Received: February 13, 2012 / Accepted: March 21, 2012 / Published: June 20, 2012. Abstract: CO2 (carbon dioxide) emission reduction, especially removal from coal-fired power plants has become the highest priority in measures to combat global warming. In China, coal-fired power is the main generating electricity style; more than 2,000 millions tons coal has been consumed in coal-fired power plants in China. In order to control CO2 emission, three technologies has been introduced, CCS (carbon capture and storage), oxy-combusion, and IGCC (integrated gasification combined cycle). CCS and IGCC technologies are expensive and need too many facilities; besides, there are some concrete problems need to resolve on the oxy-combustion technology. The energy saving work is the other pattern of CO2 emission control.
Key words: CO2, CCS, oxy-combustion, IGCC.
1. Introduction
Recently, CO2 emission reduction or removal from coal fired power plants has become the highest measures to combat global warming [1]. The power generation sector is one of the largest emitters of CO2. There is a great need to find ways to reduce CO2 emissions from these coal-fired power plants [2].
In CO2 capture researched technologies in the power plant industry, the way to drastically cut emissions from fossil fuelled power plants is to develop and apply CCS (carbon capture and storage), Oxy-fuel combustion, and IGCC technologies [1].
Several CCS technologies are currently developed and applied. IGCC (integrated gasification combined cycle) is another CO2 emission control technology. It is paid attention to as a promising technology, and to realize zero emission. The oxy-fuel process is nowadays of particular interest to CO2 emission control[1]. Oxy-fuel combustion is one of these CO2-control technologies which is believed to have a great potential for commercialization [2, 3]. Oxy-fuel combustion is an attractive option for coal-based power generation with CO2 capture [4].
2. Technologies on CO2 Emission Control
2.1. CCS Technology
CCS, alternatively referred to as Carbon capture and sequestration, is a means of mitigating the contribution of fossil fuel emissions to global warming [5]. CCS is an approach to mitigating global warming based on capturing CO2 from large point sources such as fossil fuel power plants and storing it instead of releasing it into the atmosphere [5]. Storage of the CO2 is envisaged in deep geological formations, and the most prospective of these are depleted oil and gas fields or saline aquifers [6].
CO2 from the coal-fired power plant can be collected in order to avoid to be taken into atmosphere. This technology is the effective method to slower global warming. This flow chart of technology involves CO2 capture, CO2 transport, and CO2 storage. CO2 may be transported storage spot and become isolated from air. The CCS project has been issued in the second power plant in shidongkou area in Shanghai in China.
Carbon capture and storage technology is divided into four parts. Flue gases from power plant is firstly be separated; then the gases can be compressed and dehydrated; thirdly, the gases may be transported to the appointed site; the end, it can be sequestrated. The technology route illustration is described in Fig. 1.
CCS technology is significant to reduce the CO2 quantity in atmosphere, but there are some problems to resolve in this time. It is be ensured that CO2 can be buried permanent safely. It is ensured that CO2 can not be generated negative effect to our living environment, especially in biologic diversity respect. It is ensured that international concerted procedure can be accepted to independently inspect and detect interrelated CO2 activities. And it is ensured that CCS technology cost can be reduced, in order to carry out extensively the technology.
However, the application of the technology will meet some difficulties in the future, including many difficulties in law and technology areas, and so on. In order to find the methods to resolve these problems, lots of industry practices and theory studies are need. CO2 is stored in rock layer underground or deepwater, turned into carbonate. The former is the best ideal spot instead of the latter, because it may acidify sea. The existing spot may at least store 2,000 Mt CO2, consume more than 60 years CO2 emission quantity.
2.2. Oxy-combustion Technology
Oxy-fuel combustion of pulverized coal is a promising technology for cost-effective power production with carbon capture and sequestration that has impacts on emission reductions [7]. In the oxy-fuel technology, the combustion of fuel is performed with pure oxygen and recycled flue gas instead of air [1].
Oxy-combustion is not pure oxygen combustion, is oxygen and some inert gases together. The more oxygen is in assistant combustion air, the better completion fuel combustion is. But flame temperature is too high and the life of furnace will be decreased. At the same time, the integrative benefit of boiler is declined. The density of oxygen is 26-30% normally.
There are some characters in oxy-combustion system: (a) The concentration of oxygen increases 4%-5%, and temperature of flame increases 200-300 °C. The high temperature of flame promotes high temperature of gases in furnace; (b) In ordinary combustion system, 79% N2 of air does not take part in combustion, at the same time take some of heat out of the boiler; (c) There is a great balance of combustion by fuel in air and oxygen, for example, H2 combustion velocity is 280 cm/s in air, but is 1,175 cm/s in oxygen; (d) Oxy-combustion can be used not only in boiler, but also in industrial furnace, kiln, and furnace etc.; (e) Low-carbon fuel can be burnt in the oxy-combustion system.
In oxy-fuel combustion system, the fuel is burnt with pure oxygen or rich oxygen instead of air (as in conventional combustion, 21% oxygen concentration). In order to consistent the combustion temperature, flue gas is recycled and reintroduced through the burner into the combustion zone by the high temperature fan. Thus, the combustion flue gases mostly consist of CO2 and water vapor (H2O), higher the CO2 concentration is, which makes an efficient capture of CO2 possible. The replacement of nitrogen (N2) with CO2 and H2O affects several fundamental processes, several facilities have to be replaced [8].
The CO2 is separated from water vapor by condensing the water through cooling and compression according to different vapor temperature. Further treatment of the flue gas may be needed to remove pollutants and non-condensed gases (such as nitrogen) from the flue gas before the CO2 is sent to storage. Oxy-combustion cannot be simply substituted for air combustion in existing fossil-fueled power plants due to differences in combustion characteristics. The oxygen produced from air separation would be mixed with recycled flue gas (CO2 domain) to approximate the combustion characteristics of air [4].
The technology route illustration is described in Fig. 2. Pulverized coal, oxygen and recycled flue gases are imported to the boiler; the flue gases can be removed nox and particle, then sent to boiler; the other flue gases can be cooled, condensed and removed sox, then compressed.
The firing system and in particular the burner is highly affected by the modified combustion process, which needs special attention. For testing purposes related to the oxy-fuel combustion process, a highly flexible firing system and burner concept is needed to cover the broad range of test conditions [1]. The aerodynamics in the combustion chamber is different, the reaction chemistry in combustion is affected and the radiative heat transfer differs from that of air-firing. These factors must be taken care in designing boiler. N2 is transparent to thermal radiation, but both CO2 and H2O absorb and emit radiation at thermal wavelengths [8].
2.3. IGCC Technology
Integrated gasification combined cycle is advanced power system, which combine coal gasification technology and united cycled technology. The coal gasification system involves gasification furnace and gases purification facilities, which can be used to remove sulfur, nitrogen, and dust; the united cycled system involves gas turbine, generator, waste heat boiler, and steam turbine. An IGCC (integrated gasification combined cycle) is a technology that turns coal into gas—synthesis gas, lower emissions of sulfur dioxide, particulates and mercury. Excess heat from the primary combustion and generation is then passed to a steam cycle, similarly to a combined cycle gas turbine. This also results in improved efficiency compared to conventional pulverized coal [9].
IGCC is a adding coal gasification to two power cycle system. Integrated power cycle is two absolute power cycle by different wording materials. The two power cycle is integrated by energy exchange. It is that high-temperature exhaust gases from gas turbine produce steam in waste-heat boiler, and send it to steam turbine to work. Thus, the two power cycle can be integrated by heat exchange. The ideal heat efficiency is 1—T2/T1. The heat efficiency is high if the hot source temperature is high and the cool source temperature is low. The temperature in hot source is up to 1,100-1,300 °C in integrated cycle, is higher than ordinary steam cycle temperature (540-566 °C). The temperature in cool source is low to 29-32 °C in integrated cycle, is lower than ordinary gas cycle temperature (450-640 °C). In other words, IGCC is combined by coal gasification and two high-efficiency integrated power cycles. Therefore, IGCC efficiency is very high. A new style integrated gasification combined cycle is being running in the power plant in lingang xincheng area in Shanghai in China.
Process flow on integrated gasification combined cycle is described in Fig. 3. Coal are gasified in gasification furnace; low-value gases can be drawn; gases can be cleaned by removing sulfur, nitrogen, and dust; the clean gases are imported to firebox, the heated gases can power gas turbine; the exhausted gases are introduced to boiler to heat water; the water can be turned into steam, and power steam turbine; the exhausted gases involves CO2 and H2O, and it can be turned in chemical products. The integrated gasification combined cycle technology may realize zero emission in coal utilization [9].
3. Energy Saving Pattern in Traditional Coal Fired Power Plant
Energy saving is to reduce the energy consummation in traditional coal fired power plant, is the other way to reduce the CO2 emission, because the usage of coal is reduced and the carbon content of coal is invariable. Energy saving pattern includes program, design, running and management etc.. It is described in Fig. 4.
(1) In program of coal fired power plant
Firstly, connection plan can be optimized in order to reduce net wastage; use rate of export power can be increased. Secondly, combined heat and power generation is needed. Combined heat and power generation supply heat and power at the same time, increase the efficiency of coal fired power plant. Thirdly, large capacity boiler must be applied to reduce the coal consume of per power unit.
(2) In design of coal fired power plant
The design of coal fired power plant should meet the challenge of saving money method, safety, less floor area, and less power consume.
(3) In running of coal fired power plant
Firstly, efficiency of steam turbine can be increased. Secondly, combustion efficiency of boiler can be increased. Thirdly, the quantity of coal can be improved. Fourthly, frequency conversion of fan and pump can be used. And fifthly, the quantity of steam can be improved.
(4) In management of coal fired power plant
That the energy saving consciousness is improved is necessary. The level of management should be enhanced.
4. Conclusions
Base on the analysis and discussion about carbon capture and storage, oxy-combustion, integrated gasification combined cycle technologies, and energy saving technologies in traditional coal fired power plant, the conclusion can be drawn: the three technologies are taken to resolve problem about CO2 and global warming; CCS (carbon capture and storage) and IGCC (integrated gasification combined cycle) technologies are expensive and need too much facilities; at the same time, there are some concrete problems need to resolve on the oxy-combustion technology. The energy saving work is the other pattern of CO2 emission control.
References
[1] S. Grathwohl, O. Lemp, M. Müller, U. Schnell, J. Maier, G. Scheffknecht, et al., Oxy-combustion of coal—needs, opportunities and challenges, in: AICHE, Salt Lake City, 2010.
[2] D.R. Simbeck, M. McDonald, Existing power plant retrofit CO2 control options analysis, in: 5th International Conference on Greenhouse Gas Control Technologies, Cairns, Australia, 2000.
[3] T.F. Wall, Combustion processes for carbon capture, Proceedings of the Combustion Institute 31 (1) (2007) 31-47.
[4] B.J.P. Buhre, L.K. Elliott, C.D. Sheng, R.P. Gupta, T.F. Wall, Oxy-fuel combustion technology for coal-fired power generation, Progress in Energy and Combustion Science 31 (4) (2005) 283-307.
[5] J. Gibbins, H. Chalmers, Carbon capture and store, Energy Policy 36 (12) (2008) 4317-4322.
[6] D. Hilditch, Demonstration and deployment of carbon dioxide apture and storage in Australi, Shanghai—Shell Sustainable Energy Co-operation, in: 4th Energy Forum, Shanghai, China, 2008.
[7] H. Gendy, I. Preciado, T. Ring, E. Eddings, Particle image velocimetry of pulverized oxy-coal flames, in: AICHE, Salt Lake City, 2010.
[8] R. Porter, M. Pourkashanian, A. Williams, D. Smith, Modelling radiative heat transfer in oxycoal combustion, in: ASME Summer Heat Transfer Conference, California, 2009.
[9] C. Descamps, C. Bouallou, M. Kanniche, Efficiency of an integrated gasification combined cycle (IGCC) power plant including CO2 removal, Energy 33 (6) (2008) 874-881.
