Evaluation of Newly Designed Polygeneration System with CO2 Recycle

Article pubs.acs.org/EF

Evaluation of Newly Designed Polygeneration System with CO2 Recycle Qun Yi, Bingchuan Lu, Jie Feng,* Yanli Wu, and Wenying Li* Key Laboratory of Coal Science and Technology (jointly constructed by Shanxi province and Ministry of Science and Technology), Taiyuan University of Technology, Taiyuan 030024, PR China ABSTRACT: A new coal-based polygeneration system with CO2 recycle is proposed in this paper. With the gasified coal gas containing 23 vol % CO2 and the coke oven gas containing 25 vol % CH4 as the dual gas sources, the system mainly produces methanol, dimethyl ether, and electric power. The system adopts CO2/CH4 reforming to modify the C/H ratio of the syngas. Particularly, the CO2, coming from the distillation tower, is recycled separately back to be a reactant during gasification and the resource gas in the reforming unit. As the CO2 concentration in the exhausted gas from the distillation tower is more than 95 wt %, this system does not require a CO2 separation unit. The system avoids the conventional water−gas shift reaction that is used to adjust the ratio of C/H in the syngas, but fully uses the CO2 produced from coal gasification, which solves the problems of CO2 capture and storage. The performance of the whole system’s energy, CO2 emission, and economics are analyzed by Aspen Plus 11.1 and Aspen Icarus 11.1 software. Results indicate that the new system realizes 11.5% increase of chemical energy, 1.3% increase of internal rate of return and 33.8% reduction of CO2 emission at the expense of 8.4% of power output. Especially, the new system can save about 13−18% on energy versus single product systems. The scheme in which CO2 is recycled back to the gasifier and the reforming unit plays the most significant role in the comprehensive evaluation of energy utilization, CO2 emission control, and economy benefits of the system.

1. INTRODUCTION Nowadays, anthropogenic emissions of CO2 into the atmosphere are mainly caused by combustion of fossil fuels, oil refinery, iron and steel plants, cement industry, lime industry, and natural gas.1 Coal plays an important role in the survival and evolution of human society. Total production and consumption of coal around the world was 3.73 billion tons in 2010, which is equivalent to 3.56 billion tons of oil. China’s coal production and consumption shares of global proportion are 48.3% and 48.2%, respectively.2 Global emissions of CO2 have reached a historical maximum of 30.6 billion tons in 2010, and about 44% of the CO2 is derived from the burning of coal.3 For these reasons, it is significant to have advanced technology options that will allow the continued use of fossil fuels without substantial emissions of CO2. As a kind of promising technology, the integrated gasification combined cycle (IGCC) with carbon dioxide capture and sequestration (CCS) is the type of power generation technology with higher efficiency and less carbon dioxide emission. For the purpose of CCS, different technologies of CO2 emission reduction such as physical absorption,4,5 membrane reactors,6,7 chemical looping4,8 and some other related technologies are investigated.9,10 The above technologies can achieve about 90 mass % CO2 recoveries. However, there is no such thing as a free lunch, and a substantial energy penalty must be paid for CO2 capture in most of the above technologies. The energy efficiency of IGCC with CCS decreases by about 10% as a result of the CO2 capture: the efficiency is 35−38% on the basis of the lower heating value (LHV), which is lower than that in the IGCC process without CO2 capture, which is about a 41−44% LHV.11−14 © 2012 American Chemical Society

The efficiency loss then becomes the energy penalty for CO2 recovery in the IGCC system, and the great energy penalty leads to the high cost of CO2 capture.15,16 The CCS approach consumes not only much energy but also a large amount of valuable carbon resources. In other words, the application of CCS technologies can improve CO2 recovery but greatly decreases the energy utilization and element utilization. Additionally, most CCS technologies based on the IGCC system17,18 do not consider CO2 treatment, transport, and storage after the CO2 is separated. As Hetland pointed out, “just capturing the CO2 without storage makes no sense”.19 In fact, CO2 transportation requires high quality pipeline materials and the cost of transportation rises with the prolonged distance, let alone some geopolitical risks.19,20 When considering the handling of CO2 that goes via pretreatment, transport, and injection for permanent geological storage, the cost and energy consumption might be sharply increased.21−23 A polygeneration system, which integrates the IGCC system with a chemical production process system, can be an option to solve problems associated with both the energy system and chemical production process.24 Based on the concept of a polygeneration system, Liu et al.25−27 put forward polygeneration systems with different types and configurations. With the purpose of improving the integration and performance of the systems, the multiobjective (e.g., energy, economy, environment) optimization of polygeneration systems was researched by an integrated systematic approach, which accounted for the Received: November 4, 2011 Revised: January 3, 2012 Published: January 9, 2012 1459

dx.doi.org/10.1021/ef201724m | Energy Fuels 2012, 26, 1459−1469

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2. ESTABLISHMENT OF THE NOVEL POLYGENERATION SYSTEM 2.1. Options of CO2 Circulation. In this paper, two ways for CO2 conversion are proposed. The first method is to reinject the separated CO2 into the gasifier as a gasification agent, which can react with coal at temperatures above 1073 K. The only thing we need to consider is to balance the gasifier heat flux. Generally, the temperature of the gasifier is higher than 1273 K. The higher the temperature is, the higher the chemical activity of CO2 will be. The US DOE model37 shows that the syngas production is enhanced within the gasifier as a result of CO2 recycle into gasifier, but it is also found that excess CO2 can reduce the temperature of the gasifier and the H/C ratio, which will lead to an efficiency decrease of the gasifier. So, the suitable recycling ratio of CO2 is the key to this scheme. The second method specially fits for the dual-gas sources polygeneration system;42,43 CO2 is recycled to the reforming unit where the reaction (CO2 + CH4 = 2CO + 2H2; ΔH = +247 kJ·mol−1) happens. The feature of this scheme is that CO2 takes part in the reaction as a feed reactant gas instead of being a waste gas and CO2 is converted into CO, which not only reduces CO2 emission but also increases the carbon utilization. Feng et al. found that the system with CH4/CO2 reforming had high efficiency in energy utilization (49.6%), carbon utilization (61.8%), and CO2 + CH4 conversion (52.6%).41 On the other hand, CH4/CO2 reforming is an endothermic process, which means that more energy must be supplied if we want to convert more CO2 into CO. 2.2. Modeling and Design Assumptions. The first step of modeling complex processes in Aspen Plus is to consider the important species that occur during the real processes. The most important species for polygeneration system applications are C, H2, CO, CO2, S, N2, CH4, H2S, COS, HCl, HCN, NH3, H2O, SO2, and O2. Further trace elements such as Hg, Cr, Cd, and Pb, and alkali components are neglected to improve the convergence behavior of the plant model.44 Because the coal consists of complex structured macromolecules, Aspen Plus cannot handle these components directly, either in chemical or in phase equilibria. Therefore, the feed needs to be decomposed into reactants. This is done in a “yield reactor”, in which the feed is characterized by the proximate and ultimate analysis, as well as the heating value, and converted to the corresponding composition.44,45 After finishing the definition of components, the property method of the simulation is chosen. From the numerous methods provided by Aspen Plus, the PR-BM method is chosen as the global method. However, the individual property method BWRS is used for DME synthesis subsystem, in which the reaction happens at high pressure and the feedstocks are hydrocarbons.40 The Gibbs reactor model is used for the gasifier and the combustor in the gas turbine; the corresponding reaction equations are entered, and the deviation from equilibrium is taken into account via approach temperatures. The gasifier is modeled as an ash-agglomerating fluidized bed gasifier,40 and the gas turbine is assumed to be the GE-7FA turbine. The stoichiometric reactor is used for simulating the desulfurization process using the high temperature desulfurization technology developed by the Taiyuan University of Technology (TUT).46,47 Aspen Plus RPlug reactor and Rcstr reactor are used to model CH4/CO2 reforming and DME synthesis,

complexities of very different scales, ranging from technology and plants to energy supply chain and megasystem on the basis of energy systems engineering, and the polygeneration system after optimization showed the better overall performance compared with the single chemical production system or IGCC. With multiple output streams, polygeneration systems allow for more flexibility and therefore are superior to IGCC and provide more opportunities to achieve higher efficiency and lower environmental impacts.19,28−30 High value-added chemical products can moderate the investment cost and manufacturing cost.31−34 Further, the reduction of CO2 emission based on a polygeneration system has been investigated.15,16,35 Jin et al.16 proposed a coal-based sequential polygeneration system for power generation and methanol production with CO2 recovery, in which 45−48% of the equivalent thermal efficiency could be acquired. It is not only higher than 35−37% of IGCC with CCS but also 1−4% higher than the 42−45% of IGCC without CCS, which indicates its potential in recovering CO2 at a lower operating cost.15,33,34 The world’s first polygeneration system in operation by the Yankuang Group in China producing methanol and electricity has 57.16% of the total system energy utilization, and the CO2 emission has been reduced by about one-third compared to the single product system by adopting precombustion CO2 capture.36 Based on the above-mentioned, IGCC with CCS has higher CO2 recovery but lower energy efficiency. The coproduction plant with high CO2 recovery can have higher energy utilization and economic benefits simultaneously. However, the same problem exists in both polygeneration system and IGCC: the handling of abundant CO2 after separation. CO2 produced in the production process can be seen as the waste of carbon from the coal feedstock. To solve this problem, Chapalamadugu and co-workers designed the IGCC system with CO2 recycling back into the gasifier as the gasification agent, which can be reacted with coal at temperatures above 1073 K, and the results showed that the syngas production was enhanced and CO 2 emission decreased.37,38 Oki et al. have developed innovative gasification technology with CO2 capture, which can keep thermal efficiency as high as a state of the art IGCC plant, namely, more than 40%. In this process, coal was gasified with a mixed gas of O2 and recycled CO2 flue gas. Similar to the case of an oxy-fuel combustion system, the whole system did not require a CO2 separation unit.39 CO2 circulation may be a suitable and potential way to convert CO2 into CO, which is used to synthesize oxygenated hydrocarbons, for example methanol, dimethyl ether (DME), and dimethyl carbonate (DMC). CO2 recycle will definitely consume a certain amount of energy, so it is very important to find an appropriate way to achieve higher CO2 conversion especially in the energy system and, at the same time, to avoid much energy loss. Following the above studies, we propose, in this article, a novel polygeneration system with CO2 being reinjected into a coal gasification unit and reforming unit, making use of the CO2 transformation and utilization to reduce CO2 emission. On the basis of the previous work of our research group,40,41 we designed this new coal-based polygeneration system by taking into account the factors of economics, system energy utilization, and CO2 emission reduction and tried to increase carbon resource utilization as much as possible and avoid the problems of CO2 capture and storage. 1460


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respectively, by independent reaction kinetics48−52 being written as an external Fortran subroutines. Radfrac columns are chosen for the DME and methanol (MeOH) distillations. They can be used for any type of multistage vapor−liquid interaction, internal cooling, as well as heating, and they assume equilibrium at the single stage. The other separation, compression, and heat exchange process models such as Flash, Sep, Heater, Heatx, Compr, Mcompr, and so on are used. For instance, the separation process is simulated by Flash and Sep models. Flash performs rigorous 2 (vapor−liquid) or 3 (vapor−liquid−liquid) phase equilibrium calculations. Flash2 can be used to model flashes, evaporators, knockout drums, and any other single-stage separators, with sufficient vapor disengagement space, Flash3 is employed to model any single-stage separator with sufficient vapor−liquid disengagement space as well as two liquid phase settling space. Sep combines inlet streams and separates the resulting stream into two or more streams, according to splits you specify for each component. We can use the Sep model to represent component separation operations. The heat exchanger models simulate the performance of heaters or two or multistream heat exchangers. We can use Heater to model heaters or coolers (one side of a heat exchanger). HeatX can perform shortcuts or detailed rating calculations for most types of two-stream heat exchangers. Compr and MCompr mainly simulate the compression process. Compr models can simulate a polytropic compressor, isentropic compressor, and isentropic turbine, and Compr calculates either the power requirement given an outlet pressure specification, or the outlet pressure given a power specification. MCompr simulates a multistage polytropic compressor, isentropic compressor, and isentropic turbine. 2.3. Description of the Novel Polygeneration System. On the basis of the reference system shown in Figure 1, which

Figure 2. Novel dual gas polygeneration system with CO2 recycle (stream A, light components from distillation tower; stream B, unreacted gas).

analysis), the running conditions, and technology parameters chosen in the system can all refer to the previous work.40 Figure 3 is the flow diagram of the novel polygeneration system. As illustrated in Figure 3, coal is gasified at about 1300 K with steam and oxygen agents; the oxygen is from the air separation unit. High temperature flue gas is fed into a waste heat boiler to vaporize the supply water and to produce the saturated steam for power generation system. After cooling and removal of particulate matter, the cooled gas (about 775 K) enters the high temperature desulphurization unit. H2S and COS are first reduced to the concentration of about 70 ppm in the thickdesulfurization tower,46 and then, the rest of the H2S and COS are removed in the fine-desulfurization tower, in which the concentration of (H2S + COS) in the clean gas is reduced to below 1 ppm,47 which can effectively avoid the adverse effects of sulfur on the reforming catalysts and synthesis catalysts in the subsequent process. Next, the clean gas mixed with purified coke oven gas (COG) and part of the recycled CO2 enters the reforming unit where the CH4/CO2 reforming reaction takes place. The energy for the reaction is provided by the direct combustion of partially reacted gas in a furnace called the reforming auxiliary unit (RAU) in this paper. The clean syngas coming from reforming unit is compressed to 6.5 MPa and then sent to DME synthesis unit as feedstock. After going through the synthesis reactor, the chemicals are separated out from the unreacted gas. The chemicals separation process uses three distillation towers. In the first tower, CO2 is separated out; and then DME and MeOH are separated out in the second tower and third tower, respectively. The separated CO2 (95 wt %,) is partly recycled into the gasifier and partly recycled to the reforming unit. Part of the unreacted syngas is compressed and sent back to the synthesis reactor; the rest of the unreacted gas is divided into two parts: one part as fuel is sent to the RAU for supplying the energy reforming unit required, and the other part is sent to the power generation subsystem (combined cycle). The power generation system supplies the power and the steam that are required in the former processes.

Figure 1. Dual gas polygeneration system as the reference system (stream A, light components from distillation tower; stream B, unreacted gas).

is at a pilot scale, a novel system is proposed in Figure 2. Coal properties data (such as elemental analysis and proximate 1461


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Figure 3. Flow diagram of the novel polygeneration system.

Table 1. Operation Conditions and Technology of Key Units key unit gasifier desulfurization

CH4/CO2 reforming reactor

DME synthesis reactor

gas turbine

operation condition temp./°C pressure/MPa temp./°C pressure/MPa desulfurization efficiency/% temp./°C pressure/MPa SV/L·h−1·kgcat−1 temp./°C pressure/MPa SV/L·h−1·kgcat−1 compression ratio firing temp./°C exhaust temp./°C

type 1050 0.51 500 0.1−0.8 >99.8 900 0.1 3000 260 6 5000 15.7 1288 604

ash-agglomerating fluidized bed gasifier Fe2O3/SiO2/Al2O3 (thick desulfurization) and ZnFe2O4/SiO2/Al2O3 (fine desulfurization) Ni/La/Al2O3

CuO/ZnO/Al2O3 (methanol synthesis catalyst) and γ-Al2O3 (methanol dehydration catalyst) GE-7FA

polygeneration system is a system with multiple processing units, exergy efficiency is adopted to evaluate the performance of the polygeneration system to better reflect the system energy efficiency.53−55 Exergy efficiency is defined as follows:

The operation conditions and technology of key operation units are list in Table 1. Compared with the reference system, the most distinguishing feature of the novel system is the CO2 recycle scheme shown in Figure 2. In Figure 1, it is clear that CO2 is mainly from stream B. However, comparing with stream B and stream A, the composition percentage of CO2 in stream A reaches 95 wt %, and the mass flows of stream A account for a third of stream B. That is why CO2 from stream A can be recycled without further processing.

η (%) =

out EXch + EX pout × 100 in EX fuel

(1)

out where EXch and EXpout denote exergy of the chemicals output in and exergy of the output work, respectively, and EXfuel denotes the input fuel total exergy. η represents the exergy utilization efficiency of system. The calculation of exergy can be found in Hinderink’s paper.56

3. THEORETICAL METHODS AND CALCULATIONS The system simulations and analysis are carried out by using Aspen Plus 11.1 and Aspen Icarus 11.1 software. Because the 1462


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Because the polygeneration system is a system with multiple outputs, it is difficult to evaluate the system performance by conventional criteria like efficiency. So, the energy saving ratio (ESR) is adopted to evaluate the performance of the polygeneration system, which is defined as follows:28,53

ESR =

Table 2. Boundary Conditions in the Dynamic Economic Evaluation

(Q P + Q C) − Q PL QP + Q C

(2)

where (QP + QC) represents the total energy input for the electricity generation and the methanol production and QPL represents the energy input for the polygeneration system. Thus, the term ESR represents the energy saved in the polygeneration system compared with that saved in the single product systems when they produce the same outputs. Chemical products of the system are mainly DME and MeOH; therefore, the effect of the system on the environment is estimated by eqs 3 and 4.

⎡ (F ⎤ CH3OH + 2 × FDME)out ⎥ × 100 f (%) = ⎢ ⎢⎣ (FCO + FCO2 + FCH4)in ⎥⎦

(3)

⎡ (F ⎤ CH4 + FCO2)in − (FCH4 + FCO2)out ⎥ × 100 Y (%) = ⎢ ⎢⎣ ⎥⎦ (FCH4 + FCO2)in

(4)

n

=0 (5)

t=0

where (CI − CO)t denotes net cash flows of the year t and n represents the calculation years. If IRR ≥ i, it is feasible in economy performance and the system can bring profits; if not, it is infeasible. The annual profits can be calculated by eqs 6 and 7.

AP = CAPEX × CRF + COPEX + CM + CE + CP + Co − CPS

CRF =

(6)

i 1 − (1 + i)−N

value 45 0.06 12 300 15 3 33 90% designed capacity 0.045 700 400

4. RESULTS AND DISCUSSION Effects of the option of CO2 recycle technology on system will be presented and discussed in section 4.1. Then, a comparison of systems in the respects of energy utilization, CO2 emission, and economy benefits in the whole production procedures from coal exploiting to final products use will be made in section 4.2 to investigate the effects of CO2 recycle on the system performance. 4.1. Sensitivity to the CO2 Recycle. To investigate the effect of the CO2 recycle on the output gas composition of the gasifier, a sensitivity study of parameter β has been performed. Parameter β denotes the mass flow of CO2 recycle back to gasifier over the mass flow of coal. As it can be seen from Figure 4, keeping other inlet gas flows constant while varying inlet CO2 flow, with the increase of parameter β, CO2 recycled to the gasifier as the gasification agent promotes the Boudouard reaction (C + CO2 → 2CO), in which a part of the CO2 is converted into CO. Meanwhile, CO2 will also restrain the water−gas shift reaction (CO + H2O → CO2 + H2), so the concentration of CH4 keeps nearly constant, and H2 decreases while CO and CO2 increases. These results coincide with similar research.32,35 However, if the value of β is too large, the output gas composition of the gasifier will be changed a lot, and the H/C ratio of the gasified gas will decrease. In that case, the COG flow and other conditions of the system should also be changed to satisfy the production, which increases the complexity and cost of the whole system. Further, with more CO2 recycled to the gasifier, the gasifier temperature will decline, and lower temperature decreases the amount of gasification. Therefore, considering the gasification performance and the whole system technology integration, CO2 should be controlled in a range according to the system production features. In this study, the ratio of the mass flow of all the separated CO2 from the distillation unit to the mass flow of coal is 0.244. From Figure 4, when β is 0.244, the output gas composition of the gasifier with CO2 recycle back to the gasifier is as follows: CO 21.96%, H2 32.77%, CO2 20.71%, and CH4 1.09%. Without CO2 recycle, the corresponding composition is as follows: CO 22.76%, H2 31.4%, CO2 22.16%, and CH4 1.01%. It is clearly shown that the system with CO2 recycle back to the gasifier has little effect on the output gas composition of gasifier. Therefore, the whole system conditions will not be changed when the CO2 recycle scheme is adopted. The two CO2 recycle technologies have different effects on the polygeneration system. To find the best proportion of recycled CO2 with these two technologies to satisfy the optimal performance of the whole system referring to energy,

f and Y denote system carbon element utilization and CO2 + CH4 conversion yield, respectively; FCH3OH and FDME denote products outflow rate; (FCO + FCO2 + FCH4)in represents CO, CO2, and CH4 inflow rate of system; (FCH4+FCO2)out represents CO2 and CH4 outflow rate of synthesis unit. f not only expresses carbon element utilization but also reflects CO2 emission indirectly. So, both f and Y can be used as criteria for CO2 emission reduction capacity.41 The internal rate of return (IRR), as an important economic evaluation method, is applied in this paper. The IRR is defined as the discount rate that equates the present value of the project’s future net cash flows with the project’s initial cash outlay. It can better reflect the dynamic economic benefit of one enterprise. The calculation can be expressed as

∑ (CI − CO)t (1 + IRR)−t

items coal price/US$·t−1 COG price/US$·m−3 discount rate/% run time/d·y−1 depreciation life/y construction life/y income tax rate/% production capacity feed-in tariff/US$·kWh−1 DME price/US$·t−1 MeOH price/US$·t−1

(7)

Additionally, some important boundary conditions should be set at the beginning. As it can be seen from Table 2, the other conditions, such as operation cost, maintenance cost and so on, are set according to the book and the current market.57 1463


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Figure 4. Effect of mass flow ratio of CO2 recycled to gasifier on gasified coal gas composition.

Figure 5. Effects of CO2 recycle ratio on the system performance.

CO) increase, the energy consumption of the subsequent processes such as compression, chemicals synthesis, and separation will be increased due to the increase of handling capacity. The increased chemical energy in the chemical production cannot compensate the energy consumption of the system due to the CO2 recycle. So, the system energy utilization decreases with the increase of λ. Besides, the annual profit (above US$170 M·y−1) of the system with CO2 recycles is higher than that (US $120.2 M·y−1) of the system without CO2 recycle. More CO2 recycle will increase the amount of CO2 in the syngas, and syngas rich in CO2 is not conducive to producing DME. Although the total amount of chemical products increases, MeOH increases and DME decreases due to CO2 recycle use. While the price of MeOH is lower than that of DME, the chemical production sales decrease on the whole. Meanwhile, net power output will decrease as a result of CO2 recycling that

environment, and economy, we introduce another sensitivity variable, λ, to investigate the effect of the CO2 utilized proportion on the system. λ is the mass flow of CO2 recycled to the reforming unit over the mass flow of all separated CO2 from distillation unit. Figure 5 shows the influence of the λ on the system. With the increase of λ, more CO2 will take part in the reforming reaction (CO2 + CH4 → 2CO + 2H2; ΔH = +247 kJ·mol−1) as feed gas, more CO2 will be converted into CO, which will be consumed in the DME and methanol synthesis, and finally, more chemicals will be produced. Most of the carbon from the input coal is stored in the form of chemicals, and the system realizes reduction in CO2 emission effectively. However, the reforming reaction is an endothermic reaction. When more CO2 is recycled to the reforming unit and reacts, the energy loss in the reforming process will be increased correspondingly. Furthermore, with the effective gas (H2 + 1464


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Figure 6. Evaluation of the reference system without CO2 recycle.

Figure 7. Evaluation of newly designed polygeneration system with CO2 recycle.

paper. Figures 6 and 7 show the detailed exergy distribution among the different production processes, and the numbers in the figures express the proportion of the system energy input. In Figure 6, the energy loss in coal exploitation and coal transportation process is 1.0%. In the coal conversion process, coal reacts with steam (2.2%) and O2 (0.9%) in the gasifier, and it is 7.1% exergy destruction in the gasification process. The O2 comes from air separation unit, which results in about 1.9% exergy loss. After being desulfurized (0.3%), the clean gasified gas (20.5%) mixed with COG (71.6%) is sent to the reforming

converts more H2 + CO into DME and MeOH. So, the annual profits decline slightly with the increase in λ. Therefore, considering the overall system performance, λ is selected as 0.6 in this article for the system study, which means that 60% CO2 recycle to the reforming unit and 40% CO2 recycle into the gasifier will bring better performance to the system. 4.2. Energy, Environmental, and Economic Analyses of Systems. To evaluate the system and find its potential for energy use and conversion processes, energy, environmental, and economic analyses of the system have been included in this 1465


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unit where the CH4/CO2 reforming reaction takes place. The energy for the reaction is provided by the direct combustion of partly unreacted gas (6.1%) in the RAU. It is about 7.2% exergy destruction in the reforming process. The output gas of reforming unit with appropriate H2/CO ratio then is sent to DME synthesis unit as feedstock, which results in 9.9% exergy destruction in DME synthesis and separation. All steam (10.9%) produced in the above process is sent to power generation. After going through the synthesis reactor, the chemical products (41.5%) are separated out from the unreacted gas, and about 9.4% as liquid fuels are directly used for transportation, the remainder (31.2%) is for industrial use. Unreacted syngas (31.4%) is divided into two parts: one part (6.1%) is sent to the RAU, and the other part (25.3%), as fuel gas, is sent to the power generation subsystem (combined cycle). The exergy destruction in the power generation subsystem is 12.7%, and the exhaust and steam water also takes 4.3% exergy away. 7.2% of the energy input goes to the power generation system itself. The other losses of the system sum to about 5.3%, including exhaust gas, wastewater, and so on. Compared with Figure 6, the most obvious difference in Figure 7 is the coal conversion and utilization processes. In terms of CO2 emission, with the same input, CO2 emission of the two polygeneration system is the same (33.2 kt·y−1) in the subsystems of coal exploitation and transportation. However, in the coal conversion and utilization processes, the CO2 emission of the reference system is 745.5 kt·y−1, which is 255.5 kt·y−1 more than that of the novel system. Compared with the reference system, production transportation and utilization caused a CO2 emission of more than 50.5 kt·y−1, but the novel system can reduce emission by about 200 kt·y−1 over the whole life cycle. In respect to energy use, CO2 (3.4%) coming from the separation unit is recycled to the gasifier and reforming reactor. These have been greatly changed as a result of the adoption of the CO2 recycle scheme. The exergy destruction of the novel system in the reforming process and synthesis process increases 1.4% and 2.6%, respectively; combustion exergy destruction decreases from 12.7% to 7.8%, while the chemical exergy in chemical production increases from 41.5% to 53%; the new system contributes 8.4% power output from the system itself to realize the 11.5% chemical energy increase, 33.8% CO2 emission reduction, and the system total effective energy increase from 54.5% to 57.6%. Because the CO2 concentration in exhaust gas from the distillation tower is more than 95 wt %, this system does not require a CO2 separation unit. The increased chemical exergy (11.5%) in chemical production and the exergy decrement (4.9%) of combustion in the gas turbine can almost compensate most of the system exergy loss (4.9%) and power output decrease (8.4%) due to CO2 recycle. So, the system can keep thermal efficiency as high as that of the reference system, and this result is the same as Yuso’s conclusion.39 The detailed comparison of exergy distribution between the reference system and novel system is listed in Table 3. Further, it is easy to find that the overall energy efficiencies of the two polygeneration systems are about 52.6% and 55.5%, respectively, which is 10−12% higher than that of the individual systems in Table 4. With the same output, the energy saving ratios based on IGCC and DME system (ESR1) of the two polygeneration systems are 13.2% and 16.4, respectively, and the energy saving ratios based on IGCC-CCS and DME system (ESR2) of the two polygeneration systems are both 18.0%. Compared with the reference system, ESR1 of the novel system

Table 3. Exergy Distribution of the Reference System and Novel System reference system exergy (MW)

novel system

ratio/%

exergy (MW)

ratio/%

300.3 757 9.5 23.3 11.6

28.4 71.6 0.9 2.2 1.1

20.1 64.5 13.7 90.9 108.9 23.3 59.2 23.3 65.6

1.9 7.3 1.5 8.6 10.3 2.2 5.6 2.2 6.2

48.6 560.4 609

4.6 53 57.6

9.5 1.1 11.6 586.8

0.9 0.1 1.1 55.5

Input coal COG O2 steam make-up water

300.3 28.4 757 71.6 9.5 0.9 23.3 2.2 6.3 0.6 Internal Loss 20.1 1.9 55 6.6 10.6 1.3 76.1 7.2 92 8.7 12.7 1.2 96.2 9.1 38.1 3.6 89.9 8.5 Internal Output 137.4 13 438.8 41.5 576.2 54.5 External Loss 9.5 0.9 1.1 0.1 9.5 0.9 556.1 52.6

air sep. unit coal gasification clean up reforming and RAU synthesis distil. unit gas turbine steam turbine/HRSG waste and other loss power output chem. output total output coal exploit. coal transport. prod. transport. net total output

Table 4. Comparison of Performances between Single Production System and Polygeneration System sys. for single production IGCC

IGCCCCS

coal COG

315.1

Input/MW 387.1 902.9

3259.5

power equiv. DME external consum./MW energy eff./% ESR1/% (IGCC and DME sys.a) ESR2/% (IGCCCCS and DME sys.)

137.4 16.7

Outputs/MW 137.4 438.8 24.8 47.8

1584.3 172.7

38.3

29.1

43.3

a

DME sys.

43.3

polygeneration sys. reference sys.

novel sys.

304.7 752.6

861.4 2127.8

137.4 438.8 20.1

137.4 1584.3 62.8

52.6 13.2

55.5 16.4

18.0

18.0

DME is the only product.

is 3.2% higher than that of the reference system, though the ESR2 is the same in the two polygeneration systems. The novel system has a much better overall performance, which can reduce about 22% CO2 emission according to the previous analysis. Additionally, the novel system shows good economic performance. The comparison of different system performances is shown in Table 5. To have a better comparison, the whole system is divided into two parts: the external part and the internal part. The external part includes coal mining and transportation; CO2 capture, storage, and transportation; 1466


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mental, and economic analyses of the system, the scheme using CO2/CH4 reforming technology and CO2 recycle to the gasifier and reforming unit could increase the carbon conversion. The novel system achieves CO2 conversion by means of consuming a part of the energy of the system itself and avoids the conventional water−gas shift reaction, CO2 separation and capture, and some other problems of CO2 subsequent handling, as well. In the end, it realizes maximization of resource utilization. This technology, which realizes the integration among energy utilization, CO2 emission, and economic benefits, is highly instructive for development of new CO2 emission control technology.

chemical products transportation; and so on. The internal part is the process for coal to produce the chemical products and Table 5. Comparison of Performances among Different Energy Systems items coal input/t·d−1 COG input/ × 106 m3·d−1 DME output/t·d−1 MeOH output/t·d−1 net power output/MW CO2 emission/kt·y−1 energy consum./% cost/US$ M·y−1 CO2 emission/kt·y−1 energy consum./% cost/US$ M·y−1 total capital invest./US$ million variable and fixed invest./US$ M·y−1 annual sales/US$ M·y−1 IRR/% total energy eff./% element utilization/% CO2 emission/kt·y−1 CO2 emission/kg·kgcoal−1

IGCC− CCS

reference sys.

novel sys.

2000

2000

273.8 External 60 5.3 38.5 Internal 1695.3 56.4 106.7 Total 420.2

223.3

1107 4.1 984.3 754.8 137.4

1107 4.1 1314.4 912.4 48.6

60 6.4 73.4

208.2 1.9 27.9

258.7 2.1 31

151.8 64.5 125.6

754.5 45.5 227.3

499 42.4 241.6

506.7

488.8

521.4

145.2

182.9

255.2

272.6

112.3 1.4 38.3

95.3

1755.3 1.74

211.8 0.21

300.4 16.8 52.6 53.8 962.7 0.95

361.5 18.1 55.5 69.4 724.5 0.75

IGCC

29.1

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 351 6018957. Fax: +86 351 6018453. E-mail: fengjie@ tyut.edu.cn (J.Feng); [email protected] (W.Y.Li).

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (21076136), consultation projects of Chinese Academy of Engineering (2011-ZD-7; 2011-ZD-7-11-2), and Shanxi Returned Scholarship (2010-2).

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NOMENCLATURE Capital Letters AP = the annual profits, US$ M·y−1. CAPEX = the total capital investment, US$ M. CE = the cost of coal exploitation and transportation, US$ M·y−1. (CI − CO)t = net cash flows of the year t, US$. CM = materials cost, US$ M·y−1. CO = other cost, such as sale, research, depreciation, and so on, US$ M·y−1. COPEX = operating expenditures and management cost, US$ M·y−1. CP = products transportation cost, US$ M·y−1. CPS = products sales, US$ M·y−1. CRF = the ratio of annual average investment, y−1. ESR = the energy saving ratio, %. in EXfuel = the input fuel total exergy, MW. out EXch = exergy of the chemical product, MW. EXpout = exergy of the output work, MW. F = element utilization of system, %. FCO = CO flow rate, kmol·s−1. FCO2 = CO2 flow rate, kmol·s−1. FCH4 = CH4 flow rate, kmol·s−1 FCH3OH = MeOH flow rate, kmol·s−1. FDME = DME flow rate, kmol·s−1. IRR = internal rate of return, %. N = project duration, year 0 is the first year of investment when N equals 0, y. QC = the total energy input for the chemical production, MW. QP = the total energy input for the electricity generation, MW. QPL = the total energy input for the polygeneration system, MW. Y = CO2 + CH4 conversion efficiency of system, %.

power. The external data and calculations are based on information from the National Bureau of Statistics of China and related papers,15,21,58−62 and the internal analysis and calculation is performed by Aspen Plus 11.1 and Aspen Icarus 11.1 software. As can be analyzed from the Table 5, IGCC-CCS shows the best performance in CO 2 emission (0.21 kg·kgcoal−1), but the system has no net profit and low energy efficiency (29.1%). Compared with the other three systems, IGCC has no obvious advantages in terms of energy, environment, and economy. The polygeneration system shows better overall performance. External performance of the novel system is inferior to that of the reference system in energy consumption, CO2 emission, and cost, but internal performance is superior. With the same resource input, the total capital investment of the novel system (US$521.4 million) is higher than that of the reference system (US$483.5 million). The transportation of the chemical products costs more (US$3.2 million), but the novel system sales increased US$50.9 million, carbon utilization improved 15.6%, and CO2 emissions are reduced 205 kt·y−1; even the IRR is improved from 16.8% to 18.1%. Although CO2 conversion and recycle would consume energy, the increased chemical production can not only offset this part energy destruction but also increase system sales and reduce CO2 emission, so that the system can keep high thermal efficiency and better economic effectiveness, which is the most obvious feature of the novel system.

5. CONCLUSIONS Based on commercially ready technology with CO2 recycle, we have explored the polygeneration system and propose a novel type of polygeneration system. With the aid of energy, environ-

Lowercase Letters

i = the discount rate, %. 1467


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n = the calculation years referring to net cash, y. Greek Letters

β = the mass flow of CO2 recycle back to gasifier over the mass flow of coal. η = the exergy utilization efficiency of system, %. λ = the mass flow of CO2 recycled to the reforming unit over the mass flow of all separated CO2 from distillation unit. Subscripts

in = input of system. out = output of system. Acronyms

ASPEN = Advanced System for Process Engineering. BWRS = Benedict−Webb−Rubin−Starling equation of state. CCS = carbon dioxide capture and sequence. COG = coke oven gas. Compr model = compressor/turbine. COS = carbonyl sulfur. DME = dimethyl ether. DMC = dimethyl carbonate. Flash model = flash vessel. Heater model = heater/cooler. Heatx model = two-stream heat exchanger. IGCC = integrated gasification combined cycle. LHV = lower heating value. M = million. Mcompr model = multistage compressor/turbine. MeOH = methanol. PR-BM method = Peng−Robinson equation of state with Boston−Mathias modifications. Radfrac columns = rigorous distillation tower. RAU = reforming auxiliary unit. Rcstr = continuous-stirred tank reactor. RPlug = plug flow reactor. Sep model = multioutlet component separator. TUT = Taiyuan University of Technology. US DOE = Department of Energy, the United States of America.

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