Carbon chain analysis on a coal IGCC — CCS system with flexible multi-products

Abstract

A carbon chain analysis is applied to assess a complex energy conversion system with CO₂ capture and storage (CCS). A coal integrated gasification combined cycle, with CCS, which co-produces electricity and liquid fuels (IGCC–LF–CCS) is taken as a case study. A process simulation method is used to estimate the technological data, and balance the heat and electricity for the whole system. Carbon and energy flows are calculated to evaluate the mass conversion efficiency and the energy efficiency. The results show that in the case in which one third of the coal is allotted to synthesize liquid fuels, globally 60% carbon is captured for storage, 19% transferred to liquid fuels, 19% emitted to the atmosphere, while the remaining carbon is discharged as waste. For the energy flow, 28.1% of total higher heating value of coal is converted into the liquid fuels. The net electricity efficiency is 20.7% accounting for the power demands by air separation, CO₂ capture and compression. Three scenarios with different ratio of resource to produce electricity and liquid fuels with or without CCS have been studied. This work will provide useful information for the coal utilization with CCS in a carbon-constrained world.

Introduction

Large scale CO₂ capture and storage (CCS) has been considered as one of the most promising ways of mitigating anthropogenic CO₂ emissions and reducing the carbon intensity of the electricity sectors and other industries, like steel, cement and bulk chemical producers. It is undoubted that carbon sequestration is one of the few options that would allow the use of fossil energy without the threat of dangerously altering the earth's climate system. Even greater efforts and investments are being made by many nations to increase the share of renewable energy in the primary energy supply and to foster conservation and efficiency improvements, addressing climate change concerns during the coming decades will likely require significant contributions from CCS [1]. However, current research has also revealed a lot of pending issues around CCS, such as energy penalty [2], [3], high investments [2], [4], and unknown environmental impacts [5], [6], [7], which had become big barriers to commercialization of CCS technologies. Great efforts from both academic institutes and industrial companies have been focused on the solutions to resolve the 3E problems (high Energy penalty, high Economic costs and possible Environmental impacts) which are caused by the deployment of CCS. For example, new modes of combustion for power system, such as pre/oxy-combustion [8], IGCC [9], exhaust gas recirculation [10], membrane WGS reactor [11], [12]; energy-saving CO₂ capture solvents and technologies, such as KS-1 solvent, Econamine FG^+ SM, ionic liquids, zeolitic imidazolate frameworks, membrane etc. [3], [8], [13], [14]; and also new concepts and technologies for CO₂ transport and storage [15], as well as the key issues for scaling up from megatons to gigatons [1].

Even though CCS are new facilities without enough industrial reliability compliance test until now, the necessary components of a CCS system are in commercial available and use today. The technologies for CCS are then rather well known. However, there is no CCS industry today because the components do not currently function together in the manner required for large-scale CO₂ reduction. Therefore, one of the challenges for CCS to be considered as commercial is to integrate and scale up these components [1]. Thus understanding the behavior of complex energy conversion systems with CCS as a whole chain is important before the commercialization of the CCS technologies. This behavior includes economical, technical, environmental and social acceptance issues caused by CCS. One of the important advantages is that many opportunities of energy saving and cost lowering might be found by taking the whole system into account to retrofit the current components or optimize future facilities.

Several works have been down around the concept of chain analysis. A methodology for CO₂ chain was proposed to take the joint impact of several parameters into account and support a systematic evaluation of the potentials of a CO₂ chain. It is an effective tool to assess the behavior and provide important information for decision makers, industries, investors, the public and also the researchers engaged in the technological developments of CCS [16], [17]. The similar concepts were proposed, such as European value chain for CO₂ [18], CO₂ value chain [19], CCS chain [20]. Some applied cases have been studied, such as the value of flexibility in the CCS chain of coal-fired power generation with post combustion carbon capture [21]. Damen et al. also proposed a chain analysis method (called CCS chain) to compare energy production costs and CO₂ avoidance costs in a consistent matter, taking electricity and hydrogen production systems as a case study. The system boundary of this method was extended and both a spatial and a temporal dimension were considered. The spatial dimension encompasses the infrastructural design to connect energy extraction, conversion, and end-use markets and CO₂ sources with storage reservoirs. The temporal dimension is related to the time frame considered for implementation, i.e., relatively short term and long-term chains [22], [23]. These methods mainly expected to give the techno-economic performances along the CO₂ chain, but did not investigate the detailed carbon conversion and energy utilization along the chain inside of the system.

Considering carbon is the most important element in complete CCS chain, not only existed in CO₂, but also in raw source, fuel gasses, flue gasses, liquid fuels with different chemical valencies which show different physical and chemical performances. Thus, the flow of carbon element (direction and amount) in the whole system could be helpful for understanding the efficiencies of mass conversion and energy utilization of a complex system. Hence, in this work, a carbon chain analysis is applied to assess a complex integrated system including CO₂ production, capture, transport and storage, focusing on the efficiencies of carbon conversion and energy utilization. The advantages of the method are: 1) obtaining a more comprehensive chain from resource to CO₂ storage and utilization considering the viewpoint of the whole carbon life cycle; 2) being able to compare energy efficiency and CO₂ emission in a consistent matter as they can differ strongly among technologies and primary resources; 3) knowing the CO₂ emissions over the entire chain which give an indication on the ‘climate neutrality’ of de-carbonized electricity and chemicals/products; 4) understanding how the change of a single technology influences the whole carbon chain; 5) providing insight into the trade-off between CO₂ reduction and energy consumed. However, due to the system complexity of CO₂ emitting industries with CCS facilities, a comprehensive carbon chain analysis faces a lot of challenges, such as the definition of the integrated framework of the chain, suitable boundaries, multi-disciplinary, component assessment, technological parameters, data collection and verification, and so on.

An integrated framework consisting of four sessions (resource conversion, CO₂ capture, CO₂ transport and CO₂ storage) is constructed. The model-based method with process simulation tool is used to estimate the technological data and process parameters. Energy and carbon flow analysis based on the detailed simulation results are used to look into the performance of the whole system. A coal integrated gasification combined cycle (IGCC) process with CCS which co-produces electricity and liquid fuels (IGCC–LF–CCS) is taken as a case study. The rationale of choosing this case is as follows. As coal is a carbon-intensive but abundant primary energy, the coal industry has been focusing on the development of cleaner and less carbon intensive technologies, such as the IGCC to produce electricity [24], synthesize liquid fuels or natural gas to partly mitigate the tensions of crude oil and gas markets. However, one of the main barriers faced by a coal industry is that the CO₂ emission per kW of electricity or per kg of synthesized fuel or chemical products from coal is much higher than in scenarios using crude oil as the feedstock. Recently, the IGCC with CCS has been evaluated as one of the high potential alternatives to at the same time produce electricity and liquid fuels and capture CO₂ [3], [25], while understanding this process deeply will be important for the future mitigation of global warming in the coal industry. Another key point is that synthesizing transport fuels from coal has become a possible option to limit shortages on the oil market and have a flexible supply. For example, one of the typical technologies from coal to liquid fuel is the Fischer–Tropsch process which is relatively mature and developed by Sasol© [26].