System analysis of pulping process coupled with supercritical water gasification of black liquor for combined hydrogen, heat and power production

Accepted Manuscript System analysis of pulping process coupled with supercritical water gasification of black liquor for combined hydrogen, heat and power production

Changqing Cao, Liejin Guo, Hui Jin, Wen Cao, Yi Jia, Xiangdong Yao PII:

S0360-5442(17)30860-5

DOI:

10.1016/j.energy.2017.05.104

Reference:

EGY 10912

To appear in:

Energy

Received Date:

04 August 2016

Revised Date:

25 April 2017

Accepted Date:

15 May 2017

Please cite this article as: Changqing Cao, Liejin Guo, Hui Jin, Wen Cao, Yi Jia, Xiangdong Yao, System analysis of pulping process coupled with supercritical water gasification of black liquor for combined hydrogen, heat and power production, Energy (2017), doi: 10.1016/j.energy.2017.05.104

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ACCEPTED MANUSCRIPT Highlights： -The integrated system of pulping process and SCWG of black liquor was simulated -The separator parameters were optimized by calculating the phase equilibrium -Both process using air and oxygen as oxidant in gas combustion were compared -The mass and energy flow of the integrated system was obtained -The total energy consumption of pulp production in the form of kgce was obtained

ACCEPTED MANUSCRIPT

System analysis of pulping process coupled with supercritical water gasification of black liquor for combined hydrogen, heat and power production Changqing Cao1,2,*1, Liejin Guo1,*, Hui Jin1, Wen Cao1, Yi Jia2, Xiangdong Yao1,2 1 State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an 710049, China 2 Queensland Micro- and Nanotechnology Center, School of Natural Sciences, Griffith University, Brisbane 4111, Australia

Abstract: Supercritical water gasification is an innovative black liquor treatment method for hydrogen production. In the present study, an integrated system of pulping and SCWG of black liquor was simulated. Combined hydrogen, power, MP and LP steam are produced for pulping process. The gas product after H2 extraction was burned with imported natural gas to supply more heat. For a reference pulp mill producing 1000 ADt pulp/day, potentially 37126 Nm3/h hydrogen can be produced. The generated MP and LP steam can fully meet the requirement of pulping process. Using air as oxidant in gas combustion is more energy-efficient than using oxygen for being free of oxygen production process. In the case of using air, 22604 kW power can be exported after balancing the consumptions and 219 kgce energy can be produced with 1t pulp production. While using oxygen, 10723 kW power needs be imported and 288 kgce energy can be consumed to produce 1t pulp. However, using air as oxidant may bring N2 and NOx in the exhaust gas, posing a challenge to the subsequent processing. Scaling-up of the system improved the energy efficiency, but the influence is very small when the capacity is above 250ADt/day. Keywords: supercritical water; black liquor; hydrogen; heat and power production; system integration

*1 Corresponding authors. Changqing Cao: Tel: +86 2982660996; Email: [email protected]; Liejin Guo: Tel: +86 2982669033; Email: [email protected] 1 / 19

ACCEPTED MANUSCRIPT Nomenclatures SCWG

supercritical water gasification

SCW

supercritical water

COD

chemical oxygen demand

ASU

air separating units

LP

low-pressure steam

BL

black liquor

MP

medium-pressure steam

NG

natural gas

PSA

pressure Swing Adsorption

OXI

oxidant

ADt

air-dried tonnes

COMP

compressor

LHV

lower heating value

SEP

gas-liquid separator

HX

heat exchanger

Oxy

oxygen

PR-BM

Peng-Robinson equation of state with

ce

standard coal equivalent (heating

Boston-Mathias alpha function IAPWS

value=29307kJ/kg)

International Association for the

SRK

Properties of Water and Steam

Soave–Redlich–Kwong equation of state

Introduction With the depletion of fossil fuel and pollution of environment, more attention is attracted to the clean conversion of the renewable energy resource. Biomass is a kind of abundant renewable energy resource widely distributed around the world. Its clean conversion to energy can play an important role in solving the abovementioned problems. SCWG is an innovative biomass treatment technology developed in recent decades. This technology utilized the unique physic-chemical properties of supercritical water (above 22.1MPa and 374 oC) to effectively convert biomass or organic wastes into hydrogen-rich gases. SCWG has several advantages over the conventional treatment methods, including combustion, pyrolysis and conventional gasification. For example, relatively lower gasification temperature is needed for complete gasification and the energy-intensive drying process is not needed for the wet biomass or organic wastewater. Additionally, most of the organic compounds and the gas product can dissolve in supercritical water to form a homogeneous reaction environment, which favors the reactions. As a result, SCWG attracted much attention recently and its research development states can be found in some recent reviews [1-6]. Black liquor is kind of organic wastewater generated in pulping process, which mainly contains 2 / 19

ACCEPTED MANUSCRIPT organics from raw material and the inorganic chemicals used in pulping process. About 2×108 t of black liquor solid with a fuel value of 2.4×1015 J was generated annually in the world and the amount is still growing with the development of society [7]. Nowadays, most black liquor is combusted in Tomlinson recovery boiler for energy recovery. Meanwhile, the cooking chemicals are also recovered and reused in the pulping process. Additionally, gasification is also an alternative treatment method for black liquor, which can generate electricity and biofuels. Several researchers have performed the system analysis and comparison of these treatment methods and found that gasification was an attractive option to treat black liquor [8-15]. SCWG can also be used to recover the energy in black liquor by producing hydrogen-rich gases. Compared with Tomlinson recovery method and conventional gasification, SCWG has several advantages in black liquor treatment. For example, the weak black liquor (10-20 wt% dry solids) should be concentrated to a solid content of 75-80 wt% before being combusted in the boiler [14]. While in SCWG, weak black liquor can be directly gasified and save plenty of energy for concentrating. Besides, the melting of alkalis in black liquor at high temperatures in the recovery boiler may cause some safety problems [7, 16], but the melting can be avoided for the lower temperature of SCWG. Moreover, alkalis are proved to be an effective catalyst in SCWG, which can accelerate water-gas shift reaction and promote biomass conversion instead [17-19]. Additionally, some hazardous emissions generated during combustion, such as NOx, SOx and fine particles are not generated in SCWG, which eliminate their processing cost. Therefore, SCWG is considered to be a promising treatment method of black liquor. Sricharoenchaikul [20] assessed SCWG of black liquor in a quartz capillary reactor and studied the influence of the operating parameters. The maximum carbon conversion of 84.8% was achieved at 650oC for 120 seconds. Boucard et al. [21] investigated the catalytic effect of Nano-CeO2 on sub- and supercritical water gasification of black liquor at 350~450oC. The presence of Nano-CeO2 was found to be able to improve the conversion of black liquor and reduce the coke formation. We previously studied SCWG of black liquor at 400~600oC, with residence times ranging from 4.94 to 13.71 s in a continuous tubular reactor [22]. Maximum COD removal (88.69%) was obtained at 600oC and the alkalis in black liquor are proved to play a role in catalyzing the gasification. We also performed the 3 / 19

ACCEPTED MANUSCRIPT kinetic study of SCWG of black liquor and achieved its kinetic equation. Recently, we achieved higher gasification efficiency in SCWG of wheat straw black liquor at high temperatures (600750oC) and long reaction times (10-50 min) in a batch reactor. Maximum carbon conversion of 94.1% was achieved at 750 oC when gasifying black liquor with a concentration of 9.5wt% [23]. These studies showed that SCWG can produce hydrogen from black liquor and reduce its pollution simultaneously. The system analysis of SCWG of black liquor integrated with pulping process can help to evaluate the energy consequence and provide reference for processing design in its industrialization. Some researchers have performed system analysis of SCWG of biomass or its model compounds previously, such as glycerol and microalgae [24-27]. For the complex composition, the process of SCWG of black liquor will be different especially when it is integrated with the pulping process. This is because pulping is a complex process involving several procedures, including raw materials preparation, pulping, bleaching, chemical recovery and pulp drying. Different types of energy are consumed in this process, such as power, medium-pressure (MP) and low-pressure (LP) steam. And different materials are also involved, including water, pulping chemicals and raw materials. On the other hand, SCWG of black liquor can also produce other combustible gases except H2, including CH4 and CO, which can be combusted to provide heat. Besides, the hot compressed products from SCWG can also be recovered by generating power and steam. Therefore, the integrated system can be properly arranged to supply different types of energy, including hydrogen, power, LP and MP steam to the pulping process and reduce the energy consumption of pulp production. To our knowledge, there is little information published on the system analysis of pulping process coupled with SCWG of black liquor. In the present study, we constructed an integrated system by considering the energy consumptions in pulping process and energy productions in SCWG of black liquor and simulate it with Aspen Plus based on a typical modern pulp mill. In this system, the energy in hot-pressured product of SCWG was recovered to generate power in a steam turbine and generate steam in heat exchangers. High-purity hydrogen is produced as the main product by H2 purification in PSA (Pressure Swing Adsorption). Other gas product was burned with imported natural gas in a furnace to supply more heat. Based on the analysis, the mass and energy flows of 4 / 19

ACCEPTED MANUSCRIPT this integrated process and the total energy consumption of pulp production were obtained. 1.

Methods and assumptions

1.1 The reference pulp mill A pulp mill with a capacity of 1000 air-dried tonnes (ADt) pulp production per day is used for calculation, which is in accordance with the modern pulp industry [8, 10, 11, 28, 29]. According to the literatures [8, 29] , about 7t weak black liquor (concentration=10~20 wt%) can be produced from 1 ADt pulp production, so it is assumed that 7000 t black liquor with a concentration of 15% is generated per day. The average running time was assumed to be 20 hours per day, so the production of air-dried pulp and black liquor solid are 50 and 52.5 t/h respectively. The heating value of the black liquor solid is assumed to be 11.3 MJ/kg as measured in our previous study [22]. In the pulping process, a large amount of MP steam (1.25MPa, 205 oC), LP steam (0.41MPa, 145 oC) and electric power are consumed. They are mainly used in the process of material preparation, cooking, caustification, pulp bleaching and pulp drying et al. It is worth noting that the energy consumption in black liquor evaporation was not included here because the weak black liquor can be directly gasified in SCW without concentration. The detailed energy consumptions of pulping process and the data of the reference mill are given in Table 1. 2.2 The system description A process of SCWG of black liquor with consideration of providing energy for pulping process and hydrogen production was constructed and simulated with Aspen Plus (Fig. 1). In this process, black liquor was mixed with preheated water before the reactor. Thus fast heating of black liquor can be realized, which can inhibit the side reactions occurring under lower temperatures and benefit the gasification [30]. The energy of the hot-compressed product, mainly including SCW and gas product, was recovered by a steam turbine to generate power. Then the energy was further recovered through a series of heat exchangers (HX1~3). Among them, HX1 was used to preheat the reactant (black liquor and preheated water), and HX2~3 were used to generate MP and LP steam for the pulping process. In HX3, the product was cooled to a certain temperature and separated into liquid and gas products in a separator (SEP). The targeted temperature of the separator was controlled by adjusting 5 / 19

ACCEPTED MANUSCRIPT the flowrate of stream LP-S and determined by evaluating the influence of temperature on the separating performance (Section 3.2). The liquid product of the separator mainly containing water can be recycled in SCWG and pulping process. The gas products were further purified in a PSA equipment for high-purity hydrogen production. The off-gas from PSA after H2 extraction was burned in a furnace (BURNER) for energy recovery. To fulfill the energy requirement of the pulping and SCWG process, a certain amount of the imported natural gas was also burned in the furnace. The generated heat of the furnace was mainly used to heat SCWG reactor and preheater. Then it was used to preheat the fuel and oxidant in HX4 and generate LP steam in HX5. In a word, hydrogen was produced as the main product, and LP steam, MP steam and power can also be generated for the pulping process in this system. For black liquor is an unconventional material for Aspen Plus software, the thermodynamic analysis of SCWG cannot be directly performed in a RGIBBS reactor. To solve this problem, we introduced a fictitious flow sheet to simulate the SCWG reactor (Fig. 1b). In this process, black liquor was converted into elementary components (C, H2 and O2) with a yield-type reactor (DECOMP). Then a separator was introduced to separate the ash generated in the gasification, and other components were fed into the RGIBBS reactor for thermodynamic analysis. The details of the thermodynamic study can be found in our previous study [31]. 2.3 Methods and assumptions There is no available data of the properties of black liquor in the software, so its characteristic data needs to be input to estimate its thermo-physical properties. The data used here were the measured value of the black liquor in our previous study [22]. The main parameters of black liquor solid, including the proximate and ultimate analyzing results are given in Table 2. Thermodynamic equilibrium of SCWG of black liquor was calculated in RGIBBS reactor in Aspen Plus using Gibbs free energy minimization method. In this study, PR-BM property method (Peng-Robinson equation of state with Boston-Mathias alpha function) was adopted for calculation. This method has been proved to be convincible in thermodynamic analysis of SCWG by several researchers [31-33]. The possible products are assumed to be H2, CO, CH4, CO2, C2H4 and C2H6 according to the composition of black liquor and SCWG experimental results [22]. Nitrogen and 6 / 19

ACCEPTED MANUSCRIPT sulfur elements are omitted in the gas product for their relatively low content. The cooking chemicals (mainly alkalis) in black liquor can be separated using the low solubility of inorganic salts in SCW [34-36] and recycled in pulping process. The phase equilibrium in the gas-liquid separator was modeled using an Aspen flash reactor mode. The calculation was based on the Property method of UNIF-DMD, in which the fugacity and activity coefficient of liquid phase were calculated by UNIFAC model using group interaction parameters extracted from the Dortmund data bank. And the fugacity coefficient of gas phase was calculated through Soave–Redlich–Kwong (SRK) equation of state. The similar method was used in our previous study, where the calculation is in good agreement with the experimental results [37]. The properties of water streams were derived from the data released by International Association for the Properties of Water and Steam (IAPWS) in 1997 [38]. 2.

Results and discussion

2.1 Process simulation of SCWG of black liquor The operating pressure was set to be 25MPa in SCWG according to our experimental experience. Water and black liquor were pumped to this pressure by high pressure piston pumps. The efficiency of this type of pump is in the range of 0.85~0.90 [39, 40]. Considering different properties of the streams, different efficiencies were assumed for both pumps. The efficiency for water pump is assumed to be 0.90, while that for black liquor is assumed to be 0.85. In the case of black liquor with high viscosity, some measures should be employed to realize its feeding. For example, we used a piston-type pressure vessel in our previous studies [41, 42] and Antal et al. [43] suggested cement pump to realize the feeding of biomass slurry, which will lower the pumping efficiency to some extent. The exit pressure of the turbine is set to be 3 MPa to match up the demand of the PSA unit in the downstream. Under this pressure, high-purity of hydrogen (99.996%) can be achieved from the gas product by PSA and the recovery rate of hydrogen can reach up to 80% [11, 44]. Thus, setting the exit pressure of the turbine at 3MPa can fulfill the pressure requirement of PSA and energy consumption for gas compression can be saved. When the pressure difference was set, the power produced from turbine was calculated by assuming the mechanical and isentropic efficiencies were 7 / 19

ACCEPTED MANUSCRIPT 0.8 and 0.9 respectively as recommended [40, 45]. The exit temperature of the turbine was calculated to be 409.7oC with the exit pressure set at 3 MPa. Under this condition, the vapor fraction of the water running throughout the turbine is kept at 1 and no condensate water existed, which can make sure of safe operation of the turbine. The product flow from the turbine outlet was used to preheat black liquor and water in HX1 before feeding into the reactor. The targeted temperature of black liquor is 300oC. Most reactions have not started at this temperature, so the generation of the intermediates which can negatively affect the gasification can be suppressed. Then the product was further cooled by heat exchangers HX2 and HX3, which can also help to realize the product separation in the gas-liquid separator. The targeted temperature of the product after HX3 was determined to be 50oC by evaluating the temperature dependence of the separating performance (Section 3.2). The gas product after H2 extraction in PSA including CH4, CO and unrecovered H2, along with imported natural gas were burned in a furnace to supply heat. The amount of natural gas was determined under an assumption that the furnace exit temperature was kept at 1000oC. Thus SCWG reactor and preheater can be heated to the reaction temperature (700oC). The energy flow QREACT and QPRE in Fig. 1 refer to the heat consumptions in the reactor and preheater respectively. The amount of oxidant was determined by the consumption with an excess of 5% to make sure of the complete combustion. The oxygen and imported natural gas was fed by compressors, the efficiencies of which were assumed to be 0.72 as recommended [40, 45]. The energy of the furnace flue gas was recovered in HX4 and HX5 before being discharged. And the discharging temperature was set to a typical value of stack temperature at 160oC to reduce corrosion [9, 46]. The process was simulated under the abovementioned assumptions, and the results were summarized in Table 3. The temperature, pressure, total flow rate and component flow rates of the main streams in the flow sheet were presented. The stream numbers refer to the flow sheet shown in Fig.1. It needs be mentioned that oxygen was used as the oxidant in the furnace in this case. Additionally, we also simulated a similar process with air as the oxidant instead of oxygen for comparison. It is notable that the energy consumption of ASU (Air Separation Units) for oxygen production was considered in comparing their feasibilities, which will be described below. 8 / 19

ACCEPTED MANUSCRIPT 3.2 Parameters evaluation of the separator To determine the operating parameters of the gas-liquid separator, both the influence of parameters on the separating performance and the constraint of the up- and down-stream equipment were considered. As mentioned above, the needed operating pressure of PSA was set at 3MPa, so the operating pressure of the upstream separator set at 3MPa to meet the requirement. Under a fixed pressure, operating temperature is an important parameter for phase equilibrium. We studied the influence of temperature on the composition of the gas phase product and recovery rate of each component (= the gas flow of one component in the gas product / the total flow of this component in the separator). The separator temperature was adjusted by the heat exchange capacity of HX3. Considering the room temperature and the upstream conditions, the studied separator temperatures ranged from 30oC to 180oC. The temperature shows a significant influence on the separating performance (Fig. 2). The flow rate of each component in the gas product increased with temperature. This result leads to the increase of their recovery rate, especially CO2, whose recovery rate increased from 85.94% at 30oC to 98.65% at 180oC. The recovery rate of other gases (H2, CO and CH4) increased slightly with temperature. It is worth noting that the water content in the gas phase increased more significantly than other components. Especially when the temperature was above 110oC, the water content in the gas phase increased more sharply and reached up to 17.64% at 180oC. High water content in the gas phase may negatively impact water recycling. And it can also bring challenges to the downstream operations, such as hydrogen purification in PSA and combustion in the furnace. As a result, lower temperature was preferred for the separator from the point of view of separating performance. On the other hand, the separator temperature should coordinate with up-stream equipment and operations. In this process, the separator temperature is controlled by the heat exchange capacity of HX3. Different separator temperature can be obtained by adjusting the flow rate of the stream LPS. The temperature of stream LP-S is assumed to be 30oC (room temperature) in this study, so the separator temperature should be above 30oC. And their temperature difference will determine the heat exchange area and construction cost of HX3. For the heat exchanger running in ambient pressure, a minimum temperature difference of 10oC was usually used [47]. Considering the higher 9 / 19

ACCEPTED MANUSCRIPT construction cost corresponding to high pressure of HX3, a higher temperature difference of 20oC was assumed to reduce the needed heat exchange area. Therefore the separator temperature was set to be 50oC. At this temperature, the recovery rate of H2, CH4, CO and CO2 are 99.91%, 98.94%, 99.42% and 90.57% respectively, and the water content in the gas phase is lower than 0.5%. This result ensured that most of the combustible gas product were recovered in the gas phase with a low moisture content. 3.3 The comparison of oxygen and air as oxidant For gas combustion in the furnace, both air and oxygen can be used as oxidant. Compared with using air, the energy consumption of oxygen production needed to be considered in the case of using oxygen. Nowadays, ASU is widely used to produce oxygen in industry, and the power consumption is about 0.6 kWh/Nm3 oxygen production [11]. As a result, replacing oxygen with air can save the energy consumption of ASU. On the other hand, air also contains a large amount of nitrogen. In this study, air is assumed to compose of 21% oxygen and 79% nitrogen. Thus using air instead of oxygen will increase the flow rate and the power consumption of the compressor. We simulated the SCWG processes with air and oxygen as the oxidant respectively and compared the results of (Table 4). It shows that the usage of air instead of oxygen increased the power consumption of the compressor from 1438 to 7692kW, but it saved the power consumed in oxygen production (73007kW). As a result, the usage of air instead of oxygen lowered the total power consumption. In the aspect of steam generation, SCWG of black liquor in both cases produced enough MP steam (53053kg/h) for pulping process. However, more LP steam (506580 kg/h) was generated in the case of using air than using oxygen (434021 kg/h). These result indicated that using air as oxidant is more energy-efficient than using oxygen. However, when air is used as oxidant, the introduced N2 may bring challenges and difficulties to the tail gas processing. Some nitrogen oxide (NOx) may be generated in gas combustion at such high temperature. Thus a subsequent eliminating process of NOx is essential before the tail gas being discharged. This can improve the energy consumption and the system investment. It needs be mentioned that the content of NOx was not calculated here because its generation is not our main concern in this study. We assumed that N2 was inert in the furnace and found that the fraction of N2 10 / 19

ACCEPTED MANUSCRIPT is more than 2 times of the CO2 (Table 2). Even this result can also increase difficulties and cost for CO2 capture and storage. As a result, both the energy efficiency and the tail gas processing should be considered to choose the suitable oxidant for gas combustion. 3.4 The energy and mass flow of the integrated process In the process of SCWG of black liquor, not only hydrogen was produced, but electric power, MP and LP steam were also generated for pulping process to reduce the energy consumption of pulp production. For the reference pulp mill, the potential hydrogen (purity= 99.996%) production is 37126 Nm3/h. SCWG of black liquor also generated power from the hot-compressed product through a turbine. The power can fully meet the requirement of SCWG system, including the consumptions of feeding pumps, compressor and ASU when using oxygen as the oxidant. There is also an extra power of 15858 kW can be supplied to the pulping process. For totally 26581 kW power is needed by the pulping process, 10723 kW power should be imported to meet the requirement. SCWG of black liquor can also produce enough MP and LP steam to meet the requirement of the pulping process. And an LP steam surplus can be exported for residential heating supply of the mill and the surrounding urban area. The amount of LP steam that can be exported is different when oxygen and air are used as the oxidant respectively. A surplus steam of 361575 kg/h can be exported in the case of using oxygen as the oxidant while it reached 434134 kg/h in the case of using air, which are about 5 and 6 times of the required LP steam by the pulping process respectively. Given the above results, the energy and mass flow diagram of the integrated system of pulping process with SCWG of black liquor can be obtained. Here we took the case of using oxygen as the oxidant for example to discuss (Fig. 3). To keep the running of integrated system, an external power of 10723 kW and 50813 Nm3/h natural gas need be imported. The generated MP and LP steams from SCWG of black liquor can fully meet the requirement of the pulping process, and a surplus LP steam of 361575 kg/h can be exported for residential heat supply from this process. On the mass flow, the main product is 37126 Nm3/h high-purity hydrogen. Additionally, alkalis in black liquor can also be recovered through their low solubility in supercritical water. They are probably in the forms of Na2CO3 or NaHCO3 because NaOH can react with CO2 generated from 11 / 19

ACCEPTED MANUSCRIPT SCWG [35]. Therefore they need to be transformed into NaOH through caustification process before being reused in the pulping process. The alkalis recycling can reduce the pulp cost significantly because the cost of alkali is a significant component of the total pulp-production cost. On the other hand, the liquid product of SCWG mainly contains water, which can be recycled in the pulping process. For example, 620175 kg/h of water can be produced from the gas-liquid separator. They can be used in SCWG as the preheated water (35000 kg/h) and in pulping process. The water recycling can reduce the water consumption of the pulp mill. If the water contained in the tail gas can be collected after condensation, consumption of water resource can further be lowered. 3.5 The total energy consumption of pulp production The energy consumptions of pulp production of the studied pulp mill coupled with SCWG of black liquor were summarized (Table 5). Five types of energy were involved in this process, including power, MP steam, LP steam, natural gas and hydrogen. They are different in energy quality, so the energy consumption cannot be summed up and compared directly. To solve this problem, we unified them into standard coal equivalent (ce), which refers to the coal with a heating value of 29307kJ/kg. For example, 1 kW.h of power equals to 3.6×106 kJ of energy, which equals to 0.1228 kgce quantitively. However, the current level of electricity generated from coal should be considered in its equivalent ratio to standard coal equivalent. Referring to literature [48], 1 kW.h of power is equivalent to 0.404 kgce when considering the energy efficiency of the coal-fired power plant. The coefficients of MP and LP steam are 0.09649 kgce/kg MP and 0.09360 kgce/kg LP respectively according to the Chinese national standard on energy calculation in pulp and paper mill [49]. To our knowledge, there is no available equivalent ratio of hydrogen to standard coal in the literature. In this study, we calculated its equivalent ratio based on the energy consumption of hydrogen production process. Currently, the most widely-used hydrogen production method is steam reforming of natural gas, in which the energy consumption is about 1.25~1.35 GJ/GJ-H2 [50]. Taking the mean value (1.3 GJ/GJ-H2) for reference, its equivalent ratio is calculated to be 0.4792 kgce/Nm3 H2. With above equivalent ratios of different energy types, the energy consumptions of the pulping 12 / 19

ACCEPTED MANUSCRIPT process and that coupled with SCWG can be unified and compared (Fig. 4). The total consumed energy in pulping process is 22639 kgce/h, containing power, LP and MP steam. Coupling with SCWG of black liquor introduced a large amount of energy consumption in the form of natural gas, but it produced electric power, MP and LP steam and lowered the total energy consumption. Additionally, the generation of hydrogen as a clean fuel can further reduce the energy consumption. The total energy consumption of the integrated process was 14400 kgce/h in the case of using oxygen as the oxidant. Using air as the oxidant reduced the power consumption and improved the LP steam production. Thus the total energy consumption was further reduced, and an extra energy of 11074 kgce/h can be exported from the pulp mill in this case. The capacity of the studied pulp mill is 50 ADt pulp/h, so the comprehensive energy consumption is calculated to be 452.78 kgce/ADt pulp. It was significantly reduced when coupling with SCWG of black liquor. In the case of using oxygen as oxidant, the energy consumption is 288.01 kgce/ADt pulp production. This is lower than the best energy consumptions level of China for pulp production in 2015(370 kgce/ADt) [51]. While using air as the oxidant, the energy consumption was −221.48 kgce/ADt. That is to say, a surplus energy of 221.48 kgce can be produced corresponding to 1ADt pulp production. The low energy consumptions when coupling with SCWG of black liquor can be attributed to the efficient energy conversion of black liquor. In this process, the waste black liquor was converted into high quality of energy, including power and high-purity hydrogen. The lower energy consumption than current levels may be related to the advantages of SCWG on treating black liquor over traditional methods, including Tomlinson recovery method and conventional gasification. In these traditional methods, the energy-intensive evaporation is essential because of the high moisture content of black liquor. Thus a lot of energy will be wasted and lower the energy efficiency. It is reported that about 37% of the energy consumed in a conventional pulping mill can be attributed to the evaporation of black liquor [8]. As a result, SCWG is more efficient in energy conversion of black liquor over the traditional treatment methods for being free of the energy consumption in black liquor concentration.

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ACCEPTED MANUSCRIPT 3.6 Effect of scale on the energy efficiency of the system To study the scaling effect, we calculated the consumed and produced energy of the proposed system for different-scale pulp mills. The capacities of the studied pulp mills ranged from 10 to 2000 ADt/day, which covers most of the pulp mills in the world. For different-scale pulp mills, the available black liquor as well as the energy consumed, including the power, MP and LP steam were assumed to be linearly associated with the scale. With these estimated data, the consumed and released energy was calculated through the similar simulation method as described before (Fig. 5). Most parameters and setups for simulation were the same for different-scale systems except the efficiency of the steam turbine. The rated power of the needed turbines were estimated to be about 1, 10, 25, 50, 100, 150 and 200 MW respectively when the capacities of the pulp mills were 10, 100, 250, 500, 1000, 1500 and 2000 ADt/day. According to the literatures, the relative internal efficiency of them are different corresponding to different rated powers. From current status of steam turbine in China [52], their relative internal efficiencies were estimated to be 78.0%, 81.33%, 85.5%, 85.9%, 86.1%, 87.3% and 88.5% respectively. The energy efficiency of the system with different scales were also estimated to show the scaling effects and the feasibility of this system. Referring to literature [53], the overall energy efficiency was defined as the ratio of the useful energy output divided by the useful energy input of the system. In this system, the useful energy output included the power generated by the system (W), the required heat to generate MP and LP steam (QLP and QMP), and the heating value of the produced hydrogen (LHVH2). And the useful energy input included the heating value of black liquor (LHVBL) and the imported natural gas (LHVNG). It can be determined by the following equation: ηsys =

Useful Energy Output W  Q MP  Q LP1  Q LP 2  LHVH 2 = Useful Energy Input LHVBL  LHVNG

Where W  WT - (WP1  WP 2 )  (WC1  WC 2 )

QMP  m MP hMP  m MP  S hMP  S QLP1  m LP hLP  m LP  S hLP  S QLP 2  m LP 2 hLP 2  m LP  S 2 hLP  S 2 14 / 19

(1)

ACCEPTED MANUSCRIPT The overall energy efficiencies of the system with the studied scales were calculated to be in the range of 83.76~83.78% (Fig. 5), indicating that most of the energy contained in black liquor was recovered through this integrated system. The energy loss is about 16.2%, which can be attributed to the energy loss in the reactor and preheater heating, the energy loss in the turbine, compressors and pumps as well as the energy loss for relatively high temperature of the exhaust gas (160 oC). The heat recovery of the exhaust gas can further improve the energy efficiency. The relatively high energy efficiency can be attributed to the integrated process, where combined power, heat and hydrogen can be produced. These energy efficiencies are close to those obtained by Berglin et al in simulating the combined heat and power production from conventional black liquor gasification [9]. On the other hand, the scale of the system affected the overall energy efficiencies, which increased with the system scale. Especially at the system scales below 250 ADt/day, the scale had a greater impact on the energy efficiency. While the system scale was larger than 250 ADt/day, the impact was relatively small. The small impact of the scales can also be attributed to the integrated system, which can not only produce power but also heat and hydrogen. The generated power only accounts for under 15% of the total produced energy of the system, so the overall efficiency had a very slight change (about 0.02%) even the relatively internal efficiency of the turbine had a change over 10%. In other words, the combined production system reduced the impact of the steam turbine efficiency and the influence of the system scales. Considering other parameters may also influence the impact of the scaling effect on the energy efficiencies of the energy systems. Dornburg et al. [54] studied the scaling effects on the energy efficiency of the biomass-based energy systems. They found that the system scale influenced the overall energy efficiencies, mainly by changing of the logistics and the heat distribution distance. Similarly, this system is also a distributed energy system, so the scaling up of the pulp mill will increase the distribution of the raw biomass collection and the heat utilization. Therefore, it will change the energy consumed in the logistics of biomass and the energy loss during the heat distribution. A whole life cycle analysis of the integrated system in the future may help to understand the impact of the system scales more thoroughly.

15 / 19

ACCEPTED MANUSCRIPT 3.

Conclusion: An integrated system of pulping process and SCWG of black liquor was constructed and

simulated. In this system, combined hydrogen, electric power, MP and LP steam were produced and supplied to the pulping process to reduce the energy consumption. A pulp mill with a capacity of 1000 ADt pulp /day was chosen as a reference mill in this study. The optimal operating condition (3MPa, 50oC) was obtained for the gas-liquid separator by evaluating its influence on the separating performance. It is also found that using air as the oxidant for gas combustion instead of using oxygen can lower the total power consumption for being free of the oxygen production process (ASU). However, the usage of air brings challenges to subsequent gas processing for introducing Ncontained compounds in the tail gas. From the simulation, 37126.41 Nm3/h high-purity hydrogen can be produced potentially in this pulp mill. Additionally, SCWG of black liquor can also generate enough MP steam (53053 kg/h) and LP steam (72446 kg/h) to fully meet the requirement of the pulping process and a surplus LP steam can be exported for residential heat supply. By coupling with SCWG of black liquor, the comprehensive energy consumptions of pulp production are found to be 288.01 kgce/ADt pulp when oxygen was used as the oxidant. While in the case of using air as the oxidant, an energy surplus of 221.48 kgce can be produced corresponding to 1ADt pulp production. The energy consumption for pulp production is lower than current level, which shows the advantages of SCWG on black liquor treatment over traditional treatment methods. The scalingup of the system can increase the energy efficiency of this system, but the influence was very small when the scale of the reference pulp mill is larger than 250ADt/day. Further optimization of the parameters and process may further improve the energy efficiency and reduce the energy consumption of pulp production, which will be studied in future research. 4.

Acknowledgement: This work was financially supported by the National Natural Science Foundation of China (No.

51606150), the China National Key Research and Development Plan Project (No. 2016YFB0600100), Australian Research Council (Project No. LP 110100337) and Shaanxi Science & Technology Co-ordination & Innovation Project (Contract No. 2015TZC-G-1-10) Reference: 16 / 19

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ACCEPTED MANUSCRIPT

7

HX2

( A)

W

LP

6

MP

SEP PSA

LI QUI D

2 PUMP2

PW2 TURBI NE

WT

W

4

W

R

W

REACTOR HIERARCHY

FLUE HX4

PREHEAT

9

COMP WC

3

PRODUCT MI XER

OTHERS

OXI

5

W

H2

SEP

1

PUMP

PUMP

8

HX1

PUMP1

BL

LP- S

MP- S

PW1 WATER

GAS

HX3

OXI - 1

OXI - 2

NG- 2

NG- 1

COMP2

10

WC2

RSTOIC

11 HX5 LP- S2

LP2

BURNER

Q

NG EXHAUST

ALKALI QPRE

SCW QREACT

6

QREACT( OUT)

( b) R- GI BBS SEP- P R( I N)

R

4

2

P

DECOMP

G- LI QUOR

PRODUCT( OUT)

ALKALI ( OUT) QEXCHANG

Fig. 1 Flow sheet of combined production of hydrogen, heat and power from SCWG of black liquor (a) and the fictitious simulator of SCWG reactor (b)

ACCEPTED MANUSCRIPT

H2 CO H2O

1000

CH4 CO2

100

(a)

600 400

H2 CH4 CO CO2 Water

60 40 20

200 0

(b)

80

800

Recovery rate / %

Gas mole flow in gas phase / Kmol/hr

1200

40

60

80

100

120

140

160

180

0

40

60

80

100

120

140

160

180

o

o

Temperature / C

Temperature / C

Fig. 2 Influence of operating temperature on the separating performance of the gas-liquid separator (Pressure=3 MPa): (a) composition of the gas phase product; (b) recovery rate of each components

MP steam 53053 kg/h Raw materials Pulping

Power 10723 KW

Pulp 1000adt/day

NaOH solution

Pure Hydrogen 37126Nm3/h

Black liquor (15wt%) 7000 t/day

Power 15858KW

CO2 83401 Nm3/h

SCW G o f BL

Oxygen 121678 Nm3/h

Alkali solution

Natural Gas 50813 Nm3/h

LP steam 72446 kg/h

LP steam 361575 kg/h

Caustification

Fig. 3 The energy and mass flow diagram of the pulping process coupled with SCWG of black liquor for the studied pulp mill with a capacity of 1000 ADt/day (oxygen as oxidant)

ACCEPTED MANUSCRIPT

Power

LP steam

MP steam

Hydrogen

Nature Gas

Sum

70000.00 50000.00 30000.00 10000.00 -10000.00 -30000.00 -50000.00 Pulping

Pulping-SCWG(Oxy)

Pulping-SCWG(Air)

Fig. 4 Energy consumption (+) and production (−) of the pulping process integrated with SCWG of black liquor (kgce/h) for the studied pulp mill

1400

80.0000

1200

70.0000

1000 800

Power generation H2 production NG consumed

MP+LP production BL consumed Energy efficiency Energy efficiency, %

Energy input and output, MW

ACCEPTED MANUSCRIPT

60.0000 50.0000 40.0000

600 400 200 0

30.0000 20.0000 10.0000 0.0000 10

100

250 500 System scale, ADt/day

1000

1500

2000

System scale, ADt/day

Fig. 5 the energy input and output as well as the total energy efficiency of the integrated system for different-scale pulp mills

ACCEPTED MANUSCRIPT Table 1 Input data of energy consumptions and the production of black liquor of the reference pulp mill Items

Value

Capacity

1000 ADt pulp/day

Yield of black liquor

7000 t/day

Concentration of black liquor

15%

LHV of the black liquor solids

164.7MW

Electrical power consumption

26581 kW

MP steam (1.25MPa, 205

oC)

consumption

53053 kg/h

LP steam (0.41MPa, 145 oC) consumption

72446 kg/h

Table 2. The property of the studied black liquor air-dried solid

Ultimate analysis (%)

Proximate analysis (%)

C

H

O

N

S

Moisture

Ash

Volatile

FC

33.43

2.77

32.86

0.23

0.13

3.2

27.38

49.32

20.1

ACCEPTED MANUSCRIPT Table 3 Detailed mass flow and conditions of the main streams in the simulation of SCWG of black liquor (ethane and ethylene are not shown for their low content; oxygen is used as the oxidant in the furnace) Stream No.

Temperature

Pressure

Mass flow

oC

MPa

kg/hr

Component flow, kmol/hr H2O

O2

H2

CO

CH4

CO2

1

31.3

25

350000

19428.0

2

34.6

25

350000

16513.8

3

300

25

350000

16513.8

4

300

25

350000

19428.0

5

409.7

3

685436.5

34425.0

2072.2

56.6

83.0

1321.7

6

223.3

3

685436.5

34425.0

2072.2

56.6

83.0

1321.7

7

221

3

685436.5

34425.0

2072.2

56.6

83.0

1321.7

8

50

3

685436.5

34425.0

2072.2

56.6

83.0

1321.7

9

700

0.13

61921

15.8

414.4

56.6

82.9

1315.3

10

358.9

0.13

272132

5132.9

BL

30

0.1

350000

16513.8

EXHAUST

160

0.13

272132

FLUE

1000

0.13

GAS

50

H2

30

LIQUID

493.8

3723.3

5132.9

493.8

3723.3

272132

5132.9

493.8

3723.3

3

65262

15.8

0.13

3341

50

3

620175

34409.3

LP

145

0.41

405981

22535.4

LP-S

30

0.41

405981

22535.4

LP-S2

30

0.41

28040

1556.5

LP2

145

0.41

28040

1556.5

MP

205

1.25

53053

2944.9

MP-S

30

1.25

53053

2944.9

NG

30

0.1

36392

2268.5

NG-1

700

0.13

36392

2268.5

NG-2

56

0.13

36392

2268.5

OTHERS

30

0.13

61921

OXI

30

0.1

173819

5432.0

OXI-1

62.4

0.13

173819

5432.0

OXI-2

700

0.13

173819

5432.0

PRODUCT

700

25

685437

34425.0

R

404.8

25

700000

35941.7

SCW

700

25

350000

19428.0

WATER

30

0.1

350000

19428.0

2071.8

56.6

82.9

1315.3

0.0

0.04

6.4

1657.4 0.37

15.8

414.4

56.6

82.9

1315.3

2072.2

56.6

83.0

1321.7

ACCEPTED MANUSCRIPT Table 4 The energy consumption (−) and production (+) in SCWG of black liquor (oxygen and air as the oxidant respectively) for the pulp mill with a capacity of 1000 ADt/day Power, kW Steam turbine Feeding pumps Compressor of oxidant Compressor of natural gas ASU Total in BL SCWG Steam, kg/h MP steam production LP steam production Gas, Nm3/h CO2 in tail gas N2 Oxidant flow Consumed natural gas Hydrogen

Oxygen as the oxidant

Air as the oxidant

96516 −5618 −1438 −595 −73007 15858

96516 −5618 −7692 −675 0 82530

53053 434021

53053 506580

83401 0 −121678 −50813 37126

90197 513988 −650617 −57610 37126

Table 5 The energy consumption (+) and production (−) of the reference pulp mill (capacity=1000 ADt/day) coupled with SCWG of black liquor Power Pulping process

26581 kW.h kW.h

SCWG(Oxy)

10722

SCWG(Air)

−55949 kW.h

LP steam

MP steam

Natural gas

Hydrogen

72446 kg/h

53053 kg/h

/

/ Nm3/h

−37126 Nm3/h −37126 Nm3/h

−361575 kg/h

0

50813

−434134 kg/h

0

57610 Nm3/h