Pre-combustion carbon dioxide capture by gas–liquid absorption for Integrated Gasification Combined Cycle power plants

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Abstract

Among various configurations of fossil fuel power plants with carbon dioxide capture, this paper focuses on pre-combustion capture technology applied to an Integrated Gasification Combined Cycle power plant using gas–liquid absorption. The paper proposes a detailed study and optimization of plant design (column height and packed dimensions) with CO2 capture process using different solvents as: aqueous solutions of alkanolamine, dimethyl ethers of polyethylene glycol, chilled methanol and N-Methyl-2-pyrolidone. By developing simulations in Aspen Plus, the following performance results of these physical and chemical solvents, mentioned above, are discussed: overall energy consumption (power consumption, heating and cooling agent consumption), CO2 specific emissions, net electric power output and plant efficiency. The paper presents as well, the total investment capital cost of an IGCC coal mixed with biomass (sawdust) power plant generating 425–450 MW net electricity with (70% CO2 capture, 80% CO2 capture and 90% CO2 capture) and without pre-combustion CO2 capture.

Simulation results show that for evaluated solvents for CO2 capture, the physical solvent, dimethyl ethers of polyethylene glycol, is more energy efficient that the other physical and chemical solvents investigated. Regarding the economic study, implementation of pre-combustion CO2 capture on IGCC plant, using dimethyl ethers of polyethylene glycol, leads to an increase of the capital cost with about 19.55% for 70% CO2 capture, 20.91% for 80% CO2 capture and 22.55% for 90% CO2 capture.

Highlights

► The energy-related consumption was evaluated on a IGCC-based pre-combustion capture technology. ► Different physical and chemical solvents were used to estimate heat and power integration of the main plant sub-systems. ► Selection of solvents, quantification of carbon capture energy penalty, capital costs estimations was evaluated in details. ► The evaluation was based on modeling and simulation work in Aspen Plus engineering software.

Introduction

Today, a large amount of worldwide primary energy consumption comes from fossil fuels like oil, coal and natural gas. According to International Energy Agency, about 40% of total primary energy is used for electricity generation, and of this, coal is the fuel for 40%. Forecasts of future energy consumption predict a further increase of worldwide coal utilization in the coming 20 years (International Energy Agency, 2010). But, the impact of energy production from fossil fuels on the environment is becoming a matter of growing concern. In addition to the risks for environment on a local scale, the mankind is now faced with danger of global warming caused by greenhouse gas emissions (mainly carbon dioxide) (Rejoy, 2009). Therefore, fossil fuels are the major source of carbon dioxide emissions and they cause global warming with all its negative impacts. It is generally accepted today that huge efforts have to be undertaken to limit the greenhouse gas emissions and to reduce the impact of global warming. Mitigation scenarios indicate that this can only be achieved if all options for carbon dioxide reduction are followed (Meadowcroft and Langhelle, 2009).

An important reduction of greenhouse gas emissions resulting from fossil fuels utilization can be obtained by increasing the efficiency of power plants and industrial production processes, decreasing the energy demand and replacing fossil fuels by renewable energy sources or nuclear energy combined with carbon dioxide capture and long time storage (Abu-Zahra et al., 2007). Unfortunately in practice, on a global scale, the alternative energy conversion processes cannot substitute rapidly the usage of fossil fuels, which will continue to represent a substantial share of the energy consumed for many years to come, the only feasible solution for now remaining these carbon capture and storages technologies (CCS) (Rejoy, 2009). CCS involves three basic steps: carbon dioxide capture, transport to a suitable disposal site, and long-term storage. With respect to capture, attention is primarily directed at major point source emitters, particularly fossil fuel-fired power stations, but also other large industrial facilities including those associated with the production of oil, gas, chemicals, steel and cement. According to the 2005 Intergovernmental Panel on Climate Change report (IPCC, 2005), about 60% of global fossil fuels emissions come from such stationary sources, which each release more than 0.1 megatons of carbon dioxide per year (Meadowcroft and Langhelle, 2009). For these reasons, it is important that there should be technology options that would allow continued used of solid fuels, within these as mentioned above, without substantial CO2 emissions.

Approaches to carbon dioxide capture technologies generated from fossil fuel energy conversion include post-combustion capture, pre-combustion capture and oxy-fuel combustion. All these technologies can be achieved by several ways of carbon dioxide removal processes such as solid adsorption, absorption into a liquid solvent, membranes or other physical or biological separation methods, etc. (Kohl and Nielsen, 2005). Among these techniques, carbon dioxide capture by gas–liquid absorption is one of the most common and commercially mature technologies today (e.g. carbon dioxide absorption process is applied in chemical processes like hydrogen and ammonia synthesis) and, in many cases, has been considered to be the most viable solution, compared to the other processes that are available. However, technological improvements are necessary to reduce the high capital cost and energy requirements of absorption process. It is expected that process design innovations or usage of better solvents can reduce significantly the capital and energy costs (Tobiesen and Svendsen, 2007). Carbon dioxide absorption, as it can be noticed in Fig. 1, can be divided into chemical absorption (used in both pre- and post-combustion capture techniques) and physical absorption (used in pre-combustion capture technique) (IPCC, 2005, Kohl and Nielsen, 2005).Post-combustion capture technology involves separating the carbon dioxide from exhaust gases after fossil fuels combustion. In this case, flue gas partial pressure is close to atmospheric and carbon dioxide concentrations are relatively low, e.g. 4–8 vol.% in natural gas-fired and 12–15 vol.% in coal-fired power plants, and this low concentration is the reason why it is recommended that chemical solvents such as aqueous solutions of alkanolamines, sodium hydroxide (NaOH) or ammonia (NH3) are used. Post-combustion carbon dioxide capture by gas–liquid absorption using different aqueous solutions of alkanolamines is described in detail by authors in previous articles (Padurean et al., 2010, Padurean et al., 2011, Cormos et al., 2009).Pre-combustion capture involves separating carbon dioxide before the fuel is burned and it may be applied on IGCC power plants. In case of pre-combustion capture techniques, where the carbon dioxide partial pressure is relatively high (about 10–12 bar) and its concentration is around 40%, it is increasingly becoming normal practice to use physical solvents containing for instance methanol (Rectisol® technology), N-Methyl-2-pyrolydone (Purisol® technology) or mixture of dimethyl ethers of polyethylene glycol (Selexol® technology) (Kohl and Nielsen, 2005).

No matter what solvent is used for carbon capture, the electric power consumption, the heat (steam) and the cooling duties required for the capture process are very important aspects that must be considered. From the point of view of electric power consumption, in case of chemical solvents, the solvent flow rate is lower than in the case of physical solvents, which means lower electric power consumption for solvent circulation. The situation is totally reversed for heating and cooling consumptions; the chemical solvents require much more heat for regeneration than physical solvents due to the chemical reactions involved. These issues require a careful integration analysis in terms of heat and electric power for the carbon dioxide capture plant (Aronu et al., 2009).

The present paper is focused on pre-combustion carbon dioxide capture technology using chemical and physical solvents by gas–liquid absorption applied to an IGCC power plant. Particular importance is set in the paper on analysis of technical coefficients (specific electricity, heating and cooling consumptions for each kg of captured carbon dioxide, etc.). In order to compare different solvents, it is necessary to perform on a consistent basis and to perform a process analysis of the system. The simulations have been developed by using the SRK (Soave–Redlich–Kwong) equation of state model of chemical process engineering software Aspen Plus (version 7.0) (Aspen Plus, 2011). Simulation of the carbon capture processes using various solvents yields all necessary process data (mass and molar flows, composition, temperature and pressure profiles along the absorption and desorption columns, power consumption, heating and cooling duties) needed to assess the overall technical and economic performance of the processes.

The modeling and simulation of the whole IGCC plant concept with different carbon dioxide capture methods produced the input data for quantitative evaluation of various plant schemes for analyzing the energy penalty involved by the carbon capture process. All IGCC based carbon capture technologies were compared in terms of overall plant performance, energy consumption (power consumption, heating and cooling agent consumption), CO2 specific emissions, net electric power output, plant efficiency and capital costs with conventional IGCC concept without CO2.

Section snippets

Base case scenario

The many advantages of IGCC technology, high thermal efficiency, low NOx, SOx and solids emissions as well as the possibility to process lower grade coals, are an asset that could prove commercially competitive against conventional coal power plants. However, IGCC plants produce large amounts of CO2 that are released to the atmosphere. As it was mentioned in the previous paragraphs, a significant reduction of global greenhouse gas emissions can be obtained by developing more efficient power

Main design assumptions

The syngas flow rate and compositions which have been used in the studies and other main sub-systems of the plants and their design assumptions used in the mathematical modeling and simulation, are presented in Table 3 (Starr et al., 2007).

Since the flowrate of the syngas is very high (448.60 t/h), it was decided to divide the CO2 capture operation into 4 columns. This allows the use of absorbers and strippers with diameters which are found in commercial units now (Kothandaraman et al., 2009).

Conclusions

Carbon dioxide capture by gas–liquid absorption using physical or chemical solvents is presently considered the most practical, economical and commercially mature technology for carbon dioxide capture from the power generation sector. No matter what solvent is used for carbon dioxide capture, the power consumption, heating (steam) and cooling duties required for the capture process are a very important aspect that must be considered. Integration of carbon dioxide capture in a complete coal

Acknowledgments

The authors wish to thank for the financial support provided from the programs: Investing in people! PhD scholarship, Project co-financed by the SECTORAL OPERATIONAL PROGRAM FOR HUMAN RESOURCES DEVELOPMENT 2007–2013, Contract nr.: POSDRU/88/1.5/S/60185 – “Innovative doctoral studies in a Knowledge Based Society“, to the Romanian National University Research Council (CNCSIS – UEFISCDI) projects: PNII – IDEI 2455/2008: “Innovative systems for poly-generation of energy vectors with carbon dioxide

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