Industrial energy efficiency and carbon capture: the thermodynamic cost
of going green
of going green
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| Written by Alberto Alva, Process Ecology Carbon capture and storage (CCS) is an approach that is currently being extensively researched as a means to mitigate global warming. CCS is achieved via the separation of carbon dioxide (CO2) from point sources such as fossil fuel power plant flue gases and storing it instead of releasing it into the atmosphere. CCS applied to a modern conventional power plant could reduce CO2 emissions to the atmosphere by approximately 80-90% compared to a plant without CCS. However, capturing and compressing CO2 requires significant additional energy and would increase the fuel needs of a coal-fired plant with CCS by about 25% (1). These estimates apply to purpose-built plants near a storage location: applying the technology to retrofit existing plants or plants far from a storage location will be more expensive. Evidence of the ongoing efforts to implement CCS technologies was in display during the 18th International Congress of Chemical and Process Engineering (CHISA 2008) held in Prague late August. A number of leading academics discussed various aspects of the technological options available for CCS both in power plants and in large industrial sites. Researchers from the University of Manchester (UK) discussed the efficiency implications of installing CO2 capture technologies (2). In principle, industry has three options available for CCS, pre-combustion, post-combustion and oxy-combustion, each option with its own advantages and challenges. In post-combustion a chemical absorption unit is installed to separate the CO2 from the heat recovery steam generation unit flue gas. The collected carbon dioxide is then dehydrated and compressed to liquid form, which can then be used for enhanced gas/oil recovery or sequestrated in underground geological structures. Chemical absorption is practically the only technology that can be used in this case for separating the CO2 given the very low partial pressure of CO2 in the gas stream. The pressure drop in the system becomes a key parameter since it introduces a significant penalty in power generation thus reducing the overall efficiency of the system. Because the CO2 product is at atmospheric pressure, a large amount of energy is needed to compress the CO2 for transportation. Incremental losses for thermal power plants that use amine scrubbing ranges between 10 and 30% of the total power the plant would generate if CO2 capture were not included (3). This energy penalty translates into a noticeable impact on electricity prices. In pre-combustion a reforming or gasification unit is installed to transform the original fossil fuel into a syngas; a water gas shift reaction then produces a gas stream rich in CO2 and Hydrogen. The separation of CO2 from this mixture allows a number of technologies to be considered as the partial pressure of CO2 is much higher than in a typical flue gas. These technologies include chemical absorption, physical absorption, membranes, and pressure swing absorption. Hydrogen is then used as the fuel for power generation; alternatively the syngas may be redirected to the synthesis of valuable chemical products. Compared to other combustion processes, the incremental energy penalty of pre- combustion capture is low at 6% (3); because of the relatively favorable CO2 concentrations in the process (which range from 15 to 80%) and the high pressure involved (2). Both factors make the separation and compression of CO2 in pre-combustion systems relatively efficient. It was noted that the typical efficiency of a natural gas fired power plant is in the order of 54% and that introducing a post-combustion unit would reduce this figure to 46.5% and in the best-case scenario for pre-combustion would be lowered to 46.2% (using a cryogenic system and considering the need for an air separation plant). Oxy-combustion implies that the oxidation takes place only in the presence of pure oxygen leading to a flue gas containing exclusively CO2 and steam, leading to a much simpler separation by condensing the water. However there are a number of difficulties in the use of oxygen for the combustion step and there is a need for some diluents to be introduced. The energy requirements of an air separation plant must also be considered; the efficiency penalty for this option reduces the figure to 44.4%. The net reduction of emissions to the atmosphere through CCS depends on the fraction of CO2 captured, the increased CO2 production resulting from loss in overall efficiency of power plants due to the additional energy required for capture, transport and storage, any leakage from transport, and the fraction of CO2 retained in storage over the long term. Similar strategies for large industrial sites are shown to be heavily dependent on the specific conditions of emissions, technology in use and economics. A research presentation from Chalmers University of Technology (Sweden) (4) highlighted the fact that in large industrial sites the energy requirements for CCS technologies may be supplied (at least partially) with available excess heat. Applying the principles of Process Integration leads to modified process designs where, for example, the heat required to regenerate an absorption agent in post-combustion schemes is provided by “waste heat” from the background processes. The main issue at hand is that capturing carbon dioxide in one way or another requires additional energy inputs and with current available technologies also means a decrease in overall energy efficiency (energy output/energy input). Large industrial operations are now facing the difficult challenge to evaluate the best technology and whether it is beneficial to improve their current energy efficiency or if it may make more sense to use the available heat to drive some of the carbon capture technologies via process integration. Reducing the energy demand of CCS, together with improvements in the efficiency of energy conversion processes are high priorities for technology development. It is likely that in the future CCS will be part of the mix of solutions to mitigate greenhouse gas emissions to the atmosphere; however it must not be conceived as a unique grand solution; efforts related to energy efficiency will remain to be the single most effective approach to reduce energy-related emissions.
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