Environ. Sci. Technol. 2008, 42, 2590–2593
Utilization of Ventilation Air Methane as a Supplementary Fuel at a Circulating Fluidized Bed Combustion Boiler CHANGFU YOU* AND XUCHANG XU Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Thermal Engineering, Tsinghua University, Beijing 100084, China
Received September 10, 2007. Accepted December 27, 2007.. Revised manuscript received December 18, 2007
Ventilation air methane (VAM) accounts for 60–80% of the total emissions from coal mining activities in China, which is of serious greenhouse gas concerns as well as a waste of valuable fuel sources. This contribution evaluates the use of the VAM utilization methods as a supplementary fuel at a circulating fluidized bed combustion boiler. The paper describes the system design and discusses some potential technical challenges such as methane oxidation rate, corrosion, and efficiency. Laboratory experimentation has shown that the VAM can be burnt completely in circulated fluidized bed furnaces, and the VAM oxidation does not obviously affect the boiler operation when the methane concentration is less than 0.6%. The VAM decreased the incomplete combustion loss for the circulating fluidized bed combustion furnace. The economic benefit from the coal saving insures that the proposed system is more economically feasible.
1. Introduction The rapid economic growth in China has resulted in greatly increased coal use, which has in turn led to an increasing number of coal mines. Coal mine methane is not only a greenhouse gas but also is a wasted energy resource if not utilized. Underground coal mining is by far the most important source of fugitive methane emissions, and 60–80% of all coal mining related methane is emitted to the atmosphere through mine ventilation air. This ventilation air methane (VAM) is potentially a high value energy source (1–3). The VAM recovery rate is very low in China. According to the 2004 statistics (4), China had about 650 state-owned coal mines and 2180 ventilation air shafts. The ventilation air flow rate at the mine shaft was about 9845 m3 per minute, with about 5 billion m3 of VAM released to the atmosphere annually. Zheng et al. (5) estimated that the total amount of VAM was about 10 billion m3 in all of the coal mines in China. The heating value of 5 billion m3 of VAM would be about the same as that of 6 million tons of standard coal, which equals the annual output of a large state-owned coal mine. Methane is a greenhouse gas and is 21 times more potent than CO2 in terms of its ability to trap heat in the atmosphere over a time frame of 100 years. Thus, the release of VAM results in serious air pollution and a waste of a significant amount of energy. The methane concentration in the ventilation air is usually very low, less than 1% by volume. * Corresponding author phone: +86-10-62781740; fax:+86-1062770209, e-mail:
[email protected]. 2590
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 42, NO. 7, 2008
Efficient utilization of this low quality energy has become a challenge for the coal mining industry worldwide. The VAM can be utilized as a main fuel or as a supplementary fuel (6). The exploitation of the VAM as a main fuel requires oxidation of the methane at high temperatures. One example is the thermal flow-reversal reaction technology developed by the Swedish MEGTEC system company (7). The thermal flow-reversal reaction uses a ceramic bed for heat regeneration to keep the reaction section temperature above 1000 °C, at which oxidation of the low methane content VAM is possible. The system is well insulated so that the ceramic bed stays at the temperature required to continue oxidation. The air flow direction is changed every few minutes to keep the heating zone centered in the ceramic bed. Because more heat is produced than is required to maintain the bed temperature, the energy is available to heat steam in embedded tubes in the ceramic bed. The steam is separated in a steam drum and is recirculated in another set of embedded tubes to create superheated steam suitable for a conventional steam turbine. The high temperature VAM oxidation technologies have already been expanded to largescale systems. The BHP Billiton West Colliery in Australia has contracted a large installation to convert the methane in 250 000 m3/h of coal mine ventilation air into 6 MW of electricity (8). However, some problems still exist with these technologies mainly due to fluctuation of the methane concentration and presence of dust in the ventilation air, which affect the system heat balance and increase system complexity, thus affecting the commercial viability of the process. Fluctuation of the methane concentration results in changes in reaction temperature, which requires frequent adjustment of the system heat balance. For some mines, the VAM concentration is strongly affected by the geological conditions, the mining method, and on the rate of production (9). In addition, the ventilation air inevitably contains large amounts of dusts (10), which can easily deposit on and foul the reactor and heat transfer surfaces, causing increased pressure drop across the system and adversely affecting the system operability. However, the VAM can also be used as a supplementary fuel, where the heat required by the system is generated by the combustion of a main fuel supply. Such systems can be designed for gas turbines, internal combustion engines, and boilers (11–13). The VAM is more suited as the supplementary fuel for boilers, especially coal-fired boilers, that require a great deal of air for combustion and are tolerant of the presence of dusts. The use of methane from ventilation air as a supplementary fuel for coal fired boilers has a long history. One of the latest demonstration projects was the one built at Valse Point Power Station in Australia, where a simulated ventilation air using methane extracted from a spent coal mine, with very low methane concentration but without dust, had been mixed with combustion air in a PC boiler. However, the use of the VAM in a circulated fluidized bed boiler has yet to be evaluated. This paper describes preliminary research into the utilization of the VAM as a supplementary fuel for circulating fluidized bed combustion boilers, including an economic analysis of the viability of such systems.
2. VAM as a Supplementary Boiler Fuel 2.1. Technical Principles. In coal-fired boilers, the coal mine methane can be used as a supplementary fuel by partially replacing the combustion air with the ventilation air so that the methane contained in the ventilation air is consumed in the combustion process while contributing to heat genera10.1021/es7022779 CCC: $40.75
2008 American Chemical Society
Published on Web 02/13/2008
FIGURE 1. A schematic of the experimental system for simulating methane oxidation in a circulating fluidized bed combustor.
TABLE 1. Proximate and Element Analysis of Datong Coal Vdaf (%) Cdaf (%) Hdaf (%) Odaf (%) Ndaf (%) Sdaf (%) Qar.net.p (MJ/kg) 24.7
83.0
5.28
8.32
0.82
2.58
27.8
tion. Under the high furnace temperature conditions, the low concentration methane is oxidized to release the reaction heat: CH4 + 2O2 ) CO2 + 2H2O ∆H(298) ) -802.7 kJ/mol (1) The methane oxidation reaction rate is affected by methane concentration, oxygen concentration, temperature, pressure, and residence time. In a pulverized coal fired boiler, the peak combustion temperature can exceed 1500 °C; thus, all methane will be completely oxidized. However, most minemouth power stations in China usually employ circulating fluidized bed boilers in which the combustion temperatures are only about 900 °C. Therefore, whether VAM can be completely oxidized remains an engineering question that deserves a scientific answer. The experimental system shown in Figure 1 was built to evaluate the extent of oxidation. A circulating fluidized bed combustion boiler with a height of 3.2 m, diameter of 100 mm, and operating temperature in the combustion zone controlled at 850 °C, was used to simulate methane oxidation. The air velocity in the riser was about 1.5 m/s. The coal fired was a bituminous from Datong Coal Mine, which is a typical Chinese thermal coal. Table 1 gives the coal analysis results, where the suffix daf is dry ash-free, ar.net.p is the net calorific value as received basis, and V is volatile matter. Air residence time in the furnace was about 2 s. Methane was injected to the primary air to simulate the typical VAM. The gas components were monitored before the distributor and after the cyclone. The methane oxidation rates are shown in Figure 2. Figure 2e gives the methane concentration in the primary air. From 15 to 20 min, the measurement equipment was shut down. When the methane concentration in the primary air was varied from 0.1 to 0.6%, methane in the riser outlet air was reduced to only a few ppm. This means that almost all methane was oxidized in the circulating fluidized bed boiler. It can be seen that the oxygen concentration in the outlet air decreased and the CO2 concentration increased with increasing methane concentration. The variation in the outlet oxygen concentrations was more than that from the methane oxidation. It must have been resulted from the coal combustion process because the system thermal equilibrium was not completely approached. The VAM reduced the incomplete combustion loss for the circulating fluidized bed
combustion furnace. The CO concentration increased slightly (less than 30 pm), but this is insignificant relative to the input methane concentration. During the experiments, the temperature at the circulating fluidized bed reactor outlet was kept stable, although there was a small variation. Basically, the coal feeding rate was fixed, indicating that the heat released by the VAM oxidation does not obviously affect the system heat balance in the experiments. In the current experiments, the NOx was not monitored. Therefore, more results are needed to further investigate the effect of the VAM on the NOx emission in the coal combustion. 2.2. VAM Utilization Process. The process flowsheet for use of VAM in a furnace is illustrated in Figure 3. A flow controller is needed for the boiler operation to match the air flow rate required by the boiler with the mine ventilation air flow rate. An antiexplosion device is installed in the pipeline to protect the boiler against coal mine accidents. A damper is installed to rapidly replace the ventilation gas flow with an ambient air flow stream to prevent excessive temperature when the methane concentration in the VAM increases rapidly. The flow distributor distributes the air to the different fans in the boiler. 2.3. Effect of VAM on Boiler Operations. The main effects of the ventilation air on the boiler system are related to corrosion and boiler efficiency. The main differences between the ventilation air and the ambient air are that the methane component is included, the ventilation air humidity is very high, sometime reaching 100%, and the temperature is quite stable, 22–26 °C. The high humidity does not obviously affect the coal combustion in the furnace (100% air humidity causes about 0.3% of the exhaust gas heat loss). However, it can cause corrosion of the air preheater surfaces. However, anticorrosion techniques are commonly used on air preheaters operated in humid regions, such as southern China. The ventilation air has various gaseous components, such as CO2, CO, H2S, SO2, NO2, C2H2, and O2 (14). Here, only sulfide causes corrosion of the pipeline and the heated boiler surfaces. The other gases will not harm the system. Zhang (14) noted that the gas stored in the coal rarely contains sulfide and that sulfides do not affect the electrical generation process. Many current experiences using high concentrations methane from coal mine have shown that it does not cause corrosion. Fluctuations of the methane concentrations in the ventilation air can sometimes affect the boiler combustion process. When the fluctuations are small, the boiler operations do not have to be adjusted. However, when the methane concentration changes rapidly, for example, from 0 to 1% in several seconds (although this is rare in real situations), the changes will affect the coal combustion in the furnace (6). Local temperature in the boiler can increase very rapidly when methane content rapidly increases. Therefore, the VAM concentration should be monitored during boiler operation. When the variation becomes very large, the damper in the pipeline delivering the ventilation air to the boiler should redirect ambient air into the pipeline while reducing intake of the ventilation air. The inlet temperature also affects the boiler efficiency, but the temperature variations of the ventilation air are usually small. Operating experience of boilers using just ambient air show that the effect of the small temperature changes in the ventilation air can be ignored.
3. Coal Savings The coal savings of the utilization of VAM as a supplementary fuel in a boiler is performed based on a 135 MWe circulating fluidized bed boiler. The heating value of 1 Nm3 CH4 is assumed to be 8500 Kcal, and the heating value of 1 Kg standard coal is assumed VOL. 42, NO. 7, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
2591
FIGURE 2. Methane oxidation results in the circulating fluidized bed reactor; (a) methane concentration at circulating fluidized bed combustion riser outlet, (b) O2 concentration at circulating fluidized bed combustion riser outlet, (c) CO2 concentration at circulating fluidized bed combustion riser outlet, (d) CO concentration at circulating fluidized bed riser outlet, (e) methane concentration in the primary air of the circulating fluidized bed combustion boiler, (f) temperature in the circulating fluidized bed reactor versus time Methane oxidation results in the circulating fluidized bed reactor.
FIGURE 3. Process flow diagram for use of VAM as a supplementary boiler fuel. to be 7000 Kcal. The VAM concentration is chosen as 0.33%. The boiler requires 430 000 Nm3/h of air for normal operation with about 40 ton/h of standard coal. When the air required 2592
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 42, NO. 7, 2008
by the boiler is replaced by ventilation air, the VAM will supply about 430 000 × 0.33% ) 1419 Nm3/h CH4, which would have a heat generation rate equivalent to (1419 × 0.85)/(0.7 × 1000) ) 1.72 tons/h of standard coal, which accounts for 1.72/40 ) 4.3% of the total coal consumption of the boiler. Normally, the circulating fluidized bed boiler operates each year for about 5000 h. Using VAM will reduce the standard coal usage by about 8600 tons/year. In China, the price of standard coal is about 350 RMB/ton (about 45$/ton), so the savings would be about 3 million RMB/y (about 384 000 $/y). The economic benefit from the coal saving insures the proposed system more economically feasible. If a project is
financially supported by the CDM (Clean Development Mechanism), more income can be realized (15–17).
Literature Cited (1) Zonghu., L. Coal bed methane — A kind of clean energy resource to be urgently developed and utilized. Industry Boiler (in Chinese) 2006, 3, 1–5. (2) Creedy, D.; Tilley, H. Coal bed methane extraction and utilization. In: Proceedings of the institute of mechanical engineering, Part A. J. Power Energy 2003, 217 (A1), 19–25. (3) Creedy, D. CBM business in China. World Coal 2002, 11 (6), 65–68. (4) China Development and Reform Committee. Scenario for the coal mine methane control. http://jincao.com/fa/24/ law24.156.htm, (in Chinese), 2005, 6 pp. (5) Zheng, S.; Wang, Y.; Wang, Z. Emission amount of coal bed methane in China. Coal Min. Saf. (in Chinese) 2005, 2, 29–33. (6) Mallett, C. W.; Su, S. Progress in developing ventilation air methane mitigation and utilization technologies. 3rd International Methane and Nitrous Oxide Mitigation Conference, Beijing, China, 2003, 1–18.. (7) Richard, M. Introduction of the VOCSIDIZER technology in power generation utilizing VAM. China Coal Bed Methane (in Chinese) 2004, 1, 44–46. (8) MEGTEC Systems Inc., www.megtec.com/html/news/press/ PR30June2004.pdf, 2004.. (9) Caifang, W.; Zeng, Y.; Zhang, Z. Research on unusual gush of coal bed gas in low gas concentration mines. Coal Geol. China (in Chinese) 2003, 15, 20–22. (10) Su, S.; Chen, H. W.; Teakle, P.; Xue, S. Characteristics of coal mine ventilation air flow. J. Environ. Manage. 2008, 86 (1), 44– 62.
(11) Coalbed Methane Outreach Program (CMOP), United States Environmental Protection Agency. Technical and economic assessment: Mitigation of methane emissions from coal mine ventilation air. www.epa.gov/cmop/docs/vam.pdf, 2006, 97 pp. (12) Methane to Markets, http://www.nanning.gov.cn/2385/ 2005_12_28/2385_97341_1135761300759.html (in Chinese), 2005; http://yosemite.epa.gov/ee/epa/ria.nsf/vwRef/A.2000.33? Open Document, 2006; http://www.methanetomarkets.org/events/ 2006/coal/docs/technology_table.pdf, 2006. (13) Binchuan, Z.; Shengchu, H.; Yuhong, H.; Wenge, L.; Xin, L., Clean Development Mechanism in China, http://cdm.ccchina.gov.cn/english/UpFile/File6.DOC. (14) Zhang, T. Comprehensive Control of Coal Bed Methane; Coal Industry Press: Beijing, 2001; p 498. (15) Schultz, K. H. Making CMM a reality through the flexible mechanisms of the Kyoto Protocol, the global expansion outlook. The Successful Commercialization of Global Coal Bed and Coal Mine Methane Projects. London, United Kingdom, 10–11 Nov 2004; CWC Associates Ltd.: London, United Kingdom, 2004; p 6. (16) Talkington, C. Emerging ROI opportunities in the exploration, extraction and supply of coal bed and coal mine methane — A global analysis. The Successful Commercialization of Global Coal Bed and Coal Mine Methane Projects. London, United Kingdom, 10–11 Nov 2004; CWC Associates Ltd: London, United Kingdom, 2004; p 8. (17) Jia, N.; Zhao, X.; Dong, W. Potential analysis of natural gas transmission from Western to Eastern China in the CDM Program. Environ. Prot. (in Chinese) 2005, 5, 107–111.
ES7022779
VOL. 42, NO. 7, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
2593
