Simulation Study of Thermochemical Process from Biomass to Higher

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Simulation study of thermochemical process from biomass to higher alcohols Wenwen Guo, Guoneng Li, Youqu Zheng, and Shurong Wang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01687 • Publication Date (Web): 27 Sep 2016 Downloaded from http://pubs.acs.org on October 2, 2016

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Graphical abstract Two core modules were constructed and optimum reaction conditions were found

Methane reforming

Gasification

Alcohol synthesis

Methanol

Pine chips 50 45

Alcohol yield and energy efficiency

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Gas and liquid liquid separator

Ethanol t/100t

Propanol t/100t 40

Very close

35

Gas recycled

30 25

20

Distillation towers

100% Case 1 75%

Processes Case 2(self-powered) simulation 25%

15

10 5 0

case1

case2

Alcohol yield t/100t

case1

case2

Alcohol yield %

case1

case2

Gas and steam combined cycle power unit

Energy efficiency %

/

ACS Paragon Plus Environment

Higher alcohols

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Title page

Simulation study of thermochemical process from biomass to higher alcohols Wenwen Guoa,b, Guoneng Lia,b, Youqu Zhenga,b, Shurong Wangb,*

a

Department of Energy and Environment System Engineering, Zhejiang University of Science and Technology, Hangzhou 310023, China b

State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, China

Corresponding author (Shurong Wang) Tel: +86 571 87952801 Fax: +86 571 87951616 Email address: [email protected]

2


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Simulation study of thermochemical process from biomass to higher alcohols Wenwen Guoa,b, Guoneng Lia,b, Youqu Zhenga,b, Shurong Wangb,*

Abstract: Thermo-chemical conversion from biomass to higher alcohols is a promising route for manufacture of higher alcohols in an environmentally sustainable way. In this study, a production system of biomass gasification and subsequent higher alcohol synthesis was designed and simulated. Two core modules of the processes (biomass gasification and alcohol synthesis) were constructed and the optimum reaction conditions were discussed in detail. The results showed that the lower ER (the equivalence ratio of oxygen/fuel) (ER=0.2) could increase CO content in the gasification product. By changing the mass ratio of steam to biomass (S/B), hydrogen-rich syngas could be obtained for higher alcohol synthesis. For synthesis of higher alcohols from syngas, higher temperature was conducive to CO conversion, but also can increase the content of by-product such as CO2 and CH4. Higher pressure and H2/CO (1.0-2.0) were both in favor of the production of higher alcohols. Then the process simulation was based on two cases, which were mainly divided by the difference in power supply sources. In case 1 part of the electricity demand was provided by external power source while in case 2 the process was devised to be completely self-powered by adding in a gas and steam combined cycle power unit. An assessment on the material flow, energy consumption, energy efficiency and exergy flow of the two cases was made according to the simulation results. The alcohol yield 3


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was 25.1wt% and 19.4wt% for case 1 and case 2 respectively. And the energy efficiencies of the two cases were relatively close to each other (34.1% and 33.2% respectively). About 33.5% exergy input was converted to alcohols in case 2, lower than case 1 (39.6%). The exergy loss in the power generation unit of case 2 was higher, resulting in a higher total exergy loss. For case 2 the electricity demand can be balanced by the system itself without any fossil fuel usage, which was quite attractive and promising in the aspect of environment protection and cost competitiveness. Keywords: Biomass; Thermochemical process simulation; High alcohols; H2-rich syngas; Self-powered process.

1. Introduction As a non-renewable energy resource, oil supply is running out and new reserve becomes harder to find out. It is reported that 60% world oil resource is consumed in transportation. Higher alcohols (C2+OH) are applicable alternative fuels for vehicles. It can be mixed with gasoline by a certain percentage and directly applied in the internal combustion engines without any modification on the engine or fuel system. In addition, higher alcohols are excellent octane enhancer and gasoline oxygenates and they can replace lead and methyl tertiary butyl ether added in gasoline.1 Primarily driven by the several world energy shortages, the use of higher alcohols has been promoted by the United States, Brazil and many other countries. Among the higher alcohols, ethanol is one of most widely used alcohols. Fuel ethanol production of the United States and Brazil accounts for three-quarters of the world’s production.2 4


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Currently two methods have been commonly applied in ethanol synthesis: ethylene hydration and the biological fermentation. In catalytic conversion of ethylene, feedstocks are mainly produced from crude oil. In fermentation, feedstocks can be divided into two categories: starches/sugars and cellulose. Ethanol from starches/sugars is called the first-generation biofuel,3 whose production may push up grain prices and result in food crisis. There are many studies that illustrate the contribution of biofuels to the increase in food prices.4 As an abundant resource, biomass from agriculture and forest residues could be used to produce syngas by steam gasification with high hydrogen content,5-7 followed by the catalytic synthesis of alcohols from syngas. With the advantages of short reaction time, sufficient raw material provision and environment friendliness, the process of biomass to higher alcohols is a promising and green chemical route. Currently biomass gasification is gradually put into use in industrial applications and many attentions have been paid to the development of direct synthesis of higher alcohols from syngas. In previous study, our team has done a lot of work on thermal-chemical conversion from biomass to higher alcohols. We explored various catalytic synthesis routes, catalyst preparation and characterization methods, influences of reaction parameters on alcohol yield, and the lifecycle assessment of synthesis processes.8-10 Limited by the poor performance of synthesis catalysts (low carbon monoxide conversion and low alcohol yield),11 currently synthesis of higher alcohols from syngas has yet to be commercialized. In recent years, many simulation studies concerning biomass combustion,12,13 biomass gasification,14,15 dimethyl ether 5


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synthesis from bio-syngas,16 and liquid fuels synthesis from biomass by thermo-chemical conversion process (such as methanol,17 FT fuels,18 gasoline19) were conducted by researchers. However, few process simulations had focused on higher alcohol synthesis with biomass gasification. Considering that the synthesis mechanism and reaction conditions of higher alcohol synthesis were distinct from the synthesis of other liquid fuels, our research focused on the modeling of biomass gasification followed by alcohol synthesis as well as the optimization of reaction conditions. He et al.20 made a technical and economic analysis on ethanol synthesis from biomass raw materials by thermochemical conversion method. It was indicated that whole synthesis system could be self-powered theoretically without any external power supply. Valle et al.21 compared different syngas reforming technologies in the process of ethanol synthesis from biomass and evaluated their techno-economic benefits. They found that iCFBG (indirect circulating fluidized bed gasification) with partial oxidation method exhibited the most cost-competitive potential. Actually, design of synthesis process route played an important role on higher alcohol synthesis efficiency. In this study, a production system of biomass gasification and subsequent higher alcohol synthesis (including biomass pretreatment, biomass gasification, gas reforming and purification, alcohol synthesis and separation, residual heat utilization) was designed and simulated. The ways of electricity supply could greatly impact higher alcohol synthesis efficiency. A self-powered system without any fossil fuel consumption was attractive in the aspect of environment protection and cost competitiveness. Thus two cases were built based on different ways of electricity 6


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supply (described in detail in Section 2). A process simulation of catalytic synthesis of higher alcohols from biomass of the two cases was made and parameters such as alcohol yield, energy efficiency and exergy loss were investigated in detail.

2. Process design and description A flow diagram of the thermochemical process from biomass to higher alcohols is shown in Figure 1. The process was developed based on biomass to liquid fuel process. The necessary changes, including higher alcohol synthesis reactor, methane reforming, methanol recycle, gas and steam combined cycle power unit were incorporated. In order to make full use of the potential of carbon-neutral biomass resources, a self-powered system in case 2 was devised and built. Aspen Plus was selected for the modeling the whole process. It contains various unit operation blocks (reactors, heaters, pumps, etc.) that can be combined to give a full representation of the process. The whole process could be divided into five modules: biomass pretreatment, biomass gasification, gas reforming and purification, alcohol synthesis and separation, and residual heat utilization. Each module was described by one or more blocks. In the simulation, pine chips was selected as the biomass feedstock and fed into a circulated fluidized bed gasifier after pretreatment (100t/h scale). In the circulated fluidized bed gasifier, biomass particles were pyrolyzed and gasified under the atmosphere of oxygen and steam. The main gases products were CO, H2, CO2 and light hydrocarbons. After gas reforming, the low molecular hydrocarbons in product gases (mainly CH4) could be converted into CO and H2. In the gas cleaning process, 7


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the impurities such as H2S, NH3 and CO2 were removed. The clean syngas was compressed to an appropriate pressure and fed into an alcohol synthesis reactor to produce higher alcohols. The synthesis products were cooled down and separated in the gas-liquid separator. The gas products coming from the gas-liquid separator were recycled into the methane reforming reactor, while the liquid products were fed into distillation tower to obtain pure alcohols. Processes such as biomass grinding and multistage compression of syngas accounted for a large proportion of the electricity that was consumed during the system operation. Based on the difference in electricity sources, the system process could be divided into 2 cases: Case 1: Electricity in case 1 was provided by residual heat utilization system and external power sources. The gas products from the gas-liquid separator were all recycled to the methane reforming reactor and reformed to syngas. After purification, the syngas was compressed and fed into the alcohol synthesis reactor. Case 2: Electricity in case 2 was partially provided by an internal gas and steam combined cycle power unit. The system was devised to be completely self-powered in the production of higher alcohol. That was, the demand of consumed electricity can be balanced by the system. Instead of recycling for alcohol synthesis, 25% of gas products from the gas-liquid separator were sent into the gas and steam combined cycle power unit to provide electricity for the system. As the proportion of the recycled gas changed, the energy provided by residual heat utilization system and the energy consumption for compression, synthesis reactor and steam reforming were 8


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also different from those in case 1. Based on the present simulation, no external power supply was needed in case 2. The operation parameters such as reaction temperature and pressure were kept the same in the two cases. Figure 2 displays the flowsheet of the cases. In the figure, coolers and heat exchangers are used to recover or supply heat in the process. Mixers and splitters are used to combine or split streams. Separators are used to remove substances from the main stream. Other main facilities were described in the following section, and shown in table 1. The following assumptions were made in order to simplify the simulation process: (1) The process was operated in a steady state and all parameters did not change over time. (2) The ash in biomass was thought to be inert in the gasification process. (3) In gasification process, all the nitrogen and sulfur in biomass was converted to simple chemicals: NH3 and H2S.14, 22 The volatiles released instantaneously and the products mainly consisted of H2, CO, CH4 and H2O. (4) The chemical composition of tar was defined as C6H6.23,24 Tar yield was determined according to the literature.25 (5) The impurity removal system for CO2, H2S and NH3 was simplified to be a SEP block. The impurities were assumed to be completely removed in the simulation. (6) For alcohol synthesis with MoS2-based catalysts, all the alcohols with more than 3 carbon atom numbers were ignored in the simulation for their tiny 9


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production content. And as CH4 made up the majority of the hydrocarbons produced in the reaction, higher hydrocarbons were ignored. (7) Column conditions were adjusted to recover propanol and ethanol with the purity of 99.0%. 2.1. Biomass pretreatment Pretreatment includes biomass drying and grinding. High water content in biomass feedstock can decrease the temperature in the gasifier, resulting in the insufficient reaction and poor gasification efficiency in the reaction zone. Therefore drying is a requirement before gasification. Grinding can increase the surface area of biomass and enhance heat transfer between heating medium and biomass particles. Biomass feedstock needs to be grinded to particles less than 10mm for a successful gasification in the fluidized bed gasifier. Pine chips, a typical forestry residue, were selected as the biomass feedstock in the simulation. Water content of pine chips is about 10wt%. The element and proximate analysis data (dry basis) are shown in Table 2.26 And the water content was decreased to less than 1wt% after drying. The Flash 2 block was used to simulate the drying process. Biomass was defined as an unconventional ingredient (NC). Then the pine chips were grinded to 6mm diameter particles for gasification. The energy consumption of grinding biomass was related to the grinding size and the species of biomass. According to the published literature, the energy consumption was 32.1 KWh o.d. t-1 (the specific active electric energy used by the motor per unit (metric tons) of oven-dried weight biomass) when pine chips were grinded to 6 diameter 10


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particles.27 2.2. Biomass gasification After pretreatment the biomass particles were sent into the circulated fluidized bed gasifier. High temperature gasification media (oxygen and steam) were fed in as the fluidized gas. In a reductive atmosphere, biomass particles were pyrolyzed to combustible gas, solid residue and tar. The gas products from the gasifier were purified by cyclone separator, particulate filter and water filter to remove solid particles and tar. The gasification reaction in the fluidized bed gasifier mainly consisted of thermal decomposition (pyrolysis), coke combustion and gas reforming reaction. In the dense-phase zone of fluidized bed gasifier, the biomass particles could decompose fast in a good heat transfer condition. At the bottom of the fluidized bed, the coke burnt with oxygen, providing the energy for biomass thermal decomposition and gasification. And the secondary reaction of gas intermediates proceeded in the dilute-phase zone, including tar cracking and coke reduction. In fact, the thermal decomposition and combustion reaction mainly carried out in the dense-phase zone, while the reduction reaction occurred in the dilute-phase zone. Therefore, the gasification was divided into two reaction stages in the modeling process: thermal decomposition and gasification reaction. In the working condition of circulated fluidized bed gasifier, the temperature difference of dense phase and dilute phase was only about 20°C. As a result, the temperatures of the two stages were assumed to be the same. Considering that a suitable ratio of H2/CO (usually 0.5~2) was needed in the 11


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following alcohol synthesis, a mixture of steam and oxygen was used as the gasification medium to obtain hydrogen-rich syngas with high heating value (10-16MJ/Nm3) and 30-60vol% hydrogen content.28 In the gasification model, the blocks of RYield and RGibbs were selected to simulate the gasification process. In RYield block, biomass decomposed into gas and solid products. Solid products were ash and unreacted fixed carbon. Gas products reacted with oxygen and steam in the RGibbs. The gas-gas reaction could approach chemical equilibrium with the minimization of Gibbs free energy of the reaction system, while the gas-solid reaction didn’t reach equilibrium due to diffusion limit. In the RGibbs block, a restricted equilibrium method was utilized to restrict two gas-solid reactions: C+αO2→2(1-α)CO+(2α-1)CO2 and C+H2→CH4. α (varying from 0.5 to 1) was a parameter changing with reaction temperature. At 800°C, the value of α was 0.8.25 In order to obtain syngas with applicable H2/CO, a mixture of steam and oxygen was used as the gasification medium. The oxygen/fuel ratio was expressed as ER (equivalence ratio). ER was defined as the ratio of the amount of oxygen supplied and the amount of oxygen needed for stoichiometric combustion of the fuel. The mass ratio of steam to biomass was denoted as S/B. 2.3. Gas reforming and purification The raw syngas produced from circulated fluidized bed gasifier contained impurities such as CH4, CO2, H2S and NH3, among which H2S can cause poisoning of synthesis catalyst. Different catalysts have different sulfur tolerance. The sulfur tolerability of Cu-based catalyst for alcohol synthesis is 0.1ppm. Mo-based catalyst, in 12


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contrast, has a much stronger sulfur tolerability of 100ppm.29 High content of impurities such as CO2 and CH4 would suppress the alcohol synthesis reaction, so methane reforming was utilized to transform CH4 into CO, CO2 and H2. The raw syngas could be washed with water to remove NH3. The remaining impurities, H2S and CO2 could be removed by an ethanolamine unit.29 RStoic block was adopted to simulate the methane reforming process. The reforming process was heated by the thermal energy from biomass and char combustion. The impurity removal system for CO2, H2S and NH3 was simplified to be SEP blocks. The impurities were assumed to be completely removed in the simulation. 2.4. Catalytic synthesis The purified syngas was then compressed to the pressure required for the synthesis reaction and fed into the alcohol synthesis reactor. The alcohol synthesis reactor along with the synthesis catalyst was the core part of the whole system. Two categories of catalyst were employed for higher alcohol synthesis: noble metal catalyst and non-noble metal catalyst.30 The noble metal catalyst that developed for higher alcohol synthesis mainly included Rh-based catalyst, with a defect of high production price. And the non-noble metal catalyst included modified methanol synthesis catalyst, modified Fischer-tropsch synthesis catalyst and Mo-based catalyst. The side reactions accompanied with higher alcohol synthesis were hydrocarbon generation and water gas shift reactions. In our simulation, the purified syngas was compressed to the reaction pressure by 13


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multi-stage compression. MoS2-based catalyst was employed in the simulation for its high sulfur tolerability. The chemical reaction kinetics proposed by Larson et al. was used in the higher alcohol synthesis reactor,31 as shown in Table 3. The alcohol synthesis process could be divided into five steps: R1-R5. Methanol was generated from the reversible reaction of CO and H2, controlling by the chemical equilibrium of the reaction R1. All the light hydrocarbons were produced from methanol and the main hydrocarbons was CH4 (R2). Ethanol was produced from the reaction of methanol and syngas, and propanol from ethanol and syngas (R3 and R4). In addition, water gas shift reaction during the process could produce CO2 and H2. For MoS2-based catalyst, all the alcohols with more than 3 carbon atom numbers were ignored in the simulation for their tiny production content. CO+2H2↔CH3OH

(R1)

CH3OH+H2→CH4+H2O

(R2)

CH3OH+CO+2H2→C2H5OH+H2O

(R3)

C2H5OH+CO+2H2→C3H7OH+H2O

(R4)

CO+H2O↔CO2+H2

(R5)

RPlug and REquil were used to simulate the alcohol synthesis reactor. RPlug was for the production of alcohols (R1-R4) and REquil for the water gas shift reaction (R5). LH model was used to calculate the reaction rate in RPlug, as shown in Equation 1.32 The equation was based on the catalyst mass and partial pressure of reactants. As CH4 made up the majority of the hydrocarbons produced in the reaction, the reactions of other hydrocarbons were ignored in the calculation. 14


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(1) According to R1-R5, the net production rates of methanol, ethanol, propanol and methane can be expressed as follows:

=

···················································· (2)

= ······························································(3)

= ···········································································(4)

=

················································································ (5)

Based on the kinetic model of alcohol synthesis above, the production contents of methanol, ethanol and propanol were calculated. The reaction temperature was 280-350°C, the reaction pressure was 30-160 bar, and H2/CO was 0.5∼3.0. Assuming that the alcohol synthesis was an isothermal process, the influence of temperature, pressure and H2/CO on the alcohol synthesis were evaluated. 2.5. Alcohol separation The products from the alcohol synthesis were cooled down by water or other streams and sent into a gas-liquid separator. The separated gas products included H2, CO, CH4, CO2, etc., and the liquid products were methanol, ethanol, propanol and H2O. Due to the low single-pass CO conversion, after CO2 removal the gas products were recycled to the previous process again while the liquid products were sent to the distillation tower to obtain pure methanol, ethanol and propanol. In case 1, all the gas products were recycled to a CH4 reforming reactor. The 15


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recycled gas products were reformed and purified to remove CO2 and other impurities. In case 2, some recycled gas products were sent to a gas and steam combined cycle unit to produce electricity for the system. The rest of the gas products were also recycled as in case 1. Liquid products, including alcohols and water, were dehydrated by a molecular sieve unit and then separated by a distillation system. In distillation tower D1, 99% of the methanol and ethanol were distilled off from the tower top while 99% of propanol was from the tower bottom. Then the ethanol and methanol entered D2 for further separation. In D2, 99% of the methanol was distilled off from the tower top while pure ethanol was obtained from the tower bottom. For the MoS2-based catalysts, ethanol was produced by CO insertion into methanol. The addition of methanol in some way could promote the generation of ethanol.33 Therefore, the methanol obtained from D2 was pumped into the alcohol reactor in the present simulation in order to maximize ethanol yield. 2.6. Residual heat utilization The high-temperature gases (∼800°C) exited from biomass gasification and methane reforming flowed into a heat recovery steam generator, in which feed water was heated and evaporated into high pressure steam (550°C, 150bar). And then the high pressure steam was expanded in a steam turbine to generate electricity. The isentropic efficiency and mechanical efficiency were defined as 0.85 and 0.90 respectively. While the heat released from coolers and alcohol synthesis reactor was utilized to generate low pressure steam. Part of steam was utilized as reactants for 16


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biomass gasification and methane reforming. Other part supplied heat for distillation columns.

3. Results and discussions In both two cases, biomass gasification and higher alcohol synthesis were two key processes. The composition distribution of gasification products could exert important impact on the follow-up alcohol synthesis process. Therefore, both of the biomass gasifier and the higher alcohol synthesis reactor were described and analyzed in detail. 3.1. Biomass gasification Based on the biomass gasification model above, the influence of parameters on biomass gasification was evaluated and analyzed. The gasification was operated under atmospheric pressure at 800°C. The carbon conversion would be kept at a low level with low ER.34 The influence of ER ranged from 0.2 to 0.31 on biomass gasification was studied. The results are seen in Figure 3(a) and (b). With the increase of ER, the content of CO and H2 in gas products showed a decreasing trend while CO2 presented an opposite trend, which was in accordance with the experimental result of wood chip gasification, reported by Chen et al.35 As the value of ER increased, the CO content experienced a considerable decline while H2/CO increased in the meantime. This was due to that more CO can react with O2 to produce CO2 at higher ER. When ER increased from 0.2 to 0.31, the CO2 content increased from 0.11 to 0.22, showing excess ER would also depress syngas generation. The value of O/C for pine chips was 17


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relatively high (about 0.61). At ER=0.2 the carbon in pine chips could be converted to CO effectively. So the value of ER was kept at 0.2 in the following analysis of other parameters. In order to produce syngas with high H2/CO for the subsequent alcohol synthesis, steam was added in as a gasification medium. The influence of S/B on biomass gasification is displayed in Figure 3(c) and (d). With S/B increasing from 0 to 0.23, H2 yield gradually accumulated in the gas yield distribution (Figure 3(d)), while H2 content in the gas products was kept unchanged after a slightly increase (Figure 3(c)). As the water gas shift reaction was enhanced by the increasing steam, CO2 content presented an increasing tendency, while CO presented the opposite. CH4 content stayed relatively low, possibly because that CH4 tended to decompose into H2 and CO at higher temperatures. When S/B increased from 0 to 0.23, H2/CO also showed a noticeable increase from 0.88 to 1.34. Therefore, by adjusting the value of S/B, the hydrogen-rich syngas for alcohol synthesis could be obtained. 3.2. Catalytic synthesis of alcohols from syngas According to the chemical kinetic model above, the parameters of catalytic synthesis of alcohols were evaluated. The influence of reaction temperature on the catalytic synthesis of alcohols at 80 bar and H2/CO=1.2 is displayed in Figure 4(a) and (b). The equilibrium composition of synthesis product was greatly affected by temperature. With the temperature increasing from 280 to 350°C. CO conversion was enhanced from 5.0% to 53.3% and the alcohol space time yield (STY, the quantity of product produced per unit mass of catalyst per unit of time, mainly used in the 18


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catalytic synthesis of alcohols from syngas) was from 43.1 to 179.5 g/kgcat/h. As the reaction temperature rose, production selectivity of methanol and ethanol both decreased while selectivity of propanol went up slightly. In contrast, the selectivity of CO2 and CH4 both showed a sharp growth with reaction temperature increasing, in agreement with the result from literatures.9 This can be attributed to the water gas shift reaction and CH4 production reaction enhanced by the high temperature. In the catalytic synthesis of alcohols from syngas, increasing reaction temperature was conducive to higher CO conversion and lower alcohol selectivity. From the simulation results, in the temperature range lower than 320°C the CO conversion increased dramatically while in the range higher than 320°C increased gently. As a result, 320°C was chosen as the optimum reaction temperature for the simulation. The influence of reaction pressure on the catalytic synthesis of alcohols at 320°C and H2/CO=1.2 is displayed in Figure 4(c) and (d). With the pressure increasing from 30 to 160 bar, CO conversion was enhanced from 5.8% to 36.3% and alcohol STY grew strikingly from 37.1 to 215.6 g/kgcat/h. The result was coincidence with the experiment data reported by Surisetty et al.36 They carried out tests of higher alcohols synthesis over 4.5wt% Co-Rh-Mo-K/MWCNTs and found increasing pressure can favor the formation of higher alcohols at a constant temperature. As pressure increased from 55 to 96 bar, CO conversion increased from 29.8 to 43.9%, and total alcohols STY increased from 0.121 to 0.219 g/(g of catalyst)/h. The alcohol synthesis was a reaction with volume shrinkage, so higher pressure could shift the chemical reaction equilibrium to produce more alcohols and thus could increase CO conversion 19


Energy & Fuels

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and alcohol yield. As seen in Figure 4(d), methanol selectivity was gradually reduced with pressure increasing. For ethanol, however, the selectivity tended to go up. Higher pressure was in favor of the insertion of CO and propagation of carbon chain during the synthesis reaction, resulting in a higher selectivity of higher alcohols.37 Considering the production cost of increasing reaction pressure, the optimum pressure for the simulation was kept at 100 bar. In the condition of T=320°C and P=100 bar, the influence of H2/CO on the alcohol synthesis was explored. As H2/CO changed, the partial pressure of H2 and CO would also change, thus affecting the composition of the products. It was found in Figure 5(a) that the conversion of CO increased and H2 decreased with H2/CO growth. Alcohol STY presented an increase firstly, and then decreased with the increase of H2/CO. When H2/CO =1.0, STY reached the highest value of 159.4 g/kgcat/h. At H2/CO =2.0, STY slightly declined to 157.6g/kgcat/h. As shown in Figure 5 (b), when H2/CO increased from 0.5 to 1.0, the selectivity of alcohols increased and the selectivity of CH4 and CO2 decreased. Further increase of H2/CO could only obtain little change in the selectivity of alcohols. From the simulation result, the appropriate range for H2/CO was 1.0∼2.0. 3.3. Material and energy balance of the system 3.3.1. Material balance results Based on the analysis on the thermochemical process from biomass to higher alcohols above, the simulation conditions were determined, as shown in Table 4, for both two cases. H2/CO for the synthesis reaction was set to 1.0 and thus S/B was set 20


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to 0.054. The analysis of material flow was shown in Table 4 for both two cases. The material flow of the system was divided by the following parts: biomass pretreatment module, biomass gasification module, methane reforming, gas purification, alcohol synthesis and products separation module (the methane reforming, gas purification, alcohol synthesis and products separation was seen as a whole part for gas product were recycled in these modules), power generation module. As shown in Table 4, the feed rate of biomass feedstock was 100t/h for both two cases. The heat for methane reforming was provided by combustion of coke (from biomass gasification) and biomass (fed into the methane reforming system directly). Thus the feeding rate of biomass for gasification pretreatment was 85.5t/h and 96.4t/h, for case 1 and case 2 respectively. The raw syngas flow out of the circulated fluidized bed gasifier was 78.2t/h and 88.2t/h respectively, with H2/CO≈1.0. Due to the difference in CH4 content of the raw syngas, the reforming steam flows of the two cases were also different: case 1 was 19.6t/h and case 2 was 11.5t/h. The alcohol products for case 1 were 19.2t/h ethanol and 5.9 t/h propanol. And for case 2, 14.9t/h ethanol and 4.5t/h propanol were obtained. To evaluate the production efficiency of the process, the alcohol yield was defined as the yield of alcohol production obtained from 1 kg biomass. From the calculation results, the alcohol yield of case 1 was 25.1wt%. The alcohol yield of case 2 was 19.4wt%, lower than case 1. This was because that 25% of the gas product from the gas-liquid separation unit was delivered to a gas and steam combined cycle power unit to produce electricity for the whole system. The lower recycled flow led to a lower alcohol yield, as illustrated in Figure 21


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6. 3.3.2. Energy efficiency evaluation The main energy input of the system was biomass feedstock. On one hand, the biomass was used as the raw material for the production of higher alcohols; On the other hand, it provided part of heat for the biomass to alcohol process. In case 1 some extra electricity was needed from external sources; while in case 2 the system was self-powered. The main energy output was higher alcohols, including ethanol and propanol, and tars. The methanol produced in the synthesis reactor was separated and recycled to the former process. The electricity consumption of grinding was acquired from literature. And for other processes, the energy consumption was calculated by simulation. As seen in Table 4, the main electricity consumption was from the biomass grinder, the pumps, the fans and the compressor. Due to the different recycled gas flow of the two cases, their electricity consumption was also different. The heat from gas product of biomass gasification and methane reforming could be recovered to produce high temperature steam, which could enter a steam turbine and supply part of electricity for the whole system. It was calculated that the heat recovery system could produce 13.7MW (the other 42.4MW was from external power supply) for case 1 and 11.3MW for case 2. In addition, part of the electricity in case 2 was generated by a gas and steam combined cycle unit (35.2MW) and the generating efficiency was set as 50%.38 For the two cases, all the heat needed could be supplied by biomass feedstock directly and indirectly, without any other external heat supply. The heat of methane reforming was from the combustion of biomass (70% combustion efficiency) 22


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and unreacted coke (90% combustion efficiency). The heating value of pine chips was 16.54 MJ/kg,39 the ethanol was 26.87 MJ/kg40 and the propanol was 33.63 MJ/kg. The heating value of the coke from biomass pyrolysis could be as high as 33MJ/kg.41 In the present work, the coke from gasification was taken as 30.0 MJ/kg. The heating value of CO, H2 and CH4 was 12.64, 12.74 and 39.82 MJ/Nm3 respectively. To evaluate the energy consumption and production, the energy efficiency was expressed as the ratio of heating value of desired products to energy inputs. The quality of thermal energy was lower than electricity. Divided by corresponding generating efficiency, electricity input was modified to reflect the equivalent thermal energy. Here the energy efficiency of the system can be expressed as follows: η= ⁄ = E ⁄(E + E /η ) ×100% ·········································· (6) where Ea refers to the heating value of higher alcohol products. Etotal refers to total energy input. Eb and Ee refer to energy value of biomass input and external electricity input, respectively. ηe denotes generating efficiency. The external electricity provided by traditional thermal power plant had an average generating efficiency of 0.35.42 The calculation results based on the simulation was illustrated in Figure 6. It could be found that the energy efficiency of case 2 (33.2%) and case 1 (34.1%) were very close. Although the efficiency of gas and steam combined cycle was higher than thermal power plant, the total energy efficiency of case 2 was still somewhat lower than case 1 considering the energy efficiency from biomass to syngas, which was relevant to gasification method and 23


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condition. Besides the net energy efficiency, energy efficiency to ethanol is also an important factor. It represents the ethanol energy produced from per Joule energy input. As seen from Figure 6, the efficiency to ethanol of case 2 is very close to that of case 1. The advantage of case 2 was that it could be self-powered without any energy supply from fossil fuels. All the energy input of case 2 was from biomass residues, which was quite attractive and promising in the aspect of environment protection and cost competitiveness. 3.3.3. Exergy analysis Exergy analysis is a powerful tool in the thermodynamic analysis of energy system. By evaluating exergy input and output of a system, the true magnitude of losses and their causes and locations could be determined.43 For fuels like coal, liquid fuel and natural gas, the exergy content is estimated by multiplying the net heating value by an appropriate coefficient.44 The exergy coefficient of electricity is assumed to be 1.00. The overall exergy balance of the two cases is shown in Figure 7. For case 1, the exergy inputs were biomass and electricity, the total amount was 537.6MW. The major exergy outputs were alcohols, tar, and steam (for power generation). 39.6% of the exergy in case 1 was converted to alcohols and 5.2% was converted to high quality steam in the heat exchangers. The exergy of high quality steam was then transferred into electricity in the steam turbine. Syngas compression and heat supply for methane reforming took up 6.9% and 10.8% of exergy loss respectively. Other

24


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exergy losses could be attributed to heat transfer temperature drop, exhaust smoke heat loss and etc. The total exergy loss was 53.4%. Biomass was the only exergy input of case 2. The major exergy outputs included ethanol, propanol, tar, steam, and combustible gas. Among them, steam and combustible gas were all utilized to produce electricity for the system usage. About 33.5% exergy input was converted to alcohols in case2, lower than that of case 1. The exergy of combustible gas (68.6MW) was used to generate electricity by a gas and steam combined power unit. Compared with case 1, there is more electricity generated in case 2, resulting in higher exergy loss in the power generation module. The total exergy loss of case 2 was 57.9%, higher than that of the case 1. This was due the fact that part of electricity used in case 1 was provided by external power sources and the exergy loss from electricity production was lower. For case 2, however, the exergy loss from syngas compression and heat supply for methane reforming were both lower than that of case 1, implying that the self-powered process (case 2) may be a promising path for higher alcohol synthesis from biomass.

4. Conclusions The present research was centered on the thermochemical conversion from biomass to higher alcohols. Two cases were simulated and parameters of gasification and alcohol synthesis were analyzed in order to determine the optimum operating conditions. The material balance and energy efficiency of the system was evaluated. For biomass such as pine chips, low ER value (0.2) was appropriate. The H2/CO of gasification product 25


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was adjustable with the addition of steam. The chemical kinetics of alcohol synthesis based on MoS2-based catalyst was used for the modeling of alcohol synthesis reactor. The simulation result revealed the correlation between product distribution (CO conversion and alcohol selectivity) and reaction temperature. The optimum temperature for MoS2-based catalyst was 320°C. Higher pressure and H2/CO (1.0∼2.0) were both in favor of increasing alcohol yield. The alcohol yield of case 1 (25.1 wt%) was higher than case 2 (19.4 wt%). For the two cases, the main electricity consumption was derived from the biomass grinder, the air blower and the compressor. The energy efficiency of case 2 (33.2%) and case 1 (34.1%) were very close. The exergy loss in the power generation unit of case 2 was higher, resulting in a higher total exergy loss. Although the total energy efficiency of case 2 was a little lower than case 1, the self-powered process was still cost competitive and environmental friendly in some aspect.

Author information Corresponding Author *Tel: +86 571 87952801; E-mail: [email protected] Notes The authors declare no competing financial interest. Acknowledgments We acknowledge financial support from the National Natural Science Foundation of China (No. 51476145, 51476146), and the National Science and Technology 26


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Supporting Plan through Contract 2015BAD15B06.

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Table 1. Main blocks utilized in the modules Modules

Block description

Biomass drying

Flash 2: modeling biomass drying. Biomass was defined as an unconventional ingredient.

Biomass gasification

RYield: modeling biomass decomposition by specifying reaction yields. Separator: modeling solid separation. RGibbs: modeling components gasification.

Gas reforming

RStoic: Stoichiometric reactor, modeling methane reforming.

Alcohol synthesis

MCompr: modeling syngas compression. RPlug: modeling alcohol production, Langmuir-Hinshelwood (LH) model was used to calculate the reaction rate. REquil: modeling water gas shift reaction.

Alcohol separation

DSTWU: modeling distillation towers to obtain required purity ethanol.

High-temperature

Heater: modeling heater exchangers.

gas heat recovery

Turbine: modeling steam turbine.

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Table 2. Characteristics of typical biomass Sample

Proximate analysis

Ultimate analysis

(wt% dry basis) FC Pine chips

A

17.16 0.55

(wt% dry basis) V

C

H

82.29 50.54 7.08

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O

N

41.11 0.15

S 0.57

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Table 3. Parameters for LH kinetic model setup for alcohol synthesis over MoS2 catalysts31

i

Reaction

Pre-exponential

Activation

factor

energy

A (komls-1kg-1)

E(kJ mol-1)

Driving force

Adsorption factor

k1

k2

K1

K2

CH3OH

CO+2H2↔CH3OH

4.062⋅10-6

143.472

1.396⋅10-19

1.087⋅10-20

3.870⋅10-15

C2H5OH

CH3OH+CO+2H2↔C2H5OH+H2O

8.477⋅10-7

24.986

1.237⋅10-5

0

9.111⋅10-6

0

0

C3H7OH

C2H5OH+CO+H2↔C3H7OH+H2O

5.966⋅10-8

89.333

9.869⋅10-5

0

6.006⋅10-5

0

0

CH4

CH3OH+2H2↔CH4+H2O

2.607⋅10-6

95.416

1.237⋅10-5

0

1.542⋅10-5

0

0

35


3.562⋅10-7

K3 1.235⋅10-5

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Table 4. Material Balance of the system Case Input Biomass pretreatment Output Input Biomass gasification (800 ºC, atmospheric pressure)

Output

Case 1

Case 2

Biomass(wet) 85.5 t/h

Biomass(wet) 96.4 t/h

Electricity 2.5 MW

Electricity 2.8 MW

Biomass (dry) 77.7 t/h

Biomass (dry) 87.6 t/h

Biomass 77.7 t/h

Biomass 87.6 t/h

Steam 4.2 t/h

Steam 4.7 t/h

Oxygen 28.6 t/h

Oxygen 32.3 t/h

Electricity 0.9 MW

Electricity 1.1 MW

Gas products 78.2 t/h

Gas products 88.2 t/h

Ash and unreacted coke

Ash and unreacted coke

8.3 t/h Input

9.4 t/h

Tar+H2O 24.0 t/h

Tar+H2O 27.0 t/h

Raw syngas 78.2 t/h

Raw syngas 88.2 t/h

Steam 19.6 t/h

Steam 11.5 t/h

Methane reforming, gas

Electricity 52.7 MW

Electricity 42.6 MW

purification, alcohol

Heat 105.3 MW

Heat 78.0 MW

synthesis (MoS2-based

(provided by char (7.9t/h)

(provided by char (8.8 t/h)

catalyst, 320 ºC,

and biomass (14.5 t/h)

and biomass (3.6 t/h)

100bar, H2/CO=1.0)

combustion)

combustion)

CO2 49.9 t/h

CO2 59.8 t/h

NH3 0.14 t/h

NH3 0.16 t/h

H2S 0.47 t/h

H2S 0.54 t/h

H2O 20.6 t/h

H2O 9.6 t/h

and products separation

Output

Ethanol 19.2 t/h

Ethanol 14.9 t/h

Propanol 5.9 t/h

Propanol 4.5 t/h

Tail gas 1.7 t/h

Tail gas 5.9 t/h Combustible gas 4.3 t/h (CO 15.6

kmol/h,

H2

275.3

kmol/h, CH4 211.2 kmol/h) Power generation (net output)

13.7 MW (from heat

11.3 MW (from heat

recovery )

recovery) 35.2 MW (from combustible gas)

Net input power

42.4 MW

0 MW

36


Page 37 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Figure captions Figure 1. A flow diagram of thermochemical process from biomass to higher alcohols Figure 2. Simplified process flowsheet of case1 and case 2 (solid lines, mass flows; red dotted lines, energy flows; green dot lines, work flows) Figure 3. Influence of ER and S/B on gas composition and gas yield for gasification products. Figure 4. (a) and (b) Influence of temperature on the alcohol synthesis at H2/CO=1.2 and P=80bar. (c) and (d) Influence of pressure on the alcohol synthesis at H2/CO=1.2 and T=320ºC. Figure 5. (a) and (b) Influence of H2/CO on the alcohol synthesis at T=320ºC and P=80bar. Figure 6. Alcohol yield and energy efficiency of the two cases. Figure 7. Sankey diagram of exergy streams.

37


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 44

Feeder Particulates filer Biomass Syngas compression Water filter Dryer

Pump

Methane reforming

Methanol

Grinder

Alcohol synthesis reactor

Collecting conveyor

Particles

Circulating fluidized bed gasifier

Biomass combustion 100%

Gas recycled for

Oxygen and alcohol synthesis steam

75%

Gas purification Tail gas

Case 1

Distillation towers

Case 2

Ethanol

25%

Gas and steam combined cycle power unit

Gas and liquid separators

Propanol

Figure 1. A flow diagram of thermochemical process from biomass to higher alcohols.

38


Page 39 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Case 1 W

Turbine

Q

Cooler4

Pump2

Power consumption of the auxiliary equipment W

Splitter1

Mixer2

Power ne eded

Heat exchangers

High-temperat ure gas heat recovery

M IXE R

Biomass 85.5t/h 10 0t/h

Dryer

14.5t/h for com bustion

Syngas

Steam 19.6t/h Purifier1

Q

Methanol recycled

Compre ssor

M ethane reforming

Pump1

Mixer1

Purifier2

Decompsition

Impurities

Reac tor

Impurities

Q

Q Q Q

Cooler1

P roducts

Se parator1

Oxygen 28.6t/h

Cooler2

Cooler3

WGS reactor

Alcohol synthesis

Q

Char, ash

Fan

Liquild productsSe parator4 Gasifier Raw syngas

Steam 4.2t/h

Distillation towers Ethanol 19.2t/h

Water 20.6t/h

Tail gas 1.7 t/h


D2

Se parator3

Separator2

D1

P ropanol 5.9t/h

Case 2

w

Turbine

Power consumption of the auxiliary equipment W

P ower needed

Q

Cooler4

Pump2

Splitter2

Mixer2

Heat ex change rs

High-temperat ure gas heat recovery

M IXE R

Biomass

96.4t/h Dry er

100t/h

Methane reforming

Oxygen 32.3t/h

Compre ssor

Purifier2

Q Dec ompsition

3.6t/h for combustion

Impurities

Reac tor

Separator1

Combustible gas to gas and steam combined cycle power unit (4.3t/h) Q Cooler1 Splitter1 P roducts

Char, ash

CO2 Separator5

Q

Gasifie r Raw syngas


Separator2

Q

Se parator3

Tail gas 5.9 t/h

Q WGS re actor

D2 Distillation

Separator4

Water Q 9.6t/h

Cooler3

Q

Alcohol sy nthesis

Liquild products Steam 4.7t/h

Pump1

Mixer 1

Impurities

Q

Cooler2 Fan

Methanol recycled

Syngas

Steam 11.5t/h Purifier1

t owers

Ethanol 14.9 t/h

D1

P ropanol 4.5t/h

Figure 2. Simplified process flowsheet of case1 and case 2 (solid lines, mass flows; red

dotted lines, energy flows; green dot lines, work flows)

39


Energy & Fuels

(a)

CO

H2

CH4

CO2

H2/CO 1

(b)

CO

H2

CH4

CO2

25

0.9 0.85

0.3

0.8

0.2

0.75 0.7

0.1

Gas yield (mol/kg)

0.95

0.4

20 15 10 5

0.65

0

0

0.6

0.20 0.22 0.24 0.26 0.28 0.29 0.31

0.20 0.22 0.24 0.26 0.28 0.29 0.31

ER

ER

(c)

CO

H2

CH4

CO2

0.45

H2/CO 1.6

0.4

1.4

0.35

1.2

0.3

1

0.25 0.8 0.2 0.6

0.15 0.1

0.4

0.05

0.2

0

0 0.00

0.02

0.05

0.09

0.13

0.16

0.20

(d)

CO

CH4

CO2

25 20 15 10 5 0 0.00

0.23

H2

30

Gas yield (mol/kg)

Gas composition (vol. %)

0.5

Gas composition (vol. %)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 44

0.02

0.05

0.09

0.13

0.16

0.20

0.23

S/B

S/B

Figure 3. Influence of ER and S/B on gas composition and gas yield for gasification products.

40


CO conversion%

80

Alcohol yield 200

70

180

60

160

(b)

Alcohol STY (g/kgcat/h)

CO conversion %

(a)

140

50

120

40

100

30

80

20

60

10

40

0

20 280

290

300

320

350

50 45 40 35 30 25 20 15 10 5 0

(c)

CO conversion%

25

150

20 100

15 10

50


200

30

5 0

0 70

90 110 130 150 160

Products selectivity %

250

35

50

ethanol

CO2

CH4

290

T (°C)

methanol CO2

(d)

Alcohol yield

40

30

methanol

280

T (°C)

CO conversion %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels


Page 41 of 44

50 45 40 35 30 25 20 15 10 5 0 30

50

70

300

ethanol CH4

90

propanol

320

350

propanol

110 130 150 160

P (bar)

P (bar)

Figure 4. (a) and (b) Influence of temperature on the alcohol synthesis at H2/CO=1.2 and P=80bar. (c) and (d) Influence of pressure on the alcohol synthesis at H2/CO=1.2 and T=320ºC.

41


Energy & Fuels

CO conversion Alcohol yield

(a)

H2 conversion

(b)

180

70

160

45

60

140 120

50

100

40

80

30

60


80

50


Conversion %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

propanol

25 20 15

20

5

0

0

0

3.0

CH4

30

10 2.0

ethanol

CO2

35

40

1.0

methanol

40

20

0.5

Page 42 of 44

10

0.5

H2/CO

1.0

2.0

3.0

H2/CO

Figure 5. (a) and (b) Influence of H2/CO on the alcohol synthesis at T=320ºC and P=80bar.

42


Page 43 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

40

34.1

35

33.2 30 25 20

Propa nol 5.9 Propanol 4.5

15 10

Ethanol 19.2

Ethanol 14.9

25.1

19.4

case1

case2

case1

case2

5

Ethanol efficiency Ethanol efficiency 24.2 24.7

0

Alcohol yield t/100t

Alcohol yield wt %

case1

case2

Energy efficiency %

Figure 6. Alcohol yield and energy efficiency of the two cases

43


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 7. Sankey diagram of exergy streams

44


Page 44 of 44

Simulation Study of Thermochemical Process from Biomass to Higher

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