Thermal Estimation of 300 MW IGCC Processing System

Transcription

1 Thermal Estimation of 300 MW IGCC Processing System Su-Yong Jung, Hyung-Taek Kim Division of Energy System, Graduate School, Ajou University, Suwon , Korea ABSTRACT: A computer simulation model in ASPEN PLUS has been developed to simulate the performance of IGCC (Integrated Gasification Combined Cycle) power plants, which is imminent to be founded in Korea. The simulative model was used to study the effects of design and thermal parameters on the efficiency and emissions from IGCC. In the model, entrained-flow gasification process was adapted. Some typical coals, such as Illinois No. 6 (USA), Shenhua and Zhongmei(China), and Roto (Indonesia), were fed into the gasifier respectively. Performance of the thermal process estimation was compared simultaneously, which specially included in heating values of raw gas and the efficiency of gasifier and whole IGCC power plant. Furthermore, the operational data of IGCC power plant were employed to verify the simulation results. From the comparison results, it can be concluded that the simulation model using ASPEN PLUS is valid and accurate. From heating values of raw gas and gasifier efficiency point of view, Shenhua coal was the best choice. Based on the plant efficiency, Zhongmei coal was favorite to Korea. Keywords: IGCC, ASPEN PLUS, Gasification, Efficiency INTRODUCTION IGCC is a technology that generates electric power using coal gasification and gasified fuel. Carbon conversion value of IGCC is higher and the influence on the environment is lower than the conventional pulverized coal power plant. Especially, in some nations like in Korea, where the weight of fossil fuel for power generation is remarkably high ( about 50%), IGCC stands out as an alternative plan to cope with sudden limitation for the emissions. Accordingly, IGCC plant is constructing in Korea currently. IGCC power plant system was composed by gasification process, desulfurization process, combined generation process. Figure 1 shows a simplified IGCC plant flow diagram. A design for IGCC plant was complex for many specifications, such as what's the type of gasifier and gas turbine, what's the fuel, is there CO 2 capture, is necessary to adapt the Air Separate Unit (ASU), etc. In this presented paper, it focused on heating value of the coal gas, gasifier and plant efficiency for each coal type, which may be used to trace system thermal performance and provide insights into components with the IGCC cycles, and act as pointers to system optimization trade-offs. Raw gas Figure 1 Simplified IGCC power plant flow diagram without CO 2 capture To whom all correspondence should be addressed ( htkim@ajou.ac.kr)

2 From Fig.1, it can be seen that analysis of IGCC power plants is complicated due to the large number of units involved, interaction between the units and presence of streams of diverse compositions and properties. The efforts needed to evaluate the performance and economic implication resulting for various options and a wide range of design and operating conditions for each piece of equipment require extensive computation of material and energy to conduct fast, simply and with accuracy, and optimization of these systems. Owing to the complexity of these systems and possibility of different equipment types and configurations thereof, a modular approach which allows independent development and testing of different sub-systems before integration is more suitable to model these systems. In this paper IGCC model developed within the ASPEN PLUS and simulation results from these models are presented. The model includes realistic representation of the various units used in commercial power plants reflecting pressure drops and characteristic temperature differences in heat transfer components. Mass and energy balances are constructed in ASPEN PLUS for each component using current practices and constraints. SIMULATION IN ASPEN PLUS ASPEN PLUS environment provides a flexible input language for describing IGCC plant components, connectivity, and computational sequences. Use of ASPEN PLUS leads to an easier way of model creation, maintenance and updating since small sections of complex and integrated systems can be created and tested as separate modules before they are integrated. It has an extensive physical property data base where the diverse stream properties required to model the material streams in an IGCC plant are all available with an allowance for addition of inhouse property data. Additionally, ASPEN PLUS has many built-in model blocks (such as heaters, pumps, stream mixers, stream splitters, compressors etc.), some of which can directly be used in power plant simulation. Where more sophisticated block ability is required, additional information may be added to the block in the form of FORTRAN subroutines, or entirely new user blocks may be created. In this work, a number of new blocks (e.g. turbine, compressor, combustor, etc.) were developed as the built-in blocks were found not to be sufficiently detailed to conduct accurate simulations. Thus, ASPEN PLUS was mainly used to model the stream connectivity and to provide the material property data. For these purposes, ASPEN PLUS is an excellent modeling tool which is versatile and relatively easy to use in modelling of advanced power cycles. ASPEN PLUS also incorporates an integrated costing and economic evaluation system. Using this feature, equipment size and cost, as well as plant cost and profitability analyses can be made. Inclusive are the tools to help the user to override the default base cost of major process equipment and the ability to estimate certain important factors from historical cost data. The next sections briefly discuss how ASPEN PLUS and the model blocks developed to simulate the performance of various key power plant components are used to simulate IGCC cycles. SIMULATION INPUT As a gasifier fuel, coal vary in many properties, such as its heating values, proximate analyses (Fixed carbon, volatile materials, ash content and moisture content), ultimate analyses (amounts of carbon, hydrogen, oxygen, sulfur, nitrogen, chloride and other impurities) and sulfur anlyses (type of sulfur present). It should be noted that, the type of coals imported to Korea are so complex, rangeing from lignite with approximate volatile matter of 24% to anthracite with an average of 5%. Majority of coals are belonged to subbituminous, the compositions of four typical coals were compiled in Table 1. Table 1 Four kinds of subbituminous coals composition Ultimate analysis(wt% db) Proximate analysis(wt% db) HHV C H O N S A FC VM Ash Mo MJ/kg Shenhua Zhongmei Illinois No Roto Feed (kg/hr) Table 2 Feeding amounts for the case of coals Coal (as received) Coal (dry) Oxidant (95% O 2 ) Nitrogen

3 For dry coal feeding system, coal is crushed to small size to feed into the gasifier. The particulate coal is carried and dried by nitrogen, without entering gasifier, the nitrogen is recycled back and then to the reclaim pile. It is the oxidant (95% O 2 ) that combusted with coal in the gasifier. Table 2 shows feeding amounts for the type of coals in the 300MW gasifier injecting system. From Table 2, it can be seen that Roto coal need much amount of nitrogen due to its higher moisture to be dried than others. In fact, the water content of coal, is also affected on the heating value of raw gas, it will be explained infra. The gasification model in ASPEN PLUS is assumed for this study is that of entrained-bed gasifier. The gasifier can operate at high pressure (5.5MPa) and the main factor is, the temperature of gasifier should be above melting temperature of ash. Melting temperature of ash was different according as type of coal. This paper determined gasifier temperature is about 1450 C. In the gasification model, the composition of raw gas calculated by equilibrium equations using Gibb's free energy minimization method. Hot raw gas from gasifier is cooled in radiative and convective heat exchanger. The waste heat from this cooling system is used to generate high-pressure steam. The cooling raw gas was routed into cyclone and filter sequentially. In the cyclone and filter, particulate in the raw gas is removed. Then the raw gas is mainly composed of CO, H 2, CO 2, N 2, CH 4, H 2 S and H 2 O etc. It was absorbed in desulfurization unit. Desulfurization process is working at low temperature. Acid gas from the regenerator, which includes that removed in the concentrator and the tail gas unit, is sent to the Claus plant. Tail gas from the Claus unit contains unreacted sulfur species such as H 2 S, COS, and SO 2 as well as elemental sulfur species of various molecular weight. In order to maintain low sulfur emissions, this stream is processed in a tail gas treating unit to recycle sulfur back to the Claus plant. Sweet gas passed through Claus unit was extinguished in the combustor of gas turbine. The combustion turbine selected for this application is based on the General Electric Model 7FA. This machine is an axial flow, constant speed unit, with variable guide vanes. Hot combustion products are expanded in four stage turbineexpander. Then exhaust gas is injected into HRSG (heat recovery steam generation). The HRSG thermally couples the waste heat rejected by the gas turbine and gasifier island with the steam turbine. The Rankine cycle used in this case is based on a commercially available 12.4 MPa/565.6 C/565.6 C single reheat configuration. The steam turbine is assumed to consist of tandem high-pressure (HP), intermediate-pressure (IP), and double-flow low-pressure (LP) turbine sections connected via a common shaft (along with the combustion turbine) and driving a 3600 rpm hydrogen-cooled generator. SIMULATION RESULTS AND DISCUSSIONS The results of a series of IGCC simulations focusing on effects of coal types and adjustable design parameters are presented and discussed. These analyses are also useful in illustrating the capabilities of the models. Tables 3 shows the composition of raw gas fed with these four type subbituminous coals. Table 3 Raw gas composition from subbituminous coals CO H Composition of CO Raw gas (mol %) N CH H 2 S+COS For the verification of simulation results, they were compared with the operational data for Illinois No. 6. Figure 2 shows comparison of raw gas compositions between simulated results and operational data. From Figure 2, it can be seen that simulation results and operational data are inosculated very well. Therefore, the primary conclusion can be reached that this simulative method using ASPEN PLUS is right and accurate. It can be used to estimate the performance of IGCC power plant. The gasification efficiency are determined for a gasifier as well as a gasification system. Figure 3 shows the gasifier efficiency and heat values (HV) of coals and raw gas. For gasifier, shown in Figure 3, the thermodynamic efficiency is defined as the energy increases of the rawgas divided by the energy decrease of the subbituminous coals. It can be seen from Figure 3, that the gasifier efficiency, three type of coals (Shenhua, Zhongmei, Illinois No. 6) were similar magnitude (about 75%) except Roto coal (68.12%). This phenomenon can be explained that Roto coal has much amount of water content, the water

4 of coal content is reduced to heating value of raw gas fired by Roto. Accordingly, the gasifier efficiency of Roto coal was the lowest CO H2 CO2 N2 CH4 H2S+COS Simulation Results Raw gas compositon (%) Operational Data Figure 2 Comparison of raw gas compositions between simulated results and operational data Heat Value (MJ/kg) Input of Coal Raw gas Gasifier Efficiency Gasifier Efficiency (%) 15 Shenhua 65 Zhongmei Illinois NO.6 Roto Figure 3 Comparison of Gasifier efficiency and Heat values of Coal and Raw gas with four types of subbituminou coals It is interesting to find that, simplex considering the gasifier efficiency (see Figure 3), shenhua coal has obtained the highest level. The second feedstock to favorite gasifier is Zhongmei and Illionis No.6 coals respectively. To maintain high efficiency of gasification, Roto coal is an unadvisable to be imported in Korea. The clean gas passed through desulfurization process is routed into the combustor of gas turbine, the fuel gas is combusted in 12 parallel combustors. NOx formation is limited by geometry and fuel gas dilution. The combustors are can-annular in configuration, where individual combustion cans are placed side-by-side in an annular chamber. Hot combustion products are expanded in the four-stage turbineexpander. It is assumed that the first two expander stages are steam cooled and that the third stage is air cooled. No cooling is expected in the fourth expander stage. The expander exhaust temperature is estimated as 568 C, which is 26 C lower than the ISO assumed value of 594 C for a natural gas-fired simple cycle gas turbine, is due to variations in firing temperature, flow rate, and flue gas specific heats. The exhaust gas compositions of four type coals are given in Table 4. Table 4 Compositions comparison of Exhaust gas out of the gas turbine with four type coals N O CO Composition of Flue H Coal (mol %) 2 O AR SO ppm NO Mass Flow rate of Flue Gas (kg/hr)

5 From Table 4, it can be seen that all the four type of coals, they have similar composition of exhaust gas from gas turbine. N 2 is the most important contributor in the component. It can be explained that, as inertia gas, the role of N 2 is to enhance the power output of gas turbine and avoid reacting with other chemical component. Attention should be paid on the CO2, CO2 emissions are high as would be expected from a coal plant of this IGCC power output. Also, the inclusion of CO2 removal system will be peremptorily employed in IGCC system. Table 5 shows evaluations output of oxygen-blown entrained-bed IGCC plant. Focused on the plant efficiency, all type of coal (Shenhua, Zhongmei, Illinois No. 6, Roto) are obtained more than 40%. It can be testified the advantage of IGCC power plant than conventional power plant. Table 5 Evaluations of oxygen-blown entrained-bed IGCC plant for coals Coal Energy Input (HHV, MW) Gas Turbine Power (MW) Steam Turbine Power (MW) Auxiliary Power Needs (MW) Net Power Output (MW) Plant Efficiency (%) Table 5 shows that relationship between the thermal efficiency of the IGCC increasing with the bottoming Rankine cycle and energy input from four types of subbituminous coals. Based on the quantity of IGCC power plant efficiency, Zhongmei is slightly better than others subbituminous coals. CONCLUSION Shenhua, Zhoumei, Illinois No.6 and Roto, four types of subbituminous coals were introduced in 300MW IGCC simulation model. Based on the comparative results of heating value of raw gas, efficiency of gasifier and plant, some conclusion have been reached: (1) Being as an commercial software, ASPEN PLUS is aptitude to be used to estimate the thermal performance of IGCC power plant. (2) Based on the gasifier efficiency, the gap of efficiency between Shenhua and Roto coals was more than 9%. The difference is caused by the water of coal content. It also affect on the heating value of raw gas, corresponding values of the two types of coals was more than MJ/hr. Therefore, Shenhua coal is reasonable choice to be imported to Korea. (3) Concerning about the IGCC plant efficiency, it is hard to distinguish which one of the four types of coals plays an significant role. But talking about small different, IGCC plant efficiency fed with Zhongmei coal obtains a higher value due to its carbon portion was higher than other coals. REFERENCES 1. Chris Higman and Maartem van der Burgt: "Gasification", (2003) 2. U.S. Department of Energy/NETL: "Evaluatin of innovative fossil fuel power plants with CO 2 removal", Yun-Kyoung Lee: "A study on the thermal designs of 300MW-class IGCC plant", Korea Electric Power Research Institute (2002) 4. Seung-Jong Lee: "Performance Evaluation of IGCC plants with variation in coal rank and coal feeding system", Institute for Advanced Engineering and Department of Systems Engineering (1997) 5. Ph. D. chae: "Combustion Engineering", (1994)