Polygeneration energy system based on coal gasification

Polygeneration energy system based on coal gasification

Articles Polygeneration energy system based on coal gasification Li Zheng, Ni Weidou, Zheng Hongtao, and Ma Linwei Department of Thermal Engineering,...

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Polygeneration energy system based on coal gasification Li Zheng, Ni Weidou, Zheng Hongtao, and Ma Linwei Department of Thermal Engineering, Tsinghua University, Beijing 100084, China E-mail (Li Zheng): [email protected]

Environmental pollution has become a bottleneck for the sustainable economic development of China. A ‘‘business-as-usual’’ energy system in China is not suitable for meeting sustainability needs. It is well known that China has to use coal as the main primary energy source over the long term. Under such special conditions, to plan and construct an integrated sustainable energy system with optimal benefits in resource and energy utilization and environmental emissions is urgent. By introducing international studies and new developments in sustainable energy systems, this paper puts forward the concept that a polygeneration strategy based on coal gasification is the trend for future development of China’s domestic energy industry. The framework of a polygeneration system based on oxygen-blown gasification is described and its benefits are analyzed. Finally, the starting procedure, government role, and policies for implementing polygeneration strategies in China are proposed. 1. Five major energy challenges China is facing In the 21st century, China is facing five major challenges in the energy field (discussed briefly below), which imply that ‘‘business-as-usual’’ future development of the energy system is unsustainable. Under the necessary condition of continued reliance on coal as the main primary energy source in the coming decades, it is important to develop new coal-based energy systems to meet all these challenges. 1.1. Energy supply In the 21st century, the energy economy of China is and will continue to be characterized by huge and increasing demand and limited energy resources. At present, the population of China is about 1.3 billion and increasing at over 10 million per year. By 2030, the population could reach 1.6 billion. The present annual average energy consumption is about 1.0 tonne of coal equivalent (tce) or 29.3 GJ per capita, which is much less than the 322 GJ/year/capita of the USA and 145-175 GJ/year/capita of Russia, Japan and Germany. Owing to rapid economic development and the increasing expectations of better lives among the people, energy demand will inevitably increase and reach 75 to 90 GJ/year/capita between 2030 and 2050. Consequently, China’s total energy demand will be 130 EJ, which is three times present consumption. Meanwhile, China’s energy resources are very limited. As Table 1 shows, China lacks oil and natural gas and will have to depend on imports over a long period. As for coal, although there are enough resources and coal will remain China’s main primary energy source for a long time, how to produce and use coal efficiently and sufficiently is a big problem. It is estimated that, by 2020, China can produce up to 2.0 billion tonnes (Gt) of coal per year. 1.2. Shortage of liquid fuel Along with economic development, especially with increasEnergy for Sustainable Development

ing numbers of transportation vehicles, China’s demand for liquid fuel is increasing rapidly and dependence on imports is growing. In 2000, 70 million tonnes (Mt) of oil were imported and total consumption reached 220 Mt. Transportation fuel needs will keep on being the main driver for liquid fuel demand. In 2002, China’s vehicle population was 20 million. This number will increase to ∼120-150 million by the year 2020. Figure 1 shows the estimated oil consumption and domestic oil production up to the year 2050. It can be seen that domestic oil production will decrease beginning around 2010 and the net gap in 2020 could reach 235 Mt [Zhai, 2003]. 1.3. Environmental pollution China is facing serious pollution problems. In 2000, emissions of dust and SOx were, respectively, 11.65 and 19.95 Mt. The concentration of SO2 exceeded the Second Class of the National Standard in 21.26 % of China’s cities. 63.1 % of cities suffered from dust concentration higher than the Second Class of the National Standard. Of 254 cities monitored, 157 cities (61.8 %) had acid rain [SEPA, 1996-2000]. Coal-burning is the main source of dust and SO2 emissions (respectively 70 % and 90 %). The power generation industry is the biggest coal consumer. In 2000, installed power generation capacity was 319 GW, of which 69.3 % was coal-fired capacity. It is estimated that by 2020, the total installed power capacity will be 900950 GW, about 60 % of which will be coal-fired and will consume 1.3 Gt of coal. Obviously, even if all newly-installed power units are equipped with desulfurization equipment, the total emissions of SO2 will still be high. Therefore, environmental pollution will continue to be a big problem. l

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Table 1. China’s energy reserves [BP, 2003] Proven reserves

Oil Gt

China World total China’s share of world total (%)

Natural gas R/P

[1]

Trillion cubic meters (Tm 3)

Coal R/P

[1]

R/P[1]

Gt

2.5

14.8

1.51

46.3

1145

82

142.7

40.6

155.78

60.7

9844.5

204

1.7

1.0

11.6

Note 1. R/P = ratio of reserves to annual production

Figure 1. Future oil demand and production of China [Zhai, 2003]

1.4. Greenhouse gas (GHG) emission In recent years, China’s CO2 emissions have been about 3 Gt a year and ranked second behind the USA (Table 2), although the average emission per capita is only 2.4 t/year/capita, much less than in developed countries. However, since China will continue to use coal as its main primary energy source, the reduction of GHG will be an important task to consider, the earlier the better. 1.5. Rural energy supply Biomass and coal are the main fuels for cooking and heating in rural areas in China. Their use causes not only low living standards and health problems due to poor indoor air quality, but also a serious ecological problem due to over-exploitation of plants. The need to supply clean energy to rural populations is very urgent.

home of human beings, the earth, and there are even more serious challenges emerging. Therefore, all countries and industries are looking for new systems with less resource consumption, high energy-conversion efficiency and low emission. Some examples are described in the rest of this section. The US Department of Energy (DOE) has put forward the Vision 21 energy system [PCAST, 1997] (Figure 2). Its idea is to get syngas through coal gasification. The syngas could be used to produce hydrogen as the fuel for fuel-cell automobiles. In addition, the syngas could be used in combined cycles based on high-temperature solid oxide fuel cells (SOFC) and gas turbines to produce electricity. Its electricity generation efficiency could reach 60 % for coal-based systems, traditional pollutant emissions could be negligible, 40-50 % reduction in CO2 emissions would be achieved by efficiency improvement and 100 % reduction with sequestration [CRDOAFFEC, 2000]. The Shell Company has put forward the concept of a

2. Polygeneration based on coal gasification will be the new trend for world energy utilization From the global point of view, industrial development and large-scale utilization of fossil fuels have damamged the 58

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Table 2. CO2 emissions of different countries in 1999 [International Energy Agency, 2001] Country

US

Russia

Japan

Germany

UK

Italy

China

India

Total CO2 emission (Gt)

5.585

1.486

1.158

0.822

0.535

0.421

3.051

0.904

CO2 emission per capita (t)

20.46

10.17

9.14

10.01

9.00

7.30

2.42

0.91

Figure 2. Vision 21 energy system [PCAST, 1997]

syngas park. In such a park, coal or residual oil is gasified. The syngas produced can be used to produce electricity by IGCC, and methanol and fertilizer in a ‘‘once-through’’ process, and can also be used as town gas [Anderson, 1999] (Figure 3). Some IGCC power stations have also been established in petrochemical enterprises. They use cheap petroleum residuals, pitch, petrocoke or orimulsion as feedstocks to produce syngas. Syngas can either be used as raw material for production of value-added chemicals or to provide electrical power and steam for the production processes. This closely integrates the production of electricity and heat with chemical production processes, and decreases the costs for both chemicals and electricity. Up to now, dozens of such facilities have been put into operation all over the world, and more are under development. One typical example is the Italian facility (ISAB Company) with a power output of 512 MW. The above examples show that developed countries are seeking efficient solutions to resource shortages and environmental pollution problems. However, because of the traditional separation of different industrial sectors, every sector is looking for optimal solutions in its own field. But these optimal solutions can hardly be the best for society as a whole. Polygeneration is intended to be a highly flexible and optimal integration of resource/energy/environment systems meant to benefit all of society, while also breaking down the existing separation of different industrial sectors. Energy for Sustainable Development

3. Framework of a polygeneration system based on oxygen-blown gasification The polygeneration concept, considered from the point of view of comprehensive optimization, is a highly flexible and cross-sector integrated system of resources, energy and environment. Figure 4 shows a basic framework for polygeneration. The main points are the following. 1. Using coal, petrocoke or heavy oil residues with high sulfur content as feedstock, syngas (the main components being CO and H2) is produced through oxygenblown gasification. After clean-up and purification of the syngas, elemental sulfur could be obtained as a by-product. 2. There are diverse ways of utilizing the syngas. • Town gas for cooking and heating, for distributed power, heat and cooling co-generation. • Large-scale power generation (fuel cell or gas/steam combined cycles) • Methanol production via the ‘‘once-through’’ liquid phase reactor • Liquid fuel production (synthetic fuel and dimethyl ether) via the ‘‘once-through’’ liquid phase reactor • Other chemical products (NH 3, urea, middle distillates) Syngas can also be reformed to produce hydrogen. With the development of PEM fuel-cell technology, in the long term hydrogen can be used as fuel for vehicles and solve the problem of transportation emissions l

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Figure 3. Syngas park concept by Shell Company [Anderson, 1999]

of large cities, i.e., creating near-zero emissions. From the long-term point of view, hydrogen as the energy carrier could be utilized as the cleanest fuel for distributed power, heat and cooling co-generation and for realizing local zero emissions as well. 3. The treatment of separated CO2. When the combustion process is properly controlled, burning of cleaned syngas causes much less pollution than conventional power plants do with direct firing of coal. Therefore, the key issue will be the treatment of greenhouse gases, the most important of which is CO2. For the proposed polygeneration system, because the separated CO2 is nearly pure (99 %) instead of mixed with 75 % nitrogen in the flue gas as at a conventional power plant, CO2 could be used as feedstock for different products, such as urea and dry ice. It can also be used for enhancement of plant growth and other industrial purposes. In recent years, a Canadian company has been conducting research and experiments on enhancing coal-bed methane (CBM) production by injecting CO2 into unminable coal seams (at depths of more than 2 km) in the Alberta area. Since coal, as a micro-porous substance, has a greater absorption capacity for CO2 than for CH4 (the main component of CBM), by injecting CO2, the valuable CH4 can be ‘‘squeezed out’’ and the CO2 can be sequestered. There are also several of other methods of CO2 sequestration: to the deep sea, to depleted oil and gas fields, or to saline aquifers. These concepts are only in the preliminary stages of 60

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development, and more detailed technical, environmental and economic assessments should be conducted. But anyway, pure CO2 will be much easier to treat than CO2 in the flue gas of conventional power plants. 4. Close intercoupling of production processes. The core of the proposed polygeneration concept is the close coupling of the production processes of different products. For instance, after passing through the ‘‘oncethrough’’ liquid phase reactor to produce methanol (or DME), the unreacted syngas could be directed to IGCC for power generation instead of subsequent separation and recycling to the reactor again as it is in conventional stand-alone methanol production. Therefore, the capital investment, maintenance cost and environmental impact will be significantly reduced, and consequently the cost of these products could be reduced as well. Furthermore, because of the co-production, the ‘‘peak and valley’’ of each product (especially power generation) could be adjusted more easily according to the demand. 5. ‘‘Open’’ and highly flexible. For coal-abundant areas in China, polygeneration can be quite beneficial. It could be implemented step by step or phase by phase according to technical advances and the availability of capital investment. For example, for the first phase, only power, heat and methanol could be co-produced, and some other products (e.g., downstream chemical products of methanol) could be arranged later when l

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Figure 4. Integrated system for resource, energy and environment

the financial situation improves or technical problems are solved. Figure 4 only shows a preliminary conceptual framework for polygeneration. The detailed material flow, energy flow, information flow and multi-target (technology, economics, resource utilization, environment) optimization for each related subsystem should be carried out in the future through sophisticated study of complex systems and taking the local conditions into account.

project for the 21st century. The objective is to reduce the backwardness of western regions, to develop their regional economies, to cultivate a new educated generation, and to improve the living standard of local people. However, the target can hardly be realized by only exporting in raw form the natural resources found in these regions, as with the West-East natural gas pipeline project. Some new innovative approaches should be adopted to add more value to the natural resources before export. To this end, the concept of polygeneration could be a worthy addition.

4. A quantitative example of the benefits of a polygeneration system

5. Policy support and starting steps for the development of polygeneration systems in China

The base case for comparison is the stand-alone production of power, heat, methanol and syngas by conventional power plants, industrial boilers, traditional technology for methanol production, and coal gasifiers, respectively. The outputs are respectively 400 MW of electricity, 400 MW of heat, 400 MW of methanol, and 400 MW of syngas. Compared with stand-alone production, quadrigeneration of these four products leads to the following benefits: reduction of capital investment by 38 %, reduction of unit cost for energy by 31 %, and reduction of coal consumption by 22.6 % [Williams, 2000] (see Figure 5). Though the above-mentioned results are rather simplified and should be adjusted according to the different situations, the potential benefits of polygeneration seem obvious. The Grand Western Development of China is a huge Energy for Sustainable Development

1. Because polygeneration is a large cross-sectoral system, its development depends on the support and coordination of different sectors of industry (such as chemical, coal-mining and power generation sectors). In order to benefit the whole nation, the central government must play a strong role of coordination and policy-making. The barriers between different sectors must be broken. 2. The National Development and Reform Commission (NDRC) and Ministry of Science and Technology (MOST) must formulate a coordinated plan and multilayer arrangement. As two instances, fundamental studies of this system should be conducted by the State Key Fundamental Research Program and industrial demonstrations of the system should be supported by l

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Figure 5. Benefits of investment, energy consumption and environment for quadrigeneration [Williams, 2000]

the S-863 Program. In addition, favorable policies under the guidance of the government should be implemented for the initial technology introduction period, since every new technology and system will go through a buy-down process from initial high costs. Expecting comparable economics of demonstration polygeneration projects with conventional technologies from the very beginning is not realistic and will hinder development of polygeneration systems. 3. In the new round of China’s Medium- and Long-term Science and Technology Planning, coal gasificationbased polygeneration should be emphasized as an important strategy to address the challenges to China’s sustainable energy development. 4. The power grid must loosen the regulations for grid connections so that owners of polygeneration systems can sell the electricity generated to the grid at a reasonable price. 5. It is urgent to set up one demonstration project of coalbased polygeneration to verify the concept and to demonstrate relevant technologies. The possible site of the project should be in a coal-rich area and make use of

high-sulfur-content coal. To be easy, practical and economic, proven domestic technologies should be used; for example, the gas-phase methanol synthesis reactor, syngas cleaning technology and MS6B gas turbine. References British Petroluem (BP), 2003. 2003 Statistical Review of World Energy. Zhai Guangming (PetroChina), 2003. ‘‘Prediction of China’s oil and gas supply in 2050’’, Speech delivered during panel discussion of China’s Middle and Long Term Science and Technology Planning, Sept. 23. State Environmental Protection Agency (SEPA), 1996-2000. Report of National Environmental Quality. International Energy Agency, 2001. Key World Energy Statistics from the IEA, 2001 edition. President’s Committee of Advisors on Science and Technology, Panel on Energy Research and Development (PCAST), 1997. Report to the President on Federal Energy Research and Development for the Challenges of the Twenty-first Century, pp. 4-5, Nov. Committee on R&D Opportunities for Advanced Fossil-Fueled Energy Complexes, Board on Energy and Environmental Systems, Commission on Engineering and Technical Systems, National Research Council (CRDOAFFEC), 2000. Vision 21 -- Fossil Fuel Options for the Future, National Academy Press, Washington, DC. Anderson, B., 1999. ‘‘Coal gasification: a viable and economic alternative for China’’, presented at the meeting of the China Council for International Cooperation on Environment and Development, October. Williams, R.H., 2000. ‘‘A zero emissions strategy for fossil fuels’’, paper presented at Ballard, Vancouver, BC, Canada, 8 May.

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