Energy & Fuels 2008, 22, 2997–3004
2997
Influences of Different Operation Conditions on the Emission Characteristics of O2/Recycled Flue Gas (RFG) Waste Incineration Jyh-Cherng Chen,*,† Hsiao-Lin Huang,‡ and Jian-Sheng Huang† Department of EnVironmental Engineering, Hungkuang UniVersity, Number 34, Chung-Chie Road, Shalu, Taichung County 43302, Taiwan, and Department of Occupational Safety and Health, Chia Nan UniVersity of Pharmacy and Science, Number 60, Erh-Jen Road, Section 1, Jen-Te, Tainan 717, Taiwan ReceiVed March 20, 2008. ReVised Manuscript ReceiVed July 17, 2008
This study applies the modified O2/recycled flue gas (RFG) combustion technology to waste incineration and investigates the emission characteristics of CO2, CO, NOx, SO2, and HCl at different operation conditions. The operation parameters under investigation include the O2 concentration in the feed gas, the rate of RFG, the total flow rate of feed gas, the type of incinerator, and the chemical compositions of feedstock. Experimental results show that the concentrations of CO2, SO2, and HCl in the exhausts can be enhanced by properly adjusting the O2 concentration in the feed gas and the RFG rate. This facilitates the removal of these pollutants by conventional control equipment. With 21% O2 feed gas and 75% RFG rate, the highest CO2 concentration can achieve 54%. A fluidized bed incinerator had better combustion efficiency than a fixed bed incinerator. The combustion efficiency levels were 96.68-99.99% and can be improved by increasing the RFG rate and O2 concentration. Moreover, the total flow rate of feed gas influences the mixing levels of gas and solid as well as the residence time of combusted gas in the incinerator. With a moderate total flow rate of feed gas (60 L/min), the concentrations of CO2, SO2, and HCl in the exhausts can be enhanced and those of CO and NOx were depressed.
Introduction Because global warming and climate change as a result of greenhouse effects and air pollution are a continuing serious problem, how to effectively improve the combustion efficiency and mitigate the emissions of greenhouse gas CO2 becomes an international and imperative issue. Most anthropogenic CO2 are released from combustion processes; their concentrations in the exhaust gas are usually lower than 15% because of only 21% O2 is contained in normal air for combustion. CO2 is an inert and stable gas; it is hard to dissolve in water and react with other chemical absorbents. The separation or capture of CO2 from the combustion gas is difficult by conventional control technologies, such as physical adsorption, physical absorption, chemical absorption, cryogenic distillation, and membrane separation.1-4 On the other hand, the major gas in normal air (79% N2) is useless for waste combustion. It is always emitted as exhaust gas and consumes a lot of heat energy. Therefore, this O2/recycled flue gas (RFG) or O2/CO2 combustion technology was developed and applied to coal-fired power plants in Japan and Europe. Its basic principle is separating nitrogen out of the air and mixing pure O2 with the RFG (the major gas is CO2) to serve as the feed gas. With such adjustment of feed * To whom correspondence should be addressed. Telephone: 886-426318652, ext. 4109. Fax: 886-4-26525245. E-mail:
[email protected]. edu.tw. † Hungkuang University. ‡ Chia Nan University of Pharmacy and Science. (1) Reck, A.; Hoag, K. A comparison of greenhouse gas mitigation options. Energy 1997, 22, 115–120. (2) Adams, E. E.; Golomb, D. S.; Herzog, H. J. Ocean disposal of CO2 at intermediate depths. Energy ConVers. Manage. 1995, 36, 447–452. (3) Freund, P. International collaboration on capture, storage and utilization of greenhouse gases. Waste Manage. 1997, 17, 281–287. (4) Chakma, A. Separation of CO2 and SO2 from flue gas streams by liquid membranes. Energy ConVers. Manage. 1995, 36, 405–410.
gas composition, the amount of exhaust gas can be significantly reduced and the heat efficiency as well as the emitted CO2 concentration can be greatly enhanced (even higher than 90%). These advantages can facilitate the separation, control, and recovery of CO2 and other air pollutants.5-11 Most research used this O2/RFG combustion technology for CO2 emission control in coal-fired power plants;6,8,12,13 however, fewer studies applied this technology for waste incineration. (5) Nozaki, T.; Takano, S.; Kiga, T. Analysis of the flame formed during oxidation of pulverized coal by an O2-CO2 mixture. Energy 1997, 22, 199– 205. (6) Mathieu, P. Mitigation of CO2 emissions using low and near zero CO2 emission power plants. Proceeding of the Sixth International Conference on Technologies and Combustion for a Clean Environment, Porto, Portugal, 2001; Vol. 1, pp 7-14. (7) Reinke, M.; Carroni, R.; Winkler, D.; Griffin, T. Experimental investigation of natural gas combustion in oxygen/exhaust gas mixtures for zero emissions power generation. Proceeding of the Sixth International Conference on Technologies and Combustion for a Clean Environment, Porto, Portugal, 2001; Vol. 1, pp 15-20. (8) Shimoda, H.; Okawa, M.; Arai, K. Development of CO2 recovery type pulverized coal fired power plant applied oxygen and recycled fuel gas (CO2) combustion. Proceeding of the Sixth International Conference on Technologies and Combustion for a Clean Environment, Porto, Portugal, 2001; Vol. 1, pp 21-27. (9) Iijina., M.; Mitsuoka, S.; Mimura, T.; Suda, T. Development of CO2 recovery technology from combustion flue gas. JSME Int. J., Ser. B 1998, 41, 1007–1011. (10) Kimura, N.; Omata, K.; Kiga, T.; Takano, S.; Shikisima, S. The characteristics of pulverized coal combustion in O2/CO2 mixtures for CO2 recovery. Energy ConVers. Manage. 1995, 36, 805–808. (11) Croiset, E.; Thambimuthu, K. V. NOx and SOx emission from O2/ CO2 recycle coal combustion. Fuel 2001, 80, 2117–2121. (12) Mathieu, P.; Nihart, R. J. Zero-emission MATIANT cycle. J. Eng. Gas Turbines Power 1999, 121, 116–119. (13) Yamada, T.; Kiga, T.; Okawa, M.; Omata, K.; Kimura, N.; Arai, K.; Mori, T.; Kato, M. Characteristics of pulverized-coal combustion in CO2-recovery power plant applied O2/CO2 combustion. JSME Int. J., Ser. B 1998, 41, 1017–1022.
10.1021/ef800203e CCC: $40.75 2008 American Chemical Society Published on Web 08/20/2008
2998 Energy & Fuels, Vol. 22, No. 5, 2008
Chen et al.
Figure 1. Schematic diagram of the experimental apparatus. (1) Air compressor, (2) dehumidifier, (3) float meter, (4) oxygen cylinder, (5) first combustion chamber, (6) second combustion chamber, (7) rotary feeder, (8) thermocouple, (9) induced fan, (10) fly ash collector, (11) filter, (12) water vapor condenser, (13) sampling pump, (14) HCl impinger, (15) RFG pump, (16) dilution system, and (17) NDIR stack analyze.
The operation parameters that possibly influence the emission characteristics of O2/RFG or O2/CO2 combustion technology can be summarized as the following. (a) Physical and chemical compositions of feed materials: For normal waste incineration, the required quantity of oxygen is greatly varied with different materials feeding into the incinerator. During the normal air combustion of waste with a relatively high heating value or combustible content, a great quantity of combustible organic gases will be released and the temporary oxygen deficits happen in some areas of the incinerator because the air supply is insufficient. This will cause the formation of incomplete combustion products, such as polycyclic aromatic hydrocarbons (PAHs), chlorobenzenes, chlorophenols, dioxins, and CO. With O2/RFG combustion technology, these phenomena can be mitigated. The combustion behaviors and operation stability can be improved, especially for the waste with relatively high heating values or moisture.5,14 Moreover, injecting O2 into the second combustion chamber can remove the CO and the incompletely combusted hydrocarbons produced in the first combustion chamber. (b) O2 concentration in the feed gas: With a high concentration of O2 in the feed gas, the contacts and reaction intensities between O2 and the combustibles are increased. The total flow rate of feed gas and exhausts can be significantly decreased. The residence times of combustion gases in the incinerator are therefore extended, which leads to higher decomposition efficiency of incompletely combusted pollutants. In addition, a significant reduction in the quantity of the exhausts by this O2/RFG combustion technology can save considerable cost on the required auxiliary fuels, especially for the combustion of waste with high humidity and a low heating value. It is reported that 60% auxiliary fuels can be saved if pure O2 replaces normal air as the feed gas under the condition of 1300 °C and 2% residual oxygen in the exhausts.10,11,13 (c) Combustion temperature: Because oxygen-rich combustion usually has the problem of high NOx formation because of local high temperature spots in the incinerator, the RFG in this O2/RFG system plays an important role to restrain the flame temperature and promote the mixing of combusted gases. The temperature distributions in the combustion chamber could be uniform, and the formation of NOx is depressed.8,11,16 (d) RFG rate: Recycling the flue gas not only restrains the temperature spots and NOx formation, it also recycles the heat energy and increases the total residence time of combusted gases.5
The O2/RFG combustion technology is not popular in industry and waste incineration, but it will have considerable development potentials in the future because the energy deficiency and greenhouse effects are a continuing serious problem. Our laboratory has studied the combustion characteristics of O2/RFG technology and its modification and application for 4 years. The combustion behaviors and emission characteristics of coal combustion and waste incineration by conventional O2/CO2 and O2/RFG technologies were investigated.15,16 The results showed that the combustion efficiencies were improved and the concentrations of gas emissions were enhanced. However, great amounts of pure O2 or CO2 gas were required to perform the combustion technology. A modified O2/RFG combustion technology was there developed and applied to waste incineration, and the emission characteristics of acid gases and organic pollutants were investigated by means of theoretical and experimental approaches.17,18 The results indicated that the modified O2/RFG combustion technology is more feasible and competitive than conventional O2/RFG technology. The combustion efficiencies and destruction efficiencies of organic pollutants can be increased with proper feed gas compositions and RFG rate. On the basis of the above research results and experiences, this study further investigates the effects of different operation conditions on the emissions of acid gases by the modified O2/RFG combustion technology for waste incineration. The major parameters under investigation include the O2 concentration in the feed gas, the RFG rate, the total flow rate of feed gas, the type of incinerator, and the chemical compositions of feedstock. The results are suppose to provide useful (14) Harada, E.; Uekado, M.; Takao, S.; Tanaka, T.; Suda, M. Research and development of a new coal based combined cycle power plant concept. JSME Int. J., Ser. B 1998, 41 (3), 685–690. (16) Chen, J. C.; Huang, J. S. Combustion efficiency and CO2 emission from O2/N2, O2/CO2 and O2/RFG coal combustion processes. EnViron. Eng. Sci. 2007, 24 (3), 353–362. (15) Chen, J. C.; Liu, Z. S.; Huang, J. S. Emission characteristics of coal combustion in different O2/N2, O2/CO2 and O2/RFG atmosphere. J. Hazard. Mater. 2007, 142 (1-2), 266–271. (17) Chen, J. C.; Huang, J. S. Theoretical and experimental study on the emission characteristics of waste plastics incineration by modified O2/ RFG combustion technology. Fuel 2007, 86 (17-18), 2824–2832. (18) Chen, J. C.; Huang, J. S.; Chen, J. M.; Guo, J. S. Emission characteristics of PAHs, benzene and phenol group hydrocarbons in O2/ RFG waste incineration processes. Fuel 2008, 87, 2787–2797.
Different Conditions for O2/RFG Incineration
Energy & Fuels, Vol. 22, No. 5, 2008 2999
Table 1. Chemical Composition of Feed Materials weight % run
materials
C
H
O
N
S
Cl
1-14 15-24 25-33
PE coal plus PVC (4:1) coal plus PVC (1:1)
85.7 62.9 53.7
14.3 5.3 5.1
0 11.7 7.3
0 0.6 0.3
0 0.3 0.2
0 11.4 28.4
information for the future development and application of this modified O2/RFG combustion technology. Experimental Section Apparatus. Figure 1 illustrates the schematic diagram of the experimental apparatus. The composite feed gas with different O2 concentrations was prepared by a pure oxygen cylinder, an air compressor, four mass flow meters, and a gas mixer. The incinerator consisted of a feedstock feeder, a preheated chamber, a first combustion chamber (70 cm height and 6 cm inside diameter), and a second combustion chamber (50 cm height and 10 cm inside diameter). All of the chambers are made of 3 mm thick stainless steel (AISI 310). The incinerator is fitted with a perforated stainlesssteel gas distributor. Both the fixed bed reactor and the fluidized bed reactor are located in the same position (first combustion chamber) of Figure 1. These two incinerators have the same dimensions. The only one difference between these two reactors is that silica sand was filled inside the fluidized bed incinerator, and it was fluidized during the combustion processes. Five thermocouples were inserted into the axis to determine the temperature profiles of combustion chambers and flue pipes. Silica sands were used as the fluidized media, and their particle size distributions were 15.05 wt %, 470-300 µm; 49.62 wt %, 300-214 µm; 17.35 wt %, 214-163 µm; and 12 wt %,
