SPECIAL BIOREFINERY ISSUE LIGNIN PRODUCTION PATH ASSESSMENT: ENERGY, WATER, AND CHEMICAL INTEGRATION PERSPECTIVE ZOÉ PÉRIN-LEVASSEUR, LUCIANA SAVULESCU, MARZOUK BENALI* ABSTRACT Extracting the lignin from the black liquor stream has the potential not only to generate bio-products and bio-fuels, but also to debottleneck the recovery boiler in the case of increased pulp production. Revamping a pulp mill into a lignin-based biorefinery will, however, affect significantly the resources involved in the process. To analyze the response from various factors, the key correlations among the production rate, energy, water, and chemical recovery systems have been derived. The potential recovery of from the flue gases of the recovery boiler and lime kiln and from the spent acid from the chlorine dioxide (ClO 2 ) generator has been examined for capture and for abatement of chemical demand. Water reuse opportunities have also been considered to lower the site water consumption. The need for energy improvements through heat recovery retrofit of the heat exchanger network has been raised as a key point in achieving overall biorefinery energy efficiency. INTRODUCTION Lignin is one of the primary components of wood. When wood is pulped to cellulose fibres, the lignin is dissolved into the black liquor, which is currently burned to recover pulping chemicals and to provide energy to the mill. Lignin also has the potential to serve as a feedstock chemical for various products. Therefore, advances in lignin chemical processing provide new and renewable opportunities in biomass and black liquor utilization for production of chemicals and biofuels. These opportunities create a synergy with the need to optimize the recovery boiler, which is the major controlling bottleneck in the current Kraft pulping mill. Indeed, the capacity of the recovery boiler limits pulp production. Extracting lignin from black liquor in the evaporation section of the plant is one of the convenient options to overcome this limitation. In addition, this approach will benefit traditional pulp mills by diversifying the portfolio of products that they can offer and will benefit the emerging biofuels sector and the chemical industry by providing value-added products to complement the low margins of the pulp and paper industry [1]. There are currently three main technology routes for lignin extraction from black liquor: ultrafiltration (UF) in the digester, electrolysis (EL), and acid precipitation (AP) in the evaporation sub-system. The AP process is the most developed and the most cost-effective at large scale, whereas UF and EL are still at the development and demonstration stage for large processing capacities [2]. Moreover, the AP process offers one of the promising chemical biorefinery pathways for achieving a successful integrated forest biorefinery and moving towards an optimal utilization of biomass. The purpose of this work is to evaluate the influence of lignin removal on the ZOÉ PÉRIN-LEVASSEUR Natural Resources Canada, CanmetENERGY, Industrial Systems Optimization 1615 Lionel-Boulet Blvd., P.O. 4800, Varennes, Qc Canada, J3X 1S6 LUCIANA SAVULESCU Natural Resources Canada, CanmetENERGY, Industrial Systems Optimization 1615 Lionel-Boulet Blvd., P.O. 4800, Varennes, Qc Canada, J3X 1S6 MARZOUK BENALI Natural Resources Canada, CanmetENERGY, Industrial Systems Optimization 1615 Lionel-Boulet Blvd., P.O. 4800, Varennes, Qc Canada, J3X 1S6 *Contact: marzouk.benali@nrcan.gc.ca J-FOR Journal of Science & Technology for Forest Products and Processes: VOL.1, NO.3, 2011 25
new generation of Kraft mills through integrating an acid-precipitation lignin extraction process, to assess the opportunities for and ClO 2 recycling loops within the lignin-based integrated biorefinery, and to propose energy and water savings opportunities to reduce the overall energy and water demand of the ligninbased biorefinery Kraft mill. ACID PRECIPITATION OF LIGNIN Acid precipitation (AP) is the most common and well-developed method of lignin extraction from black liquor. Because sulphuric acid (H 2 ) disturbs the chemical balance by providing an excess of sulphur, is preferred for acidification. Figure 1 illustrates the key steps in the AP process. This process starts with lignin precipitation from partially concentrated black liquor in an acidification reactor to lower the ph from 12 14 to 9 with. This acidification step is followed by filtration and washing of the lignin cake in acidic aqueous conditions using H 2. The lignin-free filtrate (containing mainly inorganics) is returned to the chemical recovery system of the Kraft pulp mill. The purity of the obtained lignin is 95% 98%, with a solids content ranging from 50% to 70% (mass) and an ash content from 1% to 3% (mass). The performance of the AP process depends mainly on ph and temperature [3]. The integration of the AP process within a generic Kraft mill offers several process integration opportunities such as energy and water savings as well as capture from the flue gases of the recovery boiler and lime kiln and spent acid recovery from the ClO 2 generator to be used Fig. 1 - General view of -acid precipitation of lignin. in the acidification reactor and filtration/ washing steps. These process integration opportunities are discussed later in this paper. LIGNIN PRODUCTION PATH ASSESSMENT (L2PA) Maximizing the proportion of biomass utilized in the production of lignin-based products is the first step in achieving a successful biorefinery system. In this regard, five main production pathways: pellets, speciality or commodity chemicals, lignin for combustion in the bark boiler, lignin for combustion in the lime kiln, and gasification have been identified as promising candidates (Fig. 2). Fig. 2 - Lignin biorefinery pathways. Therefore, to cope with the multitude of lignin-production processing pathways and the high complexity of system interactions, a conceptual approach has been introduced. This L2PA is a systematic assessment approach which looks at the implications of production variations for the energy, water, and chemical systems and their interrelations. This will facilitate decision-making to select cost-effective integrated lignin-based biorefinery designs. This paper covers the aspects related to lignin extraction and integration within the existing Kraft process. Revamping the Kraft process into a lignin-based biorefinery requires a deep understanding of the links between the new technology and the specific existing process and the implications at the level of resource allocation and management. Therefore, in the first stage, an analysis platform has been considered for the Kraft process as well as for the biorefinery technology (Fig. 3). This stage involved data collection and screening and the assembling of knowledge from all these production systems, along with identification of their resource and environmental impact issues. The characterization of system interactions was performed through a sensitivity analysis. Consequently, representative correlations have been derived to elaborate a simplified model for lignin-based scenario evaluation. The aim of the proposed assessment is to examine and evaluate the trade-offs that are hidden beneath the multitude of potential scenarios and to screen 26 J-FOR Journal of Science & Technology for Forest Products and Processes: VOL.1, NO.3, 2011
SPECIAL BIOREFINERY ISSUE only the most economically and environmentally attractive lignin-based biorefinery production and processing pathways. The general L2PA framework includes the Cadsim Plus Kraft simulation platform as well as the Aspen Plus acid precipitation simulation platform to generate the data and system knowledge required to develop the correlations to be used in the Excel-based simplified business-case evaluation platform shown in Fig. 3. Impact on the Mill Steam System The impact on the steam demand is shown in Table 1 for all cases. Because the trends were found to be similar for all cases, only Fig. 3 - L2PA framework. TABLE 1 Total steam demand increase (%) compared to reference case. Pulp production increase (%) 5 10 15 10 3.8 6.8 8.3 the 10% pulp production increase case is illustrated. Figure 4 shows the steam system variation for a Kraft pulp production increase of 10% and for lignin removal levels from 0 to 50 t/d. The increase in the steam demand compared to the reference case can be explained by two factors: Lignin extracted (t/d) 25 50 6.2 20.6 22.9 10.4 24.7 27.0 a) The increase of the process steam demand is mainly due to an increase in Kraft pulp production and to the dilution effect of the black liquor entering the evaporation train. This dilution effect comes from the recirculation of a substantial flow of washing filtrate (273 1382 t/d, or 12.4 t/t lignin extracted) from the acid precipitation process; b) Steam production losses in the recovery boiler are due to the high impact of lignin extraction on the higher heating value. Lignin extraction has a considerable impact on the higher heating value of the black liquor, as can be seen in Fig. 5. The black liquor higher heating value in the reference case is equal to 13.79 MJ/kg. For constant black liquor flow rate sent to the recovery boiler and for rates of lignin extraction varying from 10 to 50 t/d, the higher heating value decreased by 5.6%. A recent survey of Canadian Kraft pulp mills enabled the authors to identify an operational limit of the recovery boiler at 12.50 MJ/kg. Below this limit, the recovery boiler cannot operate efficiently: the presence of lesser quantities of organic compounds reduces the heat released during combustion, leading to less steam generation. Therefore, the maximum lignin that could be extracted is 66 t/d. This extraction still has an impact on the steam production flow rate at the recovery boiler because a 4% decrease in steam production occurs compared to the reference case. This can be explained by a significant change in the composition of the black liquor entering the recovery boiler, due mainly to the inorganics contained in the recycled lignin-free filtrate, and will be J-FOR Journal of Science & Technology for Forest Products and Processes: VOL.1, NO.3, 2011 27
described in more detail in the next section. It is also of interest to identify the pieces of equipment in the Kraft mill that are the most affected by the integration of lignin extraction. In this specific case, the evaporator train accounts for more than 30% of the entire steam demand of the mill. In the case of maximum lignin extraction (66 t/d), this percentage increases to 34%. The steam demand for lignin extraction is obviously negligible compared to that of the evaporation train and consists basically of the energy necessary to maintain the acidification reactor at 80 C and to heat the wash water and the chemical reactants. Fig. 4 - Impact on the steam system for 10% pulp production increase. Fig. 5 - Targeting the potential recovery of lignin. Impact of Lignin Extraction on the Inorganics Recirculation Loop In the scope of this preliminary impact assessment of lignin extraction in a Kraft pulp mill, particular attention has been devoted to the filtrate exiting the acid washing section. This filtrate is partly recirculated to the weak black liquor tank upstream of the evaporation train. The large amount of water (between 3.5 and 5 tonnes per tonnes of lignin extracted depending on black liquor composition) used to wash the precipitated lignin generates a large amount of inorganic-loaded filtrate recirculated to the evaporation section. Figure 6 shows that the ratio between carbonates (Na 2 ) and sulphates (Na 2 ) in the black liquor for a solids concentration of approximately 51% (mass) is increasing, which is likely to lead to carbonate build-up with a risk of burkeite formation in the evaporation train. Burkeite (2Na 2 Na 2 ) appears when the critical solids ratio between sodium carbonate and sodium sulphate in the black liquor is reached. Because Na 2 content increases when lignin is extracted, this ratio must be carefully monitored to avoid scaling due to the deposition of crystalline precipitates on the evaporator walls [4]. Consequently, the black liquor directed to the recovery boiler will be more diluted, and its composition will vary, leading to inefficient combustion within the recovery 23 28 J-FOR Journal of Science & Technology for Forest Products and Processes: VOL.1, NO.3, 2011
SPECIAL BIOREFINERY ISSUE boiler. The Na 2 excess will impose a supplementary load on the recausticizing unit, increasing energy demand and production in the lime kiln. PROCESS INTEGRATION OPPORTUNITIES Process integration tools can be applied at two levels: to decrease the chemical reactant demand by integration of chemical recycling loops, and to perform energy benchmarking and analysis to identify potential heat and water recovery options, reduce overall energy and water consumption, and evaluate the impacts of lignin extraction on the energy and water profiles of the whole process. Fig. 6 - Impact of lignin extraction on the inorganics recirculation loop for a 10% increase in pulp production. Fig. 7 - ASPEN Plus block diagram of absorption process. Chemical Recycling Loop Integration Variation in the cost of chemicals such as and H 2 will strongly affect the profitability of the acid precipitation process. An interesting approach to absorb the cost could be its capture from the flue gases of the recovery boilers or the lime kiln. For low pressure and concentration in flue gases, chemical absorption is a relevant separation technique and requires no design modifications to the recovery boiler [5]. Monoethanolamine (MEA) is generally used as an absorbent. MEA desorption for regeneration still represents a high energy demand: 2.9 to 4.5 MJ/kg of [5, 6] are needed. The energy impacts on the system must be determined and thermal process integration ultimately applied to generate lowpressure steam for the desorption unit [5]. The chlorine dioxide generator produces a large quantity of waste liquid containing H 2, which can be recovered and used to wash extracted lignin. Figure 7 shows the ASPEN Plus block diagram used for absorption process assessment. The flue gases are cooled to 40 C and absorbed using MEA in the counter-current absorption column (8 m diameter). The -rich solvent stream is then pre-heated to 70 C using a heat exchanger with the hot existing stream from the desorber unit (5 m diameter). It is then further heated to 125 C to separate the from the solvent in the desorber unit; this operation needs a large amount of energy. In the ASPEN Plus simulation, the extraction rate of was fixed at 85%. Absorption is performed at atmospheric pressure, while desorption occurs at 202.65 kpa and 125 C to avoid amine degradation. The purity of the extracted is 98%. For this case study, Table 2 shows that the stream from the lime kiln is richer than that from the recovery boiler. However, the flow rate is nine times less, although still sufficient to extract 50 t/d of lignin. Obviously, the selection of one of these capture opportunities should be performed based on the trade-off J-FOR Journal of Science & Technology for Forest Products and Processes: VOL.1, NO.3, 2011 29
between the investment cost and the level of extra energy demand to make each of these opportunities viable. Potential Heat and Water Recovery Options The energy profile of the Kraft process with a 10% production increase corresponding to 50 t/d lignin production has been evaluated. The overall steam demand has been increased by 24.7%: 1.2% due to the acid precipitation process itself, 3.7% due to evaporation, 15.8% due to the pulping and drying process, and 4.0% due to the loss of recovery boiler steam production. Consequently, steam-saving projects should be considered to compensate for this energy increase. Wash water pre-heating is the main energy demand in acid precipitation, accounting for 58% of the total energy demand. This load can be reduced through process integration opportunities. A combined energy and water analysis indicated a dual benefit of water reuse for washing and heating purposes. On the one hand, the wash water could be entirely replaced by water recovered from the Kraft pulp mill. Potential sources include vacuum pump water, condensate from the evaporation train, and white-water filtrate coming from the brown-stock washing. On the other hand, energy consumption is diminished because the water sources are hotter. Furthermore, pre-heating of the chemical reactants entering the acid precipitation process will decrease the energy demand of this process. In the evaporation plant, a rearrangement of the evaporation train with different steam and recirculating filtrate injection points should be considered, together with the potential integration of a mechanical vapour compression and preevaporation system for the recycled filtrate TABLE 2 Chemical fractions and fl ow rates as available in Kraft process. Chemicals from the recovery boiler from the lime kiln Available fraction (%) 22.3 35.6 Flow Rate (kg/s) 14.7 1.7 from the acid precipitation process. In the same vein, heat-exchanger network retrofit projects to improve the Kraft mill heat recovery system will be considered to avoid the use of fossil fuel. Waste heat recovery and upgrading projects will complement and improve the biorefinery energy efficiency system. All these possibilities are of critical importance for the selection of the most promising production paths. The associated trade-offs, both economic and environmental, will be accounted for. Opportunities to reduce water and energy demands have been investigated as a move towards an advanced process integration methodology. Preliminary assessment of those opportunities led to 16 MW steam savings and 1.8 Mm 3 /y water reduction, which compensate for the extra steam and water demands incurred when increasing pulp production by up to 15%. CONCLUSIONS Lignin extraction can be a profitable solution to debottleneck the recovery boiler in the context of a pulp production increase. The interaction between the Kraft process and the acid precipitation of lignin has been explored for a large production range as a move towards a novel L2PA approach. A systematic identification of the relations between production, energy, water, and chemical recovery systems and their integration potential has been proposed. It has been demonstrated that integration of acid precipitation within a pulp production process has a significant impact on the steam system demand, which may increase up to 27% for the highest rate of lignin extraction and removal. This increase could be compensated for by internal heat recovery within the Kraft process, water reuse, and chemical recovery. ACKNOWLEDGEMENTS The authors acknowledge the financial support provided by the Program on Energy Research and Development of Natural Resources Canada. REFERENCES 1. 2. 3. 4. 5. 6. Lindgren, K., Potential Lignin Applications Beyond Energy, Proceedings, 2nd Nordic Wood Biorefinery Conference, Helsinki, Finland (2009). Davy, M., Uloth, V., and Cloutier, J., Economic Evaluation of Black Liquor Treatment Processes for Incremental Kraft Pulp Production, Pulp and Paper Canada, 99(2): 35 39 (1998). Loufti, H. and Blackwell, B., Lignin Recovery from Kraft Black Liquor: Preliminary Process Design, Tappi Journal 74(1): 203 210 (1991). http://www.tappi.org/ Downloads/unsorted/UNTITLED--- 91jan203pdf.aspx Adams, T., Sodium Salt scaling in Black Liquor Evaporators and Concentrators, TAPPI Journal 84(6):1-18 (2001). http://www.tappi.org/downloads/ unsorted/untitled--- 01JUN70pdf.aspx. Hektor, E. and Berntsson, T., Future CO2 Removal from Pulp Mills Process Integration Consequences, Energy Conversion and Management 48: 3025-3033 (2007). Möllersten, K., Gao, L., and Yan, J., Efficient Energy Systems with CO2 Capture and Storage from Renewable Biomass in Pulp and Paper Mills, Renewable Energy 29(9):1583-1598 (2004). 23 30 J-FOR Journal of Science & Technology for Forest Products and Processes: VOL.1, NO.3, 2011