Energy & Fuels 2006, 20, 575-578
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Combustion Characteristics and Pollutant Control by Eco-fuel from Polyurethane Foam Heejoon Kim,* Wataru Minami, and Tianji Li Department of Ecological Engineering, Toyohashi UniVersity of Technology, Tempaku-cho, Toyohashi, 441-8580, Japan ReceiVed June 24, 2005. ReVised Manuscript ReceiVed NoVember 5, 2005
Around the world, thousands of tons of polyurethane (PU) foam waste accumulate in municipal dumps from sources such as car disposal industries. Due to its low density and high volume, polyurethane foam waste is difficult to treat and dispose of in landfill. In this study, we developed a novel and environmentally friendly disposal method for PU waste by mixing it with coal to produce a new composite fuel called Ecofuel. Eco-fuel is made by briquetting a mixture of PU with coal (10:90 by weight) under high pressure. The combustion characteristics and emission gases of Eco-fuel were compared with those of biobriquette (a 20:80, by weight, briquette of biomass and coal). The combustion characteristics of Eco-fuel briquette fuel were improved with the addition of PU foam. Although PU has a higher nitrogen content (6 wt %) compared to biomass (1.4 wt %) and coal (1 wt %), the emission of NOx from the combustion of Eco-fuel did not increase. We discovered that the combustion of PU results in the conversion of nitrogen to NH3, and then NH3 reacts with NOx to neutralize to N2.
Introduction Polyurethane (PU) foam is used worldwide in industries such as automobile, construction, and packaging. Each year, large amounts of PU foam waste are generated. This waste is hard to treat due to its low-density and high-volume properties, and it does not easily decompose in landfill. Biobriquette is a Japanesedeveloped composite solid fuel that utilizes agriculture wastes and low-grade coal for energy.1,2 It is composed of about 20 mass % of biomass and 80 mass % of coal and is briquetted under high pressuresa briquetting technology commonly used in some developing countries.3-6 We developed a new artificial solid fuel, Eco-fuel, that uses both waste PU foam and coal, while controlling the emissions of environmental pollutants from coal combustion. Eco-fuel is composed of about 10 mass % of PU foam and 90 mass % of low-grade coal, briquetted under high pressure (3 ton/cm2). In this study, we compared the combustion characteristics and emissions (particularly, NOx) of coal, biobriquette, and Ecofuel. Experimental Section Our first experimental system (Figure 1) consisted of an electrically heated batch furnace, temperature controllers, a digital balance, and a flue-gas analyzing system. The sample briquette was * Corresponding Author. Tel: (86) 532-44-6908. Fax: (86) 532-44-6929. E-mail:
[email protected]. (1) Maruyama, T.; Mizoguti, C. Coal in Asia-Pacific. NEDO (Japan) 1992, 4, 32. (2) Maruyama, T.; Takemichi, S. Coal Combust. (USA) 1988, 987-994. (3) Lu, G.-Q.; Toyama, T.; Kim, H. J.; Naruse, I.; Ohtake, K.; Kamide, M. Kaguku Kogaku Ronbunshu 1997, 23, 404-412. (4) Lu, G.-Q.; Kim, H. J.; Naruse, I.; Yuan, J. W.; Ohtake, K.; Kamide, M. Energy Fuels 1998, 12, 689-696. (5) Naruse, I.; Kim, H.-J.; Lu, G.-Q.; Yuan, J.-W.; Ohtake, K. 27th Symp. (Int.) Combust. 1998, 2973. (6) Kim, H.-J.; Lu, G.; Li, T.; Sadakata, M. EnViron. Sci. Technol. 2002, 36, 1607-1612.
placed in a basket linked to the upper digital balance, and the basket was then positioned in the center of the furnace along the vertical axis. The injected air was preheated to a desired temperature by a packed bed of alumina balls located at the bottom of the furnace. Samples were 5 g with different PU content. The properties of tested coals, biomass, and PU are listed in Table 1. The combustion characteristics of different samples were tested at a temperature range of 973-1173 K. The NOx emission of Eco-fuel at a temperature range of 873-1273 K was measured. Changes in both the sample mass and in the concentration of SO2, NOx, CO, CO2, and O2 in the flue gas were continuously measured during the combustion process, by the digital balance and the flue-gas analyzing system, respectively. Our second experimental setup apparatus (Figure 2) consisted of a horizontal electrical furnace and was used to investigate the pyrolysis characteristics of PU foam. PU foam was cut into 1-mm pieces and placed in a holding vessel in the center of the quartz reactor. The reactor was moved horizontally so that the vessel with the sample was in the center of the furnace, which was preheated to 1073 K. Air or Ar gas was used as the medium, and the flue gas was analyzed continuously by a gas chromatography-mass spectrometry system (GC-MS).
Results and Discussions The combustion characteristics of coal briquette, biobriquette, and Eco-fuel were investigated by comparing their combustion profiles over time (Figure 3). Samples of biobriquette and Ecofuel contained 3 wt % of biomass and urethane, respectively. The results showed that both biomass and PU additions improved the coal combustion characteristics and that PU was a little more effective than biomass. The effect of PU waste content on the burning profile was also investigated (Figure 4). As PU content was increased, the burning time was reduced remarkably. The burning time decreased by 50% when PU content was 20 mass %. Thus, PU in Eco-fuel greatly improved the briquette combustion rate. As reported in our previous studies,3-6 combustion of coal briquette
10.1021/ef050186t CCC: $33.50 © 2006 American Chemical Society Published on Web 01/24/2006
576 Energy & Fuels, Vol. 20, No. 2, 2006
Kim et al.
Figure 1. The experimental setup for combustion tests.
Figure 2. The experimental setup for pyrolysis tests. Figure 3. Comparison of burning profiles of different samples.
is clearly divided into two combustion stages, namely, volatile combustion and char combustion. Comparison of the burning time of the last half fractions of various samples in Figure 4 showed that PU promoted char combustion. The last 50 mass % of all samples contained the same weight (2.5 g) of char (because the volatile fraction of PU is very high), but their burnout times were quite different. For example, burnout of the
last 50 mass % of a coal briquette took 55 min, while the Ecofuel with 20 mass % addition of PU took 35 min (Figure 4). As shown in Table 1, the volatile fraction of PU foam was 98%. The combustion of PU in the volatile combustion stage generated a large amount of pores in the Eco-fuel. These pores enhanced oxygen diffusion in the following stage of char
Table 1. Properties of Tested Coal, Biomass, and Polyurethane proximate analysis [mass %, dry basis] sample coal biomass polyurethane
NL BJ cornstalk ECS
ultimate analysis [mass %, daf]
sulfur [mass %, daf]
ash
VM
FC
fuel ratio [-]
C
H
N
total combustible
17.2 20.5 4.2 0
36.6 19.6 79.6 97.9
46. 2 59. 9 16. 2 2.1
1.26 3.05 0.20 0.02
79.2 83.9 43.6 59.8
4.9 4.2 5.6 8.4
1.7 1.0 0.7 6.0
0.8 2.2 0.0 0.5
0.3 1.6 0.0 0.5
calorific value [kJ/kg] QH
QL
26700 16745 27969
26000 15291 26101
Combustion of Eco-fuel from Polyurethane Foam
Figure 4. Burning profiles of samples with different amounts of polyurethane.
Figure 5. Conversion of N to NOx using different samples at various temperatures.
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Figure 6. Conversion of N to NOx versus polyurethane content.
Figure 7. Emission gases from pyrolysis of polyurethane.
combustion. This is probably the reason PU foam improved the combustion of Eco-fuel. Since the nitrogen content of PU foam (6 mass %) is fairly high, the NOx emission from its combustion process was expected to be very high. The conversion of N to NOx during the combustion of coal briquette, biobriquette, and Eco-fuel at different temperatures was investigated (Figure 5). Samples of biobriquette and Eco-fuel contained 3 mass % of biomass and PU, respectively. The conversion was defined as
η ) NNOx/Nall where NNOx was the mole number of nitrogen emitted as NOx, and Nall was the mole number of all nitrogen in the sample. The conversion of N to NOx for PU-containing samples performed a little bit lower than the other samples at various temperatures. Conversion also decreased with increasing temperature for all samples. The effect of PU content in Eco-fuel on the conversion of N to NOx was also studied using GC-MS. Conversion of N to NOx decreased a little with the increase of PU content (Figure 6). The above results indicate that emission of NOx is not increased by adding PU, despite its high nitrogen content. On the contrary, PU showed the ability to denitrify. To understand the denitrification mechanism of PU in Eco-fuel combustion, we studied the pyrolysis behavior and combustion characteristics of PU foam. Experiments were performed using the horizontal apparatus shown in Figure 2. One gram of PU foam was placed in the reactor preheated to 1073 K in the atmosphere of Ar. Emission gas was analyzed by GC-MS; the results are shown
Figure 8. Emission gases from combustion of polyurethane.
in Figure 7. Reducing gases such as H2, NH, NH2, and NH3 were detected in the emission gas mixture. In fact NH and NH2 were produced from decomposition of NH3 in the GC-MS system. This means that H2 and NH3 were the main reducing gases emitted following PU pyrolysis. As discovered in some studies,7,8 reducing gases such as NH3 react with NOx to produce N2. This is probably the denitrification mechanism occurring with Eco-fuel. To further explain the denitrification mechanism, we also analyzed the flue gas of the combustion of PU by GCMS. Even during the combustion process of PU, we detected reducing gases such as H2 and NH3 (Figure 8). Thus, the (7) Zamansky, V. M.; Sheldon, M. S.; Maly, P. M. 27th Symp. (Int.) Combust. 1998, 3001-3008. (8) Zamansky, V. M.; Ho, L.; Maly, P. M.; Seeker, W. R. 26th Symp. (Int.) Combust. 1996, 2075-2082.
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complete mechanism of denitrification in the combustion of Ecofuel most probably involves the formation of NH3, as shown both in the pyrolysis and combustion processes of PU, and the NH3 then reacts with NOx to produce N2.
Kim et al.
The combustion of PU does not emit extra NOx, although the nitrogen content of PU is high. The denitrification mechanism of PU is due to the reaction of NOx with NH3 that formed in the pyrolysis and combustion process of PU.
Conclusion PU-form waste can be effectively treated by mixing it with coal to produce an environmentally friendly fuel, the Eco-fuel. PU improves the combustion characteristics of coal briquette.
Acknowledgment. We thank INOAC Corporation Anjo Plant for providing PU foam samples. EF050186T
