Simulation and exergoeconomic analysis of a dual-gas sourced polygeneration process with integrated methanol/DME/DMC catalytic synthesis
Introduction
Fast increasing greenhouse gas (GHG) emissions and depletion of fossil fuels have become two challenging global issues. Polygeneration is considered a promising option to tackle these challenges due to improved energy conversion efficiency and reduced environmental impacts.
A polygeneration process produces chemical products, usually synthetic liquid fuels, and electricity via integrated chemical production and power generation. A typical polygeneration process starts from gasification of a certain type of feedstock, usually coal or biomass, where crude synthetic gas, or syngas is produced. The crude syngas, consisting mainly of carbon monoxide and hydrogen, goes through a series of cleanup units, where sulphur compounds, ash and other hazardous components are removed. After the cleanup unit, clean syngas is split into two streams. One stream goes to a chemical synthesis process to produce chemical products. The flue gas, together with the other stream of the clean syngas, is fed into a power generation process to produce electricity.
Polygeneration has many advantages over conventional stand-alone processes. Firstly, due to tight integration between power generation and chemical synthesis processes, the energy conversion efficiency of a polygeneration plant is usually higher than a combination of stand-alone plants producing the same amount of products. Secondly, the power generation process and the chemical synthesis process within a polygeneration plant share a single gasifier of a larger size, and the unit investment cost can be reduced as a result of economies of scale.
Configuration of a polygeneration plant may differ from one another, depending on the specific types of feedstock available to the polygeneration plant and chemical products required by the market Li et al., 2003, Yamashita and Barreto, 2005. From previous studies, many types of polygeneration configurations have been proposed, for instance, co-production of Fischer-Tropsch fuels and electricity (Zheng, Ma, Xiao, Li, & Ni, 2002), coal-based hydrogen and electricity production Chiesa et al., 2005, Cicconardi et al., 2008, co-production of methanol and electricity Liu et al., 2010b, Liu et al., 2007, Liu et al., 2009, Liu et al., 2010a, Liu et al., 2010, and natural gas-based methanol and electricity production (Gao et al., 2008). They differ mainly in types of chemical products produced and arrangement of chemical synthesis and power generation (sequential or parallel). However, most of these approaches focus on coal gasification or natural gas reforming as the only sources of syngas, research on utilization of other sources of syngas in a polygeneration plant is rather limited.
Coke oven gas, a major by-product in the coking industry, is produced in a huge amount worldwide but not yet efficiently used as an energy resource. Currently, more than half of the world’s coke oven gas production is in China, with an annual production rate of 140 billion cubic meters (Sun, Jin, Gao, & Han, 2010). Approximately only 50 percent of the coke oven gas is burned to produce heat in the coking industry, while the remaining is either emitted to the atmosphere directly or via a simple burning device. It is a waste of resources from an energy conversion viewpoint, and it is also not environmentally friendly due to the huge amount of GHG emissions.
Coke oven gas mainly consists of hydrogen, methane, and carbon monoxide, and these components can be easily converted to syngas in a reforming unit, making it an ideal syngas source for a polygeneration plant. This provides a great opportunity to improve the current inefficient way of coke oven gas utilization. In this paper, we present a novel polygeneration process design featuring using coke oven gas and coal gasification gas as gas sources and producing three chemical products in an integrated catalytic synthesis procedure. Usually, a polygeneration plant is equipped with a water-gas shift reactor to adjust the mole composition of syngas, but the polygeneration process proposed in this work uses a reformer to perform the mole composition adjustment. In the reformer, coke oven gas and coal gasification gas react in a series of oxidized reforming reactions, and syngas with appropriate mole composition is produced for downstream chemical synthesis. The reforming procedure makes good use of methane-rich coke oven gas and carbon dioxide-rich coal gasification gas to adjust the mole composition of the syngas within the same procedure. Three types of chemical products, namely methanol, dimethyl ether (DME), and dimethyl carbonate (DMC) are synthesized in an integrated catalytic synthesis procedure. This is also different from most polygeneration designs, where usually only one type of chemical product is produced. The integrated catalytic synthesis procedure is established based on a terraced synthesis mechanism, where different chemicals are synthesized according to changes in the mole composition of reactant gases.
This paper is organized as follows. First, configuration of the proposed dual-gas sourced polygeneration process with the integrated catalytic synthesis procedure is presented. Secondly, detailed mathematical models of the chemical synthesis reactions are provided, based on which a process simulation is conducted. After that, results of an exergy analysis are provided, and energy conversion efficiency of the proposed process is illustrated. Finally, the methodology of an exergoeconomic analysis is illustrated, followed by results of the exergoeconomic analysis and an exergy cost distribution diagram, which indicates economic losses over each functional block in the proposed process.
Section snippets
Process configuration
An illustrative process configuration of the proposed dual-gas sourced polygeneration process with integrated catalytic synthesis is shown in Fig. 1, where CG represents coal gasification gas, COG represents coke oven gas, TG represents tail gas, GTCC represents gas turbine combined cycle, ME and MeOH represent methanol, and PSA represents pressure swing adsorption. Here, the purpose is to illustrate the major difference of the proposed process from existing polygeneration processes, thus the
Mathematical model and process simulation
The proposed polygeneration process is simulated using Aspen Plus (Aspen Technology, 2010). Detailed mathematical models based on chemical kinetics of methanol, DME, and DMC catalytic synthesis Ng et al., 1999, Wang et al., 2000 have been developed for the corresponding synthesis reactors. Sub-models for the three synthesis reactors are presented next.
Exergoeconomic analysis
The proposed polygeneration process produces three types of chemical products, therefore it is necessary to develop a quantitative pricing system to allocate production costs amongst these products. Methodologies of exergoeconomic analysis (Lozano & Valero, 1993) are used in this work to quantify economic losses over each functional block on an exergy basis. These methodologies have been successfully applied in co-generation plants (Alvarado & Gherardelli, 1994), general thermal systems (Kim,
Conclusions
A process configuration of a polygeneration system is proposed in this paper, featuring the utilization of coal-derived syngas and coke oven gas, and production of methanol, DME, and DMC via an integrated catalytic synthesis procedure. The proposed polygeneration system provides a great opportunity for more efficient and cleaner utilization of coke oven gas. Detailed mathematical models for the chemical synthesis reactions are developed, based on which a process simulation is carried out on
Acknowledgement
Financial support from BP company (BP-Tsinghua University Co-operation - Phase 2) is gratefully acknowledged.
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