heat integration methodology has been extended and developed by numerous contributions [30], not least the development of several methodologies for retrofit situations [31,32]. In this paper, however, only the basic concepts of pinch analysis have been used, as described in e.g. [21], since the latter methodologies require detailed knowledge about the heat exchanger networks that was not available in the mill and bioethanol models. In this study no economic optimization for the retrofits has been carried out due to lack of data (e.g. piping distance and area for existing heat exchangers). Still, different reasonable retrofits have been identified based on general pinch analysis heuristics, e.g. to solve the largest pinch violations, heatexchange streams located close to each other, re-use existing equipment, etc. In order to investigate the consequences of extracting hemicellulose with green liquor, sodium and sulphur balances were set up for different process units. The procedure to develop the Na/S balances is outlined in Fig. 1. First, the sodium and sulphur content in green liquor was calculated. This corresponds to the expected sodium deficit at the mill. In the case of sulphur, this represents the
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V. Lundberg et al. / Applied Thermal Engineering 43 (2012) 42e50
Use Total Titratable Alkali (TTA) [gNaOH /lGL] to calculate Na content in green liquor: Na GL. Use Active Alkali (AA) [ gAA /lGL] and Sulphidity [kgNa 2 S/kgAA ] to calculate S content in green liquor: S GL
Na and S in the extraction vessel is: Na GL, S GL
Calculate the Na and S content in the hydrolysate solution: Na extract =Na GL Is S covalently bound to lignin? Yes S extract =0; No Sextract =SGL Calculate the S input during acid hydrolysis, S H2 SO4 =Sneutr +SpH •Use TTA [gNaOH /lGL] to calculate the protons (H 2 SO 4 ) needed to neutralize green liquor components, Sneutr •Use pH=-log[H+] to calculate the protons (H 2 SO 4 ) needed to decrease the pH to 1, S pH Calculate the Na and S content in the delignified hydrolysate: Na delig=Na extract Sdelig=Sextract +SH2 SO4
The S precipitated as gypsum is: S pH
Calculate the Na and S found in the stillages: Na still =Na delig, Sstill =Sdelig-SpH Fig. 1. Method to develop Na/S balances.
maximum deficit of sulphur that can be expected, considering that none of the sulphur is bound to lignin (see Section 7). Accordingly, this is also the sodium and sulphur present in the diluted extract (Fig. 2). Thereafter, the sulphur input during acid hydrolysis was estimated. The consumption of sulphuric acid during acid hydrolysis corresponds to the protons needed to acidify green liquor components (according to the stoichiometry of the neutralization reaction) plus the protons needed to achieve the specified pH conditions (according to the definition of pH ¼ log[Hþ]). It was finally assumed that the total sodium and sulphur input (i.e. from green liquor and sulphuric acid) was eventually found in the stillages of the bioethanol plant. Thus, the net sodium and sulphur balance at the mill depends on whether the stillages are returned to
the mill or not, which in turn has important consequences for the steam balance of the mill. Finally, the consequences of implementing near-neutral extraction for the water balance of the mill were estimated by calculating the steam required to concentrate the diluted stillages of the bioethanol plant to the same dry solid content of the strong black liquor (72%), in the existing evaporation plant (5.5 effects). 4. The studied pulp mill The studied kraft pulp model was developed within the Swedish national research programme “Future Resource Adapted Pulp Mill” (FRAM) and represents an average hardwood
Green liquor Antraquinone Wood chips
Extracted wood chips Pre- extraction
Kraft process
Wash water Flash steam
Flash H2SO4
Diluted extract Lime
Acid hydrolysis Hydrolyzate Lignin filtration Lignin
CO2
Solvent
Raffinate L/L Liming extraction
Fermentation
Ethanol upgrading
Stillage Acetic acid Stillage
Gypsum
Fig. 2. Block diagram of the near-neutral process (image adapted from Mao [28]).
Ethanol
V. Lundberg et al. / Applied Thermal Engineering 43 (2012) 42e50
Scandinavian mill [27]. The mill produces 1250 ADt/d bleached market pulp from 2328 ADt/d of hardwood (Table 1). Wood logs are de-iced in the wood yard with steam-heated water (2 MW). After debarking and chipping, the wood chips are steamed with LP steam (5 MW) and black liquor flashed steam (14 MW). Steamed wood chips are then cooked in a conventional two-flash digester, where MP steam is used in the digester circulation and in the hiheat washing zone (10 MW). The pulp is then bleached and dried. In the recovery cycle, the black liquor is flashed in two steps; the first black liquor flash is used for steaming wood chips (14 MW) and the second flash to produce hot water (14 MW). In a 5.5-effect evaporation plant, black liquor is evaporated to 72% dry solid content. Strong black liquor is burned in the recovery boiler to produce steam and recover the inorganic compounds as a smelt. The steam production in the recovery boiler (216 MW) is not sufficient to satisfy all the steam demand of the mill (221 MW) and consequently, 5 MW of supplementary steam need to be produced in the bark boiler. Excess bark is sold. The back-pressure steam turbine is not sufficient to accommodate all the high-pressure steam, and about 25% of the steam is passed through let-down valves. The hot and warm water system (HWWS) consists of a heat exchanger network where the required warm and hot water for the mill is produced from excess secondary heat in the process. In the HWWS, black liquor flash steam and excess heat of low temperature is used to preheat process water and make-up water (for the recovery and bark boilers). Due to poor measuring and controlling and/or insufficient heat exchanger area, the HWWS is unable to handle stops and unbalanced operation at the mill. Consequently, the heat from filtrates, condensers, effluents etc. is not enough to heat the water, and heating with steam is required (8 MW).
5. Description of the bioethanol plant The studied bioethanol plant is based on the near-neutral preextraction method. This method has first been suggested by van Heiningen and co-workers [28,33]. In the near-neutral method, Table 1 Steam production and consumption at the pulp mill before/after hemicellulose preextraction, when no process integration is carried out. HP 61 bar, 450 C Production [MW] Recovery boiler Bark boiler
MP 11 bar, 200 C
LP 4.5 bar, 150 C
221/197
44
12/8 16
56
Sub-total
44
43/39
134/128
2 5/0
(14/13)
10
5
8 7 56/55
0
0/14
0/20
0/34 TOTAL
2.5 bar, 127 C 1.2 bar, 105 C
(28/27)
221/211
*
216/192 5
Consumption [MW] MILL Woodyard Steaming Hi-heat washing zone digester Hot water production (HWWS) Combustion air Evaporation plant Digester Other steam uses
BIOETHANOL PLANT Sub-total
Flashed steam
221/245
The numbers in brackets refer to non-net users of steam.
(14)
(28/27)
45
wood chips are steamed and exposed to a solution of green liquor (Na2CO3 and Na2S, 3% on wood as Na2O) and anthraquinone (0.05% on wood) (Fig. 2). Extraction of hemicellulose is done in a preextraction vessel, where the temperature is kept constant at 160 C by heating with steam. Approximately 10% of the wood mass is extracted, mainly hemicellulose but also some lignin. The extracted wood chips are washed and sent to the kraft process. Some of the diluted extract is recirculated back to the extraction vessel to raise the solid content of the extract, resulting in a liquidto-wood ratio of 4:1. The rest of the diluted extract is concentrated by flashing before further processing. Hydrolysis is assumed to be carried out under strongly acidic conditions (pH ¼ 1) with an efficiency of 90% conversion of oligomeric carbohydrates into monomeric sugars [28]. Under these conditions it is assumed that all the lignin is precipitated and separated by filtration [28]. The delignified hydrolysate is then cooled down before further purification and separation stages. In the first step, acetic acid and furfural are removed by liquideliquid (LeL) extraction followed by distillation. Then, lime is added in order to raise the pH to that required for fermentation and to detoxify the solution by precipitating sulphate ions as gypsum (CaSO.42H2O). Some complications with the liming operation are discussed in Section 7. The detoxified sugar solution is thereafter fermented. An important assumption regarding fermentation is that the same fermentation yields are considered for both pentoses and hexoses (as originally assumed in [28]). Finally, the diluted ethanol solution is upgraded by distillation. Previous studies [34] suggest that integrated membrane systems may be a more cost-efficient alternative to upgrade diluted ethanol-water mixtures than the classical distillation process due to its high energy demand. However, in this study no attempts were made to modify the described process. Based on the described process, with adaptation to the mill studied in this paper, a total of 43 t/d (35 kg/ADt) of ethanol and 45 t/d (36 kg/ADt) of acetic acid can be produced, while the wood input and produced pulp rate remain constant. 6. Consequences of implementing the biorefinery concept for steam balance of the mill 6.1. Base case e No process integration The extraction of hemicellulose alters the steam balance in several ways listed in the following, resulting in an increase of the steam demand by 48 MW (Table 1, Fig. 3): a) Both the flow rate and organic content of black liquor are decreased [17], which leads to a reduction in steam production in the recovery boiler (27 MW). It should be mentioned that in the literature an assumption of 10% reduction in steam production in the recovery boiler was considered for 10% extraction [25,28]. In this paper, however, the steam reduction is calculated based on the changes in heat value of black liquor, which results in a larger reduction in steam production (12%, i.e. 27 MW instead of 22 MW). Still, some of this deficit can be compensated for by burning the extracted lignin (from delignifying the hydrolysate) in the recovery boiler (3 MW). Consequently, the steam production at the mill is decreased (221e24 ¼ 197 MW). b) The decrease in black liquor flow rate would, moreover, decrease the steam demand of the evaporation plant (56e1 ¼ 55 MW). c) The wood chips are steamed at a lower temperature. Thus, the heat available in the black liquor flashes is enough to steam the wood chips and no LP steam is necessary for this purpose (5e5 ¼ 0 MW).
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V. Lundberg et al. / Applied Thermal Engineering 43 (2012) 42e50
221-27+3=197 MW
221-1-5-4=211 MW
Pulp mill Recovery boiler and bark boiler
34 MW
Bioethanol plant
48 MW Increased steam demand
Fig. 3. Steam balance when no process integration is carried out.
d) The incoming wood (extracted wood) is partially cooked during hemicellulose extraction; therefore the steam demand in the digester is decreased (12e4 ¼ 8 MW). e) The bioethanol plant has its own steam demand (34 MW).
6.2. Heat integration on a stand-alone basis In order to assess the potential for steam savings, Grand Composite Curves of the mill and the bioethanol plant were constructed (Fig. 4A, B, Appendices A, B). The theoretical minimum heating demand (QH,min) of the mill is 175 MW and therefore it has a potential of 36 MW of steam savings (the current steam demand of the mill with hemicellulose pre-extraction is 211 MW, Table 1). The bioethanol plant has a minimum heating demand of 17 MW which corresponds to a steam saving potential of 17 MW (the nonprocess-integrated plant has a steam demand of 34 MW, Table 1). Accordingly, there is a total steam saving potential of 53 MW, which would theoretically be enough to compensate for the originally increased steam demand (48 MW). In the following sections, a reasonable suggestion of a possible retrofit is presented, with the potential to save 45 MW of steam (85% of the theoretical potential). This level of solved pinch violation (i.e. inefficiencies) is in accordance with other, more detailed, studies on process integration potential in similar mills [35,36]. To further decrease the steam demand, complex heat integration is needed, and was judged unprofitable. 6.2.1. Studied mill In the warm and hot water system (HWWS) of the mill, process water is heated to 50 C, 75 C and 85 C. Black liquor flash steam (at 105 C), LP steam (8 MW) and excess heat of low temperature, e.g. condensers and filtrates, are used as heat sources. However,
some other heat sources, e.g. effluents and stripper condenser, are not used for water heating and are therefore just cooled down, which can be considered a pinch violation (cooling above the pinch). In practise it is possible to heat all the water without black liquor flash steam by using the other heat sources, as well as by increasing the heat exchanger area (an increase corresponding to an average mean temperature difference in the heat exchangers of DTlm ¼ 18 C instead of 21 C). Since the heat available in black liquor flash steam is no longer required for heating the water, it is possible to use it somewhere else. In fact, heat can be recovered directly from black liquor before flashing. By heat-exchanging black liquor with the hi-heat washing zone of the digester, it is possible to reduce the MP steam demand of the digester by 4 MW (Table 2). Additional steam savings are achieved by using the black liquor flash steam to preheat white liquor to the digester (6 MW). Conversely, the steam demand of the evaporation is slightly increased (2 MW) due to decreased flashing and consequently a decrease in the dry solid content of the black liquor. In a redesigned HWWS, a slightly higher warm and hot water flow rate can be produced, whichein combination with additional storage capacity e ensures that the required amount of warm and hot water is always available. In this way, it is possible to eliminate the need for LP steam (8 MW) due to imbalances in production and consumption of hot and warm water. Moreover, it is possible to produce significant amounts of excess warm water (55 C) for different purposes, for example to de-ice the wood chips instead of using LP steam (2 MW). A part of the excess water could also be further heated to 78 C and used thereafter to preheat combustion air to the recovery boiler instead of LP steam (4 MW). In addition, the flue gases of the recovery boiler can preheat combustion air to a higher temperature (which is a retrofit independent of the preceding one). By heat-exchanging flue gases with
Fig. 4. Grand composite curve (with individual DTmin ¼ 0.5e8 C). 3A, Left: Pulp mill. 3B, Middle: Bioethanol plant. 3C, Right: foreground-background analysis.
V. Lundberg et al. / Applied Thermal Engineering 43 (2012) 42e50 Table 2 Possible steam-saving measures on a stand-alone basis. Additional steam-saving measures for the total integration case are presented between parenthesis. Steam saving measures at the mill Use black liquor to heat hi-heat washing zone (In total integration black liquor heat can be used to a larger extent) Preheat white liquor with black liquor (In total integration the flash steam from the bioethanol plant can also be used to preheat white liquor) Increased evaporation load due to modified flashing General improvements in the hot and warm water system Use warm water to de-ice wood logs Use stripper and terpentine condenser to preheat combustion air to recovery boiler Use flue gases to preheat combustion air to the recovery boiler
[MW] 4 (6) 6 (6)
2 8 2 4 6 28 (30)
Steam saving measures at the ethanol plant Preheat washing water to extraction vessel (In total integration excess water from the HWWS can be used instead of fresh water) Use flash steam and hydrolysate to run reboilers in distillation towers
12 (13)
5 17 (18) 45 (48)
Total steam savings
combustion air, it is possible to save additional 6 MW of MP and LP steam. In this heat exchanger, air could be heated from 90 C to 159 C while flue gases are cooled from 175 C to 128 C. It has been shown elsewhere that corrosion problems in recovery boilers due to acidic condensates are not measurable at these temperature levels [37]. By implementing the previously described retrofits, it is possible to reduce the steam demand by 28 MW (Table 2). 6.2.2. Bioethanol plant By designing a cooling water circuit with an average DTlm of 25 C, fresh water can be heated from 18 C to 109 C, with the excess heat available in different process streams and condensers. The pre-heated water can then be used to wash the extracted wood chips instead of fresh water. In this way, it is possible to reduce the steam consumption by 12 MW (Table 2). In addition, some of the flash steam (from diluted extracted flash tank) can be used as a heat source for the distillation of acetic acid and furfural (2 MW), and the delignified hydrolysate stream can be used as a heat source for ethanol distillation (3 MW). Thus it is possible to reach the theoretical steam saving potential and reduce the steam demand by 17 MW (Table 2). The previously described measures at the mill and bioethanol plant all together represent a steam saving potential of 45 MW, which is nearly sufficient to compensate for the originally increased steam demand (48 MW) (Fig. 5). The net deficit of 3 MW can be taken care of by burning excess bark in the bark boiler (currently, the bark boiler provides 5 MW to the mill, in the case of integrated biorefinery; the boiler capacity can possibly be increased to 8 MW). Alternatively, additional steam savings may be achieved by exporting heat from the mill to the bioethanol plant and vice versa, i.e. by total integration.
197 MW
211-28=183 MW
Pulp mill Recovery boiler and bark boiler
47
6.3. Total integration Total integration makes it possible to achieve higher energy savings compared to the stand-alone cases. To investigate the potential for further energy savings, a background/foreground curve [22] is constructed (Fig. 4C). According to the background/foreground analysis, excess heat at 60 C can be exported from the mill to the bioethanol plant. Moreover, there is a potential to export heat at a higher temperature from the bioethanol plant to the mill. In practise, it is possible to preheat white liquor with hightemperature heat available at the bioethanol plant, e.g. flash steam (127 C), reducing the use of black liquor for this purpose by 2 MW. This is however only possible if washing water for the extraction vessel could be heated with excess heat from the mill (as shown in Fig. 4C). For example, excess warm water produced at the mill (55 C) could be top-heated with some of the heat sources available at the bioethanol plant (similarly to stand-alone integration) and be used as washing water to the extraction vessel. As a consequence of this retrofit, black liquor can be used to a larger extent in the hi-heat washing zone of the digester, resulting in additional MP steam savings (2 MW). Moreover, this retrofit eliminates the need of steam for water heating (1 MW), resulting in a reduction of the combined steam demand by 3 MW. By implementing the preceding measures (summarized in Table 2), it is therefore possible to decrease the steam deficit by additional 3 MW. Accordingly, the original steam produced at the pulp mill is enough to satisfy the steam demand of both the mill and bioethanol plant (Fig. 6). 7. Consequences of implementing the biorefinery concept for the Na/S balances of the mill Despite the promising potential of heat integration between the mill and the bioethanol plant, other process disturbances may prevent the implementation of this biorefinery concept. Here, two critical issues will be discussed: consequences for Na/S and water balances. 7.1. Na/S balance The results of our calculations are shown in Table 3. A deficit of 31 kg/ADt of sodium and between 0 and 4 kg/ADt of sulphur is expected at the mill, due to the export of part of the green liquor from the mill to the hemicellulose extraction process. It is important to notice that at different parts of the ethanol plant with low pH, sulphur could be covalently bound to lignin as thiolignin. If sulphur is covalently bound to the lignin during extraction, then it may be reincorporated in the chemical recovery cycle of the mill. Whether and to what extent sulphur will be bound to lignin must be further investigated. Regardless, a deficit of sodium is expected. By using make-up chemicals the deficit can be compensated. Using Na2SO4, for example, could potentially compensate for the loss of sodium; however, it would insert an excess of sulphur in the 34-17=17 MW
Bioethanol plant
3 MW Increased steam demand
Fig. 5. Steam balance when process integration is carried out on a stand-alone basis.
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V. Lundberg et al. / Applied Thermal Engineering 43 (2012) 42e50
197 MW
17-1=16 MW
183-2=181 MW
5 MW
Pulp mill Recovery boiler and bark boiler
Bioethanol plant
0 MW
2 MW
Fig. 6. Steam production and consumption at the mill and bioethanol plant when total integration is carried out.
cycle (22-18 kg/ADt). If a sulphur-free chemical, i.e. NaOH, were added as make-up, the loss of sodium could be compensated for, without introducing new sulphur into the system. But the increase in chemical costs when using NaOH as make-up would be nearly twice the cost for Na2SO4 due to higher market prices. It can easily be shown that the cost of make-up chemicals is significant as compared to the revenue from the value-added products, i.e. ethanol and acetic acid, unless alternative products with high market prices are targeted. Alternatively, the process could be designed so that these ions are recovered and sent back to the mill [24]. This is, however, complicated from a practical point of view, since the lost sodium and sulphur are found in much-diluted stillages which would require large steam consumption in order to be concentrated before being recirculated, thus causing an increase in evaporation demand. Moreover, the amount of sulphur recycled back to the mill would be excessive if all the sodium is to be recovered, i.e. 22 kg/ADt. This is due to the use of sulphuric acid in the acid hydrolysis unit. Additionally, other costs may worsen the profitability of the studied process. We found that only w16% of the sulphuric acid is consumed to decrease the pH of the hydrolysate, whereas the rest of the acid is used to acidify green liquor components (corresponding to the Total Titratable Alkali) with subsequent formation of sodium sulphate. In order to detoxify the raffinate prior to fermentation, purchased lime (or produced at the lime kiln) is added, forming excessive amounts of gypsum with an associated disposal fee. For the mill described in this paper, it is estimated that as much as 27 tons of gypsum will be generated daily, i.e. 22 kg/ADt. To the knowledge of the authors, the issue of the Na/S balance remains unresolved for the near-neutral extraction process. 7.2. Water balance Large amounts of water are used to wash the extracted wood chips before they are sent back to the mill. Some of the washing water will thereafter follow the wood chips through digesting and the rest will remain at the bioethanol plant, diluting the Table 3 Possible solutions to the Na/S balance problem and associated costs.
Na, kg/ADt Sa, kg/ADt Increased steam demand, MW Revenues from ethanol and acetic acid salesb, V/Adt Make-up chemicals/ steam costsb, V/ADt a
Deficit
Na2SO4 is added
NaOH is added
Stillages are recirculated
31 0/4
0 þ22/þ18 0
0 0/4 0
0 þ22/þ22 þ 13
45
45
45
12
22
12
The first number refers to the case in which all the S from green liquor is bound to lignin. The second case refers to the case in which S is not bound to lignin. b The following prices were assumed: 533 V/ton EtOH, 727 V/ton HAc, 125 V/t Na2SO4, 205 V/t NaOH 50 w% sol, 38 V/MWh steam.
hydrolysate and subsequent process streams. This affects the size of downstream equipment as well as the steam required to upgrade the products, e.g. ethanol distillation. Eventually, the water will be found in the stillages of the L/L extraction process or from the ethanol upgrading step. As mentioned in the previous section, recirculation of these stillages back to the mill might be advantageous from a sodium balance point of view. However, the large content of water in these streams would increase the steam demand of the evaporation plant. In a previous study by Kautto et al. [17], an increase of 27% in steam demand for the evaporation has been reported for a biorefinery with water extraction of hemicellulose. In the case of the mill studied in this paper, an increase by approximately 25% has been estimated (13 MW, Table 3). The difference can be explained by different wood input to the biorefineries. Van Heiningen and coworkers did not consider any pulp yield losses as a consequence of near-neutral extraction. Accordingly, the wood input to the biorefinery remains constant at a given pulp production. On the other hand, Kautto and co-workers consider pulp yield losses when extracting via water-based method, which results in an increase of wood consumption by 16%. Thus, the load of the chemical recovery cycle would be comparatively higher than for the mill studied in this paper. 8. Conclusions By implementing hemicellulose pre-extraction in the studied mill, it is possible to co-produce ethanol (43 t/d) and kraft pulp (1250 ADt/d). In addition, acetic acid can be produced as a byproduct (45 t/d) with a considerably good market value. Nevertheless, such implementation alters the steam and Na/S balances of the mill. It was shown that the increased steam demand can only be compensated with proper heat integration. Indeed, by totally integrating the mill with the bioethanol plant, it is possible to completely eliminate the need for additional steam. However, economic analysis should be carried out to estimate the profitability of the proposed retrofits and to find the optimal degree of process integration. Important consequences for Na/S and water balances have been presented. It was shown that a significant deficit of sodium is expected at the mill if no recirculation of stillages is done from the bioethanol plant to the mill. On the other hand, a large excess of sulphur, and a significant increase in the steam demand of the evaporation plant, are expected if the stillages are recirculated to the mill. Regardless of the promising heat integration potential found in this study, the disruptions in the Na/S balance of the mill may prohibit the implementation of this biorefinery concept unless products with higher market value (than ethanol) are targeted from the extracted hemicellulose. Acknowledgements The authors would like to acknowledge financial support from the “ Södra Foundation for Research, Development and Education”.
V. Lundberg et al. / Applied Thermal Engineering 43 (2012) 42e50
We gratefully thank Christian Hoffstedt (Innventia) for the supply of mill data and helpful discussions, and Jean-Florian Brau for fruitful cooperation during his Master thesis. We would also like to thank Lennart Delin and Fredrik Lundqvist (ÅF) for their valuable input regarding Na/S balance, and Jon van Leuven for editing. Appendix A. Stream data for the pulp mill.
Hot streams General cooling Chemical preparation Surface condenser Stripper condenser Stripper sec. Cond. Steam smelt dissolver BL flash steam 1 BL flash steam 2 Subcooling terp cond Black liquor Wash liquor blow tank D0-stage filtrate D1-stage filtrate (D0þD1)-stage effluent (2) EOP-stage effluent (D0þD1)-stage effluent (1) Flue gases recovery boiler Flue gases lime kiln Cold streams ww50 ww75 ww85 Make-up boiler w. Building heating (LP) Wood yard (LP) Wood chips (flash steam þ LP) Dryer (LP) LP to stripper LP to evap LP to rest Digester circ. (MP) Hi-Heat (MP) MP to oxygen stage MP to bleach plant MP to rest HP steam Combustion air recovery boiler Combustion air bark boiler White liquor to digester White liquor to impregnation
Tstart [ C]
Ttarget [ C]
Q [MW]
DT/2 [ C]
40 48 61 100 90 76 127 105 104 105 86 72 64 60 97 63 175 170
35 38 60 99 89 75 120 104 70 89 82 67 61 42 35 60 106 106
13.17 1.87 57.25 6.25 0.69 3.42 14.47 13.89 0.83 6.87 2.06 3.41 2.51 10.63 16.07 2.21 7.48 0.82
3.5 3.5 2 2 4 2 0 2 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 8 8
18 50 75 18 150 18 119 95 150 150 150 158 123 200 200 200 449 35 35 85 85
50 75 85 75 151 30 120 120 151 151 151 163 157 201 201 201 450 165 165 155 124
41.43 29.49 7.24 6.00 1.04 1.93 19.52 39.63 7.72 55.91 7.53 4.91 10.48 2.75 11.00 2.46 43.53 10.84 1.41 1.59 5.55
2.5 2.5 2.5 2.5 0.5 4 0 8 0.5 0.5 0.5 3.5 3.5 0.5 0.5 0.5 0.5 8 8 3.5 3.5
Appendix B. Stream data for the bioethanol plant.
Hot streams Flash steam Cooling hydrolysate for further treatment Hac and furfural condensers Cooling for liming Cooling for fermentation EtOH condenser Cold streams MP to extraction Preheating (wash water) Heat losses digesting Non identified/other heat losses Hac and furfural distillation EtOH distillation
DT/2 [ C]
Tstart [ C]
Ttarget [ C]
127 126
126 100
5.47 4.35
2 3.5
101 100 50 79
100 50 41 78
2.43 2.05 0.43 2.61
2 3.5 3.5 2
200 18 150 150
201 109 151 151
13.99 13.01 0.59 0.90
0.5 2.5 0.5 3.5
119 97
120 98
2.70 2.90
3.5 3.5
Q [MW]
49
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Pendse, Near-neutral pre-extraction before hardwood kraft pulping; a biorefinery producing pulp, ethanol and acetic acid, in: The International Conference on pulping, Papermaking and Biotechnology 2008, Icppb ’08, Nanjing, China, Vol I, November 46, 2008 pp. 145-156. [19] H. Mao, J.M. Genco, S.H. Yoon, A. van Heiningen, H. Pendse, Technical economic evaluation of a hardwood biorefinery using the "Near-Neutral" hemicellulose pre-extraction process, Journal of Biobased Materials and Bioenergy 2 (2008) 177e185. [20] U. Wising, T. Berntsson, P. Stuart, The potential for energy savings when reducing the water consumption in a kraft pulp mill, Applied Thermal Engineering 25 (2005) 1057e1066. [21] B. Linnhoff, D.W. Townsend, D. Boland, G.F. Hewitt, B.E.A. Thomas, A.R. Guy, R.H. Marsland, A User Guide on Process Integration for the Efficient Use of Energy, Institution of Chemical Engineers, Rugby, UK, 1982. [22] I.C. 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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Process integration of near-neutral hemicellulose extraction in a Scandinavian kraft pulp mill e Consequences for the steam and Na/S balances Valeria Lundberg*, Erik Axelsson, Maryam Mahmoudkhani, Thore Berntsson Chalmers University of Technology, Department of Energy and Environment/Heat and Power Technology, Kemivägen 4, SE-412 96 Göteborg, Sweden
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 August 2011 Accepted 5 March 2012 Available online 29 March 2012
While in a conventional kraft pulp mill, most of the hemicellulose and lignin fraction of the wood is burned in the recovery boiler to produce steam, in a biorefinery it can partially be used to produce added-value products. In this paper, the most important consequences of integrating a bioethanol production plant with a model pulp mill are presented in terms of steam and Na/S balances. The model mill represents an average Scandinavian hardwood kraft pulp mill, and the bioethanol plant is based on the “near-neutral” hemicellulose pre-extraction method. Regarding the steam balance, a comprehensive heat integration study is performed. Implementing hemicellulose extraction increases the net steam demand by 48 MW. However, process integration at the mill and the bioethanol plant individually leads to significant steam savings, and a corresponding net increase of steam by only 3 MW. Additional steam savings can be achieved if the total integration of the two processes (between the pulp mill and the bioethanol plant) is considered (3 MW), resulting in a biorefinery with no increase of steam demand. As regards the Na/S balance, it is shown that green liquor export from the mill to the bioethanol plant results in severe disruptions in the sodium and sulphur balance of the mill. Different attempts to solve this problem are discussed, but are very costly and/or negatively affect the water and steam balance of the mill. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Pulp mill Process integration Near-neutral Hemicellulose extraction Pinch analysis Hardwood Biorefinery Bioethanol
1. Introduction The Nordic pulp and paper industry is currently facing several challenges such as rising energy costs, strong competition from countries with significantly lower feedstock and production costs, and decreasing demand for some paper grades. Consequently, efforts have been made to increase the energy efficiency of mills and to diversify the mix of products. Previous studies have shown that it should be possible to reduce the steam demand and/or to create a steam surplus in both chemical and mechanical pulp mills via process integration [1e6]. Excess heat at the mill facilitates the integration of biorefinery concepts, thus enabling additional products which can increase both the profitability and sustainability of mills. An overview of several biorefinery concepts in pulp mills has been presented previously [7,8], for example hemicellulose extraction to produce ethanol, lignin extraction to produce new chemicals or materials, and biomass gasification to produce transportation fuels. However, integration of biorefinery concepts is challenging since the steam
* Corresponding author. Tel.: þ46 31 7723011; fax: þ46 31 821928. E-mail address: [email protected] (V. Lundberg). 1359-4311/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2012.03.037
production capacity at the mill is diminished as wood components are withdrawn from the pulp line. Moshkelani et al. [9] have developed a methodology to perform high levels of process integration and intensive energy efficiency, and concluded that the sustainability of biorefinery concepts depends upon the successful implementation of intensive energy integration and optimization measures. As previously mentioned, large potentials for process integration in kraft pulp mills have been identified in earlier studies. The importance of process integration for different biorefineries has also been discussed by several authors; for example, Jönsson et al. [10] compared the profitability and carbon dioxide emissions of different technology pathways for using excess steam in a typical Scandinavian pulp mill. Garcia et al. [11] studied the consequences of integrating the organosolv process (for fractionating wood components) into a mill. Fornell et al. [12,13] studied the total conversion of a pulp mill into a bioethanol plant as well as the production of dimethyl ether (DME) by gasification of black liquor. A biorefinery concept that continues to gain attention is lignocellulosic bioethanol production, as it may replace fossil fuels in the transport sector. According to Hämäläinen et al. [14], the increasing price of oil is the most important incentive for forest biorefineries in Scandinavia, North America and South America.
V. Lundberg et al. / Applied Thermal Engineering 43 (2012) 42e50
The simultaneous production of pulp and bioethanol is advantageous since pulp mills generally have existing infrastructure, logistics and know-how about processing large amounts of lignocellulosic materials. Nevertheless, production of bioethanol is a complex and energyintensive process. Hence, several researchers have investigated different routes for ethanol production as well as how to increase the energy efficiency of the process. Bioethanol can be produced both in thermomechanical pulp mills (TMP) and in chemical pulp mills. In the case of kraft mills, bioethanol can be produced by extracting carbohydrates prior to the digestion and subsequent fermentation into ethanol. Hemicellulose can be extracted by different methods, such as alkaline extraction [15], acid extraction [16], water extraction [17] and green liquor extraction, so-called “near-neutral” pre-extraction [18]. Hemicellulose extraction will affect the mill in various ways; for example, the severe conditions in water and dilute acid extraction can have a negative impact on the fibre properties. On the other hand, it has also been shown that, when using milder conditions, as in the near-neutral extraction method, hemicellulose can be extracted while maintaining the quantity (i.e. pulp yield) and quality (e.g. tear index) of the pulp [19]. In addition to possible changes in pulp quality and quantity, extraction of hemicellulose affects the operation and performance of process equipment at the mill. In the case of near-neutral extraction, for example, the continuous export of green liquor from the mill to the bioethanol plant could potentially destabilize the Na/S balance of the mill, if the sodium and sulphur ions present in green liquor are not reincorporated back in the recovery cycle of the mill (see Section 7). Moreover, the water use at the mill is significantly increased due to the washing of the extracted wood chips. It has previously been shown that the potential for energy savings at a mill, as well as the best approach for steam savings, is highly dependent on the water consumption [5,20]. Another effect of extracting hemicellulose from the wood chips is a reduction in the organic material burned in the recovery boiler and consequently a decrease in the steam production at the mill. This, in combination with the relatively high steam demand of the bioethanol plant, results in a net steam deficit, assuming that the wood input is kept constant. The deficit could in principle be compensated for by burning extra fuel at the mill, but this could jeopardize the economic and environmental performance of the biorefinery. Alternatively, the steam demand can be reduced by increasing the energy efficiency via process integration [21e23]. However, process integration studies focusing on hemicellulose extraction, and particularly on the near-neutral method, have seldom been presented. Van Heiningen et al. [24] studied hemicellulose extraction based on the near-neutral method in kraft pulp mills, but no heat integration studies were performed. Marinova et al. [25] identified energy efficiency measures to reduce the steam demand of a Canadian kraft mill with near-neutral hemicellulose extraction and bioethanol production, but the consequences for the steam demand of different parts of the mill were not taken into account (except for the change of steam production in recovery boiler). In addition, the consequences for the Na/S balance of extraction using the near-neutral method have not been thoroughly presented in the literature. The authors have therefore evaluated the heat integration potential for integration of an average Scandinavian hardwood model mill with near-neutral pre-extraction in a previous article [26]. In the present paper, the consequences of such integration for the steam balance are presented in a more comprehensive way. In addition, the scope of the study is extended to include the consequences for the Na/S balance, as this also may have a high impact on the energy and/or materials consumption.
43
2. Aims One aim of this paper is to analyse the consequences of integrating a bioethanol production plant (based on green liquor extraction, also called near-neutral hemicellulose extraction) with a kraft pulp mill, in terms of the steam (i.e. steam demand and production) and Na/S balances. Another aim is to identify the potential for steam savings within the mill, as well as within the bioethanol plant, and whether it is advantageous to export heat from the mill to the biorefinery or vice versa, i.e. to have total integration. Furthermore, the paper aims at discussing different practical retrofits that could result in steam savings. Concerning the Na/S balance aspects, the intention is to investigate the resulting Na/S deficit at the mill upon extraction of hemicellulose with green liquor, as well as to discuss the drawbacks of different attempts to compensate for this deficit (e.g. in the water balance of the mill or the profitability of the project). 3. Method In this study, relevant process data have been gathered from simulation models of the mill and bioethanol plant. In the case of the kraft mill, process data were available from a WinGEMS simulation of a hardwood Scandinavian mill [27]. The model represents a typical Scandinavia mill as regards equipment and level of resource utilization. For the bioethanol plant, mass and energy balances for different process units were generated by the authors with the help of data available in literature [19,28]. In order to study the consequences of integrating a bioethanol plant in a kraft mill, a base case has been defined to allow further comparisons. The base case represents a mill in which changes in process streams that result from implementing hemicellulose preextraction (i.e. changes in composition, mass or enthalpy flow rate) have been taken into account, but no attempt has been made to improve the system by process integration. Thereafter, two cases with different levels of process integration were studied. In the simplest case, the possibility to save steam and release excess heat on a stand-alone basis (i.e. separately, within each of the processes) was investigated. In the second case, the benefits of total integration were studied; i.e. heat/steam may also be transferred between the mill and the bioethanol plant. The process integration potential at the mill and bioethanol plant was analysed via pinch analysis [21e23]. Since the introduction of pinch analysis, i.e. heat integration, a number of different industries including the pulp and paper industry have benefited from it [29]. Moreover, the heat integration methodology has been extended and developed by numerous contributions [30], not least the development of several methodologies for retrofit situations [31,32]. In this paper, however, only the basic concepts of pinch analysis have been used, as described in e.g. [21], since the latter methodologies require detailed knowledge about the heat exchanger networks that was not available in the mill and bioethanol models. In this study no economic optimization for the retrofits has been carried out due to lack of data (e.g. piping distance and area for existing heat exchangers). Still, different reasonable retrofits have been identified based on general pinch analysis heuristics, e.g. to solve the largest pinch violations, heatexchange streams located close to each other, re-use existing equipment, etc. In order to investigate the consequences of extracting hemicellulose with green liquor, sodium and sulphur balances were set up for different process units. The procedure to develop the Na/S balances is outlined in Fig. 1. First, the sodium and sulphur content in green liquor was calculated. This corresponds to the expected sodium deficit at the mill. In the case of sulphur, this represents the
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V. Lundberg et al. / Applied Thermal Engineering 43 (2012) 42e50
Use Total Titratable Alkali (TTA) [gNaOH /lGL] to calculate Na content in green liquor: Na GL. Use Active Alkali (AA) [ gAA /lGL] and Sulphidity [kgNa 2 S/kgAA ] to calculate S content in green liquor: S GL
Na and S in the extraction vessel is: Na GL, S GL
Calculate the Na and S content in the hydrolysate solution: Na extract =Na GL Is S covalently bound to lignin? Yes S extract =0; No Sextract =SGL Calculate the S input during acid hydrolysis, S H2 SO4 =Sneutr +SpH •Use TTA [gNaOH /lGL] to calculate the protons (H 2 SO 4 ) needed to neutralize green liquor components, Sneutr •Use pH=-log[H+] to calculate the protons (H 2 SO 4 ) needed to decrease the pH to 1, S pH Calculate the Na and S content in the delignified hydrolysate: Na delig=Na extract Sdelig=Sextract +SH2 SO4
The S precipitated as gypsum is: S pH
Calculate the Na and S found in the stillages: Na still =Na delig, Sstill =Sdelig-SpH Fig. 1. Method to develop Na/S balances.
maximum deficit of sulphur that can be expected, considering that none of the sulphur is bound to lignin (see Section 7). Accordingly, this is also the sodium and sulphur present in the diluted extract (Fig. 2). Thereafter, the sulphur input during acid hydrolysis was estimated. The consumption of sulphuric acid during acid hydrolysis corresponds to the protons needed to acidify green liquor components (according to the stoichiometry of the neutralization reaction) plus the protons needed to achieve the specified pH conditions (according to the definition of pH ¼ log[Hþ]). It was finally assumed that the total sodium and sulphur input (i.e. from green liquor and sulphuric acid) was eventually found in the stillages of the bioethanol plant. Thus, the net sodium and sulphur balance at the mill depends on whether the stillages are returned to
the mill or not, which in turn has important consequences for the steam balance of the mill. Finally, the consequences of implementing near-neutral extraction for the water balance of the mill were estimated by calculating the steam required to concentrate the diluted stillages of the bioethanol plant to the same dry solid content of the strong black liquor (72%), in the existing evaporation plant (5.5 effects). 4. The studied pulp mill The studied kraft pulp model was developed within the Swedish national research programme “Future Resource Adapted Pulp Mill” (FRAM) and represents an average hardwood
Green liquor Antraquinone Wood chips
Extracted wood chips Pre- extraction
Kraft process
Wash water Flash steam
Flash H2SO4
Diluted extract Lime
Acid hydrolysis Hydrolyzate Lignin filtration Lignin
CO2
Solvent
Raffinate L/L Liming extraction
Fermentation
Ethanol upgrading
Stillage Acetic acid Stillage
Gypsum
Fig. 2. Block diagram of the near-neutral process (image adapted from Mao [28]).
Ethanol
V. Lundberg et al. / Applied Thermal Engineering 43 (2012) 42e50
Scandinavian mill [27]. The mill produces 1250 ADt/d bleached market pulp from 2328 ADt/d of hardwood (Table 1). Wood logs are de-iced in the wood yard with steam-heated water (2 MW). After debarking and chipping, the wood chips are steamed with LP steam (5 MW) and black liquor flashed steam (14 MW). Steamed wood chips are then cooked in a conventional two-flash digester, where MP steam is used in the digester circulation and in the hiheat washing zone (10 MW). The pulp is then bleached and dried. In the recovery cycle, the black liquor is flashed in two steps; the first black liquor flash is used for steaming wood chips (14 MW) and the second flash to produce hot water (14 MW). In a 5.5-effect evaporation plant, black liquor is evaporated to 72% dry solid content. Strong black liquor is burned in the recovery boiler to produce steam and recover the inorganic compounds as a smelt. The steam production in the recovery boiler (216 MW) is not sufficient to satisfy all the steam demand of the mill (221 MW) and consequently, 5 MW of supplementary steam need to be produced in the bark boiler. Excess bark is sold. The back-pressure steam turbine is not sufficient to accommodate all the high-pressure steam, and about 25% of the steam is passed through let-down valves. The hot and warm water system (HWWS) consists of a heat exchanger network where the required warm and hot water for the mill is produced from excess secondary heat in the process. In the HWWS, black liquor flash steam and excess heat of low temperature is used to preheat process water and make-up water (for the recovery and bark boilers). Due to poor measuring and controlling and/or insufficient heat exchanger area, the HWWS is unable to handle stops and unbalanced operation at the mill. Consequently, the heat from filtrates, condensers, effluents etc. is not enough to heat the water, and heating with steam is required (8 MW).
5. Description of the bioethanol plant The studied bioethanol plant is based on the near-neutral preextraction method. This method has first been suggested by van Heiningen and co-workers [28,33]. In the near-neutral method, Table 1 Steam production and consumption at the pulp mill before/after hemicellulose preextraction, when no process integration is carried out. HP 61 bar, 450 C Production [MW] Recovery boiler Bark boiler
MP 11 bar, 200 C
LP 4.5 bar, 150 C
221/197
44
12/8 16
56
Sub-total
44
43/39
134/128
2 5/0
(14/13)
10
5
8 7 56/55
0
0/14
0/20
0/34 TOTAL
2.5 bar, 127 C 1.2 bar, 105 C
(28/27)
221/211
*
216/192 5
Consumption [MW] MILL Woodyard Steaming Hi-heat washing zone digester Hot water production (HWWS) Combustion air Evaporation plant Digester Other steam uses
BIOETHANOL PLANT Sub-total
Flashed steam
221/245
The numbers in brackets refer to non-net users of steam.
(14)
(28/27)
45
wood chips are steamed and exposed to a solution of green liquor (Na2CO3 and Na2S, 3% on wood as Na2O) and anthraquinone (0.05% on wood) (Fig. 2). Extraction of hemicellulose is done in a preextraction vessel, where the temperature is kept constant at 160 C by heating with steam. Approximately 10% of the wood mass is extracted, mainly hemicellulose but also some lignin. The extracted wood chips are washed and sent to the kraft process. Some of the diluted extract is recirculated back to the extraction vessel to raise the solid content of the extract, resulting in a liquidto-wood ratio of 4:1. The rest of the diluted extract is concentrated by flashing before further processing. Hydrolysis is assumed to be carried out under strongly acidic conditions (pH ¼ 1) with an efficiency of 90% conversion of oligomeric carbohydrates into monomeric sugars [28]. Under these conditions it is assumed that all the lignin is precipitated and separated by filtration [28]. The delignified hydrolysate is then cooled down before further purification and separation stages. In the first step, acetic acid and furfural are removed by liquideliquid (LeL) extraction followed by distillation. Then, lime is added in order to raise the pH to that required for fermentation and to detoxify the solution by precipitating sulphate ions as gypsum (CaSO.42H2O). Some complications with the liming operation are discussed in Section 7. The detoxified sugar solution is thereafter fermented. An important assumption regarding fermentation is that the same fermentation yields are considered for both pentoses and hexoses (as originally assumed in [28]). Finally, the diluted ethanol solution is upgraded by distillation. Previous studies [34] suggest that integrated membrane systems may be a more cost-efficient alternative to upgrade diluted ethanol-water mixtures than the classical distillation process due to its high energy demand. However, in this study no attempts were made to modify the described process. Based on the described process, with adaptation to the mill studied in this paper, a total of 43 t/d (35 kg/ADt) of ethanol and 45 t/d (36 kg/ADt) of acetic acid can be produced, while the wood input and produced pulp rate remain constant. 6. Consequences of implementing the biorefinery concept for steam balance of the mill 6.1. Base case e No process integration The extraction of hemicellulose alters the steam balance in several ways listed in the following, resulting in an increase of the steam demand by 48 MW (Table 1, Fig. 3): a) Both the flow rate and organic content of black liquor are decreased [17], which leads to a reduction in steam production in the recovery boiler (27 MW). It should be mentioned that in the literature an assumption of 10% reduction in steam production in the recovery boiler was considered for 10% extraction [25,28]. In this paper, however, the steam reduction is calculated based on the changes in heat value of black liquor, which results in a larger reduction in steam production (12%, i.e. 27 MW instead of 22 MW). Still, some of this deficit can be compensated for by burning the extracted lignin (from delignifying the hydrolysate) in the recovery boiler (3 MW). Consequently, the steam production at the mill is decreased (221e24 ¼ 197 MW). b) The decrease in black liquor flow rate would, moreover, decrease the steam demand of the evaporation plant (56e1 ¼ 55 MW). c) The wood chips are steamed at a lower temperature. Thus, the heat available in the black liquor flashes is enough to steam the wood chips and no LP steam is necessary for this purpose (5e5 ¼ 0 MW).
46
V. Lundberg et al. / Applied Thermal Engineering 43 (2012) 42e50
221-27+3=197 MW
221-1-5-4=211 MW
Pulp mill Recovery boiler and bark boiler
34 MW
Bioethanol plant
48 MW Increased steam demand
Fig. 3. Steam balance when no process integration is carried out.
d) The incoming wood (extracted wood) is partially cooked during hemicellulose extraction; therefore the steam demand in the digester is decreased (12e4 ¼ 8 MW). e) The bioethanol plant has its own steam demand (34 MW).
6.2. Heat integration on a stand-alone basis In order to assess the potential for steam savings, Grand Composite Curves of the mill and the bioethanol plant were constructed (Fig. 4A, B, Appendices A, B). The theoretical minimum heating demand (QH,min) of the mill is 175 MW and therefore it has a potential of 36 MW of steam savings (the current steam demand of the mill with hemicellulose pre-extraction is 211 MW, Table 1). The bioethanol plant has a minimum heating demand of 17 MW which corresponds to a steam saving potential of 17 MW (the nonprocess-integrated plant has a steam demand of 34 MW, Table 1). Accordingly, there is a total steam saving potential of 53 MW, which would theoretically be enough to compensate for the originally increased steam demand (48 MW). In the following sections, a reasonable suggestion of a possible retrofit is presented, with the potential to save 45 MW of steam (85% of the theoretical potential). This level of solved pinch violation (i.e. inefficiencies) is in accordance with other, more detailed, studies on process integration potential in similar mills [35,36]. To further decrease the steam demand, complex heat integration is needed, and was judged unprofitable. 6.2.1. Studied mill In the warm and hot water system (HWWS) of the mill, process water is heated to 50 C, 75 C and 85 C. Black liquor flash steam (at 105 C), LP steam (8 MW) and excess heat of low temperature, e.g. condensers and filtrates, are used as heat sources. However,
some other heat sources, e.g. effluents and stripper condenser, are not used for water heating and are therefore just cooled down, which can be considered a pinch violation (cooling above the pinch). In practise it is possible to heat all the water without black liquor flash steam by using the other heat sources, as well as by increasing the heat exchanger area (an increase corresponding to an average mean temperature difference in the heat exchangers of DTlm ¼ 18 C instead of 21 C). Since the heat available in black liquor flash steam is no longer required for heating the water, it is possible to use it somewhere else. In fact, heat can be recovered directly from black liquor before flashing. By heat-exchanging black liquor with the hi-heat washing zone of the digester, it is possible to reduce the MP steam demand of the digester by 4 MW (Table 2). Additional steam savings are achieved by using the black liquor flash steam to preheat white liquor to the digester (6 MW). Conversely, the steam demand of the evaporation is slightly increased (2 MW) due to decreased flashing and consequently a decrease in the dry solid content of the black liquor. In a redesigned HWWS, a slightly higher warm and hot water flow rate can be produced, whichein combination with additional storage capacity e ensures that the required amount of warm and hot water is always available. In this way, it is possible to eliminate the need for LP steam (8 MW) due to imbalances in production and consumption of hot and warm water. Moreover, it is possible to produce significant amounts of excess warm water (55 C) for different purposes, for example to de-ice the wood chips instead of using LP steam (2 MW). A part of the excess water could also be further heated to 78 C and used thereafter to preheat combustion air to the recovery boiler instead of LP steam (4 MW). In addition, the flue gases of the recovery boiler can preheat combustion air to a higher temperature (which is a retrofit independent of the preceding one). By heat-exchanging flue gases with
Fig. 4. Grand composite curve (with individual DTmin ¼ 0.5e8 C). 3A, Left: Pulp mill. 3B, Middle: Bioethanol plant. 3C, Right: foreground-background analysis.
V. Lundberg et al. / Applied Thermal Engineering 43 (2012) 42e50 Table 2 Possible steam-saving measures on a stand-alone basis. Additional steam-saving measures for the total integration case are presented between parenthesis. Steam saving measures at the mill Use black liquor to heat hi-heat washing zone (In total integration black liquor heat can be used to a larger extent) Preheat white liquor with black liquor (In total integration the flash steam from the bioethanol plant can also be used to preheat white liquor) Increased evaporation load due to modified flashing General improvements in the hot and warm water system Use warm water to de-ice wood logs Use stripper and terpentine condenser to preheat combustion air to recovery boiler Use flue gases to preheat combustion air to the recovery boiler
[MW] 4 (6) 6 (6)
2 8 2 4 6 28 (30)
Steam saving measures at the ethanol plant Preheat washing water to extraction vessel (In total integration excess water from the HWWS can be used instead of fresh water) Use flash steam and hydrolysate to run reboilers in distillation towers
12 (13)
5 17 (18) 45 (48)
Total steam savings
combustion air, it is possible to save additional 6 MW of MP and LP steam. In this heat exchanger, air could be heated from 90 C to 159 C while flue gases are cooled from 175 C to 128 C. It has been shown elsewhere that corrosion problems in recovery boilers due to acidic condensates are not measurable at these temperature levels [37]. By implementing the previously described retrofits, it is possible to reduce the steam demand by 28 MW (Table 2). 6.2.2. Bioethanol plant By designing a cooling water circuit with an average DTlm of 25 C, fresh water can be heated from 18 C to 109 C, with the excess heat available in different process streams and condensers. The pre-heated water can then be used to wash the extracted wood chips instead of fresh water. In this way, it is possible to reduce the steam consumption by 12 MW (Table 2). In addition, some of the flash steam (from diluted extracted flash tank) can be used as a heat source for the distillation of acetic acid and furfural (2 MW), and the delignified hydrolysate stream can be used as a heat source for ethanol distillation (3 MW). Thus it is possible to reach the theoretical steam saving potential and reduce the steam demand by 17 MW (Table 2). The previously described measures at the mill and bioethanol plant all together represent a steam saving potential of 45 MW, which is nearly sufficient to compensate for the originally increased steam demand (48 MW) (Fig. 5). The net deficit of 3 MW can be taken care of by burning excess bark in the bark boiler (currently, the bark boiler provides 5 MW to the mill, in the case of integrated biorefinery; the boiler capacity can possibly be increased to 8 MW). Alternatively, additional steam savings may be achieved by exporting heat from the mill to the bioethanol plant and vice versa, i.e. by total integration.
197 MW
211-28=183 MW
Pulp mill Recovery boiler and bark boiler
47
6.3. Total integration Total integration makes it possible to achieve higher energy savings compared to the stand-alone cases. To investigate the potential for further energy savings, a background/foreground curve [22] is constructed (Fig. 4C). According to the background/foreground analysis, excess heat at 60 C can be exported from the mill to the bioethanol plant. Moreover, there is a potential to export heat at a higher temperature from the bioethanol plant to the mill. In practise, it is possible to preheat white liquor with hightemperature heat available at the bioethanol plant, e.g. flash steam (127 C), reducing the use of black liquor for this purpose by 2 MW. This is however only possible if washing water for the extraction vessel could be heated with excess heat from the mill (as shown in Fig. 4C). For example, excess warm water produced at the mill (55 C) could be top-heated with some of the heat sources available at the bioethanol plant (similarly to stand-alone integration) and be used as washing water to the extraction vessel. As a consequence of this retrofit, black liquor can be used to a larger extent in the hi-heat washing zone of the digester, resulting in additional MP steam savings (2 MW). Moreover, this retrofit eliminates the need of steam for water heating (1 MW), resulting in a reduction of the combined steam demand by 3 MW. By implementing the preceding measures (summarized in Table 2), it is therefore possible to decrease the steam deficit by additional 3 MW. Accordingly, the original steam produced at the pulp mill is enough to satisfy the steam demand of both the mill and bioethanol plant (Fig. 6). 7. Consequences of implementing the biorefinery concept for the Na/S balances of the mill Despite the promising potential of heat integration between the mill and the bioethanol plant, other process disturbances may prevent the implementation of this biorefinery concept. Here, two critical issues will be discussed: consequences for Na/S and water balances. 7.1. Na/S balance The results of our calculations are shown in Table 3. A deficit of 31 kg/ADt of sodium and between 0 and 4 kg/ADt of sulphur is expected at the mill, due to the export of part of the green liquor from the mill to the hemicellulose extraction process. It is important to notice that at different parts of the ethanol plant with low pH, sulphur could be covalently bound to lignin as thiolignin. If sulphur is covalently bound to the lignin during extraction, then it may be reincorporated in the chemical recovery cycle of the mill. Whether and to what extent sulphur will be bound to lignin must be further investigated. Regardless, a deficit of sodium is expected. By using make-up chemicals the deficit can be compensated. Using Na2SO4, for example, could potentially compensate for the loss of sodium; however, it would insert an excess of sulphur in the 34-17=17 MW
Bioethanol plant
3 MW Increased steam demand
Fig. 5. Steam balance when process integration is carried out on a stand-alone basis.
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V. Lundberg et al. / Applied Thermal Engineering 43 (2012) 42e50
197 MW
17-1=16 MW
183-2=181 MW
5 MW
Pulp mill Recovery boiler and bark boiler
Bioethanol plant
0 MW
2 MW
Fig. 6. Steam production and consumption at the mill and bioethanol plant when total integration is carried out.
cycle (22-18 kg/ADt). If a sulphur-free chemical, i.e. NaOH, were added as make-up, the loss of sodium could be compensated for, without introducing new sulphur into the system. But the increase in chemical costs when using NaOH as make-up would be nearly twice the cost for Na2SO4 due to higher market prices. It can easily be shown that the cost of make-up chemicals is significant as compared to the revenue from the value-added products, i.e. ethanol and acetic acid, unless alternative products with high market prices are targeted. Alternatively, the process could be designed so that these ions are recovered and sent back to the mill [24]. This is, however, complicated from a practical point of view, since the lost sodium and sulphur are found in much-diluted stillages which would require large steam consumption in order to be concentrated before being recirculated, thus causing an increase in evaporation demand. Moreover, the amount of sulphur recycled back to the mill would be excessive if all the sodium is to be recovered, i.e. 22 kg/ADt. This is due to the use of sulphuric acid in the acid hydrolysis unit. Additionally, other costs may worsen the profitability of the studied process. We found that only w16% of the sulphuric acid is consumed to decrease the pH of the hydrolysate, whereas the rest of the acid is used to acidify green liquor components (corresponding to the Total Titratable Alkali) with subsequent formation of sodium sulphate. In order to detoxify the raffinate prior to fermentation, purchased lime (or produced at the lime kiln) is added, forming excessive amounts of gypsum with an associated disposal fee. For the mill described in this paper, it is estimated that as much as 27 tons of gypsum will be generated daily, i.e. 22 kg/ADt. To the knowledge of the authors, the issue of the Na/S balance remains unresolved for the near-neutral extraction process. 7.2. Water balance Large amounts of water are used to wash the extracted wood chips before they are sent back to the mill. Some of the washing water will thereafter follow the wood chips through digesting and the rest will remain at the bioethanol plant, diluting the Table 3 Possible solutions to the Na/S balance problem and associated costs.
Na, kg/ADt Sa, kg/ADt Increased steam demand, MW Revenues from ethanol and acetic acid salesb, V/Adt Make-up chemicals/ steam costsb, V/ADt a
Deficit
Na2SO4 is added
NaOH is added
Stillages are recirculated
31 0/4
0 þ22/þ18 0
0 0/4 0
0 þ22/þ22 þ 13
45
45
45
12
22
12
The first number refers to the case in which all the S from green liquor is bound to lignin. The second case refers to the case in which S is not bound to lignin. b The following prices were assumed: 533 V/ton EtOH, 727 V/ton HAc, 125 V/t Na2SO4, 205 V/t NaOH 50 w% sol, 38 V/MWh steam.
hydrolysate and subsequent process streams. This affects the size of downstream equipment as well as the steam required to upgrade the products, e.g. ethanol distillation. Eventually, the water will be found in the stillages of the L/L extraction process or from the ethanol upgrading step. As mentioned in the previous section, recirculation of these stillages back to the mill might be advantageous from a sodium balance point of view. However, the large content of water in these streams would increase the steam demand of the evaporation plant. In a previous study by Kautto et al. [17], an increase of 27% in steam demand for the evaporation has been reported for a biorefinery with water extraction of hemicellulose. In the case of the mill studied in this paper, an increase by approximately 25% has been estimated (13 MW, Table 3). The difference can be explained by different wood input to the biorefineries. Van Heiningen and coworkers did not consider any pulp yield losses as a consequence of near-neutral extraction. Accordingly, the wood input to the biorefinery remains constant at a given pulp production. On the other hand, Kautto and co-workers consider pulp yield losses when extracting via water-based method, which results in an increase of wood consumption by 16%. Thus, the load of the chemical recovery cycle would be comparatively higher than for the mill studied in this paper. 8. Conclusions By implementing hemicellulose pre-extraction in the studied mill, it is possible to co-produce ethanol (43 t/d) and kraft pulp (1250 ADt/d). In addition, acetic acid can be produced as a byproduct (45 t/d) with a considerably good market value. Nevertheless, such implementation alters the steam and Na/S balances of the mill. It was shown that the increased steam demand can only be compensated with proper heat integration. Indeed, by totally integrating the mill with the bioethanol plant, it is possible to completely eliminate the need for additional steam. However, economic analysis should be carried out to estimate the profitability of the proposed retrofits and to find the optimal degree of process integration. Important consequences for Na/S and water balances have been presented. It was shown that a significant deficit of sodium is expected at the mill if no recirculation of stillages is done from the bioethanol plant to the mill. On the other hand, a large excess of sulphur, and a significant increase in the steam demand of the evaporation plant, are expected if the stillages are recirculated to the mill. Regardless of the promising heat integration potential found in this study, the disruptions in the Na/S balance of the mill may prohibit the implementation of this biorefinery concept unless products with higher market value (than ethanol) are targeted from the extracted hemicellulose. Acknowledgements The authors would like to acknowledge financial support from the “ Södra Foundation for Research, Development and Education”.
V. Lundberg et al. / Applied Thermal Engineering 43 (2012) 42e50
We gratefully thank Christian Hoffstedt (Innventia) for the supply of mill data and helpful discussions, and Jean-Florian Brau for fruitful cooperation during his Master thesis. We would also like to thank Lennart Delin and Fredrik Lundqvist (ÅF) for their valuable input regarding Na/S balance, and Jon van Leuven for editing. Appendix A. Stream data for the pulp mill.
Hot streams General cooling Chemical preparation Surface condenser Stripper condenser Stripper sec. Cond. Steam smelt dissolver BL flash steam 1 BL flash steam 2 Subcooling terp cond Black liquor Wash liquor blow tank D0-stage filtrate D1-stage filtrate (D0þD1)-stage effluent (2) EOP-stage effluent (D0þD1)-stage effluent (1) Flue gases recovery boiler Flue gases lime kiln Cold streams ww50 ww75 ww85 Make-up boiler w. Building heating (LP) Wood yard (LP) Wood chips (flash steam þ LP) Dryer (LP) LP to stripper LP to evap LP to rest Digester circ. (MP) Hi-Heat (MP) MP to oxygen stage MP to bleach plant MP to rest HP steam Combustion air recovery boiler Combustion air bark boiler White liquor to digester White liquor to impregnation
Tstart [ C]
Ttarget [ C]
Q [MW]
DT/2 [ C]
40 48 61 100 90 76 127 105 104 105 86 72 64 60 97 63 175 170
35 38 60 99 89 75 120 104 70 89 82 67 61 42 35 60 106 106
13.17 1.87 57.25 6.25 0.69 3.42 14.47 13.89 0.83 6.87 2.06 3.41 2.51 10.63 16.07 2.21 7.48 0.82
3.5 3.5 2 2 4 2 0 2 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 8 8
18 50 75 18 150 18 119 95 150 150 150 158 123 200 200 200 449 35 35 85 85
50 75 85 75 151 30 120 120 151 151 151 163 157 201 201 201 450 165 165 155 124
41.43 29.49 7.24 6.00 1.04 1.93 19.52 39.63 7.72 55.91 7.53 4.91 10.48 2.75 11.00 2.46 43.53 10.84 1.41 1.59 5.55
2.5 2.5 2.5 2.5 0.5 4 0 8 0.5 0.5 0.5 3.5 3.5 0.5 0.5 0.5 0.5 8 8 3.5 3.5
Appendix B. Stream data for the bioethanol plant.
Hot streams Flash steam Cooling hydrolysate for further treatment Hac and furfural condensers Cooling for liming Cooling for fermentation EtOH condenser Cold streams MP to extraction Preheating (wash water) Heat losses digesting Non identified/other heat losses Hac and furfural distillation EtOH distillation
DT/2 [ C]
Tstart [ C]
Ttarget [ C]
127 126
126 100
5.47 4.35
2 3.5
101 100 50 79
100 50 41 78
2.43 2.05 0.43 2.61
2 3.5 3.5 2
200 18 150 150
201 109 151 151
13.99 13.01 0.59 0.90
0.5 2.5 0.5 3.5
119 97
120 98
2.70 2.90
3.5 3.5
Q [MW]
49
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