ARTICLE pubs.acs.org/EF
Coal and Biomass Partial Gasification and Soot Properties in an Atmospheric Fluidized Bed Guoyan Chen,*,†,‡,§ Yanguo Zhang,† Jiulong Zhu,§ Yan Cao,‡ and Weiping Pan‡ †
School of Mechanical Engineering, Tsinghua University, Beijing 100084, People’s Republic of China Institute for Combustion Science and Environmental Technology (ICSET), Western Kentucky University (WKU), Bowling Green, Kentucky 42101, United States § Fengquan Environmental Protection Company, Fuzhou 350007, People’s Republic of China ‡
ABSTRACT: Using air as the gasification medium, one kind of coal and one kind of biomass partial gasification studies were carried out in a laboratory-scale atmospheric fluidized bed with the various operating parameters. The effects of the O/C ratio, biomass/(coal þ biomass) (Fb/Fm) ratio, and bed temperature on the fuel gas compositions and the high heating value (HHV) were reported in this paper. The results show that the concentrations of carbon monoxide, hydrogen, and methane and the HHV decrease with the rise of the O/C ratio and the decline of the Fb/Fm ratio for coal and biomass partial gasification. A rise of the bed temperature favors the semi-gasification reaction of coal, but the concentrations of carbon monoxide and methane and the HHV decrease with the rise of the bed temperature, except hydrogen. In addition, the gas HHVs are between 1.0 and 3.3 MJ N1 m3. The gas yield and carbon conversion increase with the O/C ratio, Fb/Fm ratio, and bed temperature. The atomic percentage of carbon increases and the atomic percentage of oxygen decreases with the rise of the O/C ratio in fly ash. The concentration of soot decreases along with the O/C ratio in fly ash.
1. INTRODUCTION As a clean coal technology, coal gasification technology is widely used in the world. There has been abundant literature published on coal or char gasification at the temperatures lower than 1000 °C, where the gasification reaction is thought to be controlled by chemical kinetics.17 Coal partial gasification technology can realize the stage conversion of coal, which is difficult to gasify completely at low temperature and low pressure. This technology does not pursue the high heating value (HHV) of fuel gas and carbon conversion. The remaining char is burned to produce heat and electric power in a fluidizedbed combustor. The carbonizer fuel gas is filtered to remove particulate, and the cleaned fuel gas is then fired in a combustion turbine to increase the temperature of flue gas at the entrance of the gas turbine.8 Coal partial gasification in fluidized bed has been considered as a key technology of the integrated coal gasification combined cycle (IGCC) and the advanced pressurized fluidizedbed combustion combined cycle (APFBC-CC) to realize the high efficiency and good environmental performance for electricity generation, replacing existing coal-fired power plants.9 Up to now, Ocampo et al., Tomeczek et al., Watkinson et al., Kawabata et al., and Saffer et al. have studied the coal gasification characteristics in a fluidized bed and reported the gas heating values between 2.9 and 3.5 MJ N1 m3 using air and between 1.6 and 4.5 MJ N1 m3 using airsteam mixtures at atmospheric pressure.10 However, few researchers reported the coal partial gasification in a fluidized bed.11 Soot is one of the main pollutants generated in most of the combustion and gasification processes. It is generated in the hightemperature fuel-rich areas of flames and is usually partially oxidized as well as combusted within the combustion chamber; however, it may also appear in the flue gas, becoming an air r 2011 American Chemical Society
pollutant. Soot is an important pollutant itself as a low size particle and, therefore, breathable. Because of its structure, it may act as a condensation nucleus of polyaromatic hydrocarbons (PAHs) and other organic substances.12 Additionally, soot may represent a fouling problem in many combustion and gasification devices. Apart from the undesired effects of soot, it has also been mentioned to be interesting as a possible NOx reduction agent.13 Soot has been understood to form in many hydrocarbon flames principally from the combination and condensation of acetylene, benzene, or other aromatic hydrocarbons. Soot formation has been extensively studied in the past few years in different experimental devices, such as flames, shock wave reactors, and flow reactors, using different fuels as soot precursors, such as methane and other hydrocarbons, and mainly also diesel fuels,1417 but because of its complexity, a number of uncertainties still remain. While significant progress has been achieved in relation to the determination of its physical and chemical characteristics, significant uncertainty still remains concerning the formation, growing, and conversion under different conditions. In particular, the formation of soot implies a number of complex physical and chemical processes that control the conversion of fuel into solid particles that are not well-known at present.18 The most accepted theory for soot formation is welldescribed by Haynes and Wagner,19 in which the pyrolysis of hydrocarbons produces smaller hydrocarbons, acetylene in particular. The initial step is the formation of the first aromatic species from the aliphatic hydrocarbons, followed by the addition of other aromatic and alkyl species to give higher species, Received: December 27, 2010 Revised: April 11, 2011 Published: April 13, 2011 1964
dx.doi.org/10.1021/ef101754v | Energy Fuels 2011, 25, 1964–1969
Energy & Fuels
Figure 1. Schematic diagram of the coal gasification test system.
i.e., PAHs. The continued growth of these PAHs results into the generation of the smallest soot particles with diameters of the order of 1 nm and a mass of around 5002000 amu. The formation of PAHs is a key issue and, today, is of huge interest. It is believed to follow the H abstraction and C2H2 addition (HACA) route,20,21 even though there are some other different theories for PAH growth and soot formation.22 The contribution of the particulate from the combustion of wood is considered to be important because, globally, it is a major domestic fuel and a considerable number of deaths are attributed to the effects of wood smoke.2325 Smoke can also be formed from accidental and forest fires, where the toxic products form a major hazard.26 Consequently, the properties of smoke from biomass combustion have been studied over many decades. It has been established that combustion-generated particulates have an important impact on the climate and rainfall.27,28 In atmospheric modeling, it is recognized that smoke consists of soot, volatiles, and ash and the soot itself consists of two components, black soot and organic soot. Recently, it has been recognized that the black soot consists of two components, elemental soot and condensed organic compounds.2931 For aerosols produced from biomass burning, the organic carbon content is high and is thought to have a huge impact on the warming potential of the elemental carbon, even to the extent of neutralizing the warming effect.29,30 For primary emissions from fossil fuel, the organic carbon/elemental carbon ratio is lower, and thus, the soot has an overall warming effect.2933 Very little information regarding soot formation from coal is available in the literature.34This paper presents the results obtained in the partial gasification of coal and biomass in a laboratory-scale fluidized bed at atmosphere pressure with the presence of air and steam.
2. EXPERIMENTAL SECTION 2.1. Experimental Apparatus. Figure 1 shows a schematic diagram of the fluidized-bed system for coal partial gasification in this study. The whole system consists of oxygen and N2 cylinders, coal feed subsystem, fluidized-bed gasifier, dust-capture system, collect ash system, sample test system, and control subsystem. The fluidized-bed gasifier is made of a 3 mm thick, stainless-steel cylinder of 50.8 mm diameter and
ARTICLE
1.2 m height. The coal feed subsystem consists of a frame, a hopper, and a drive subsystem consisting of an electric motor and a speed controller. The gasifier is heated by the electric motor and obtains the aim temperature. Then, coal and biomass will be sent to the furnace and reactor with oxygen from the cylinder. At the bottom of the gasifier, the primary cyclone allows for the recovery of entrained particles. The temperature probes and pressure gauges are placed along the height of the gasifier and fuel gas pipe. There is a sampling port of fuel gas downstream of the filter. 2.2. Experimental Materials. Coal and biomass were used in the fluidized-bed gasifier in this study. Their properties are shown in Tables 1 and 2, respectively. 2.3. Experimental Procedure. Each run was started with the filling of the bed of quartz sand up to the required height. The screw feeder was turned on, and the minimum fluidizing oxygen and N2 flow rate required to fluidize the bed materials in the gasifier was supplied through the electric motor. The startup period was necessary to preheat the bed up to the required temperature before the commencement of coal feeding. When the bed temperature in the gasifier reached the demand of coal gasification, the O2, N2, and coal flow rates were adjusted to give the desired equivalence ratio. The fluidized-bed gasifier operated at the steady-state condition for an hour. The pressure loss and the temperature were monitored and registered at a 10 min interval. After the bed temperature stabilized, the fuel gas sample was collected for analysis. The coal feeder was stopped once the sample collection and data recording were finished. The whole fluidized bed coal gasification system was shutdown after the temperature dropped below the safe temperature. 2.4. Method of Analysis. The fuel gas sampled downstream of the filter was sent for analysis. The composition of fuel gas was analyzed by gas chromatography (America Agilent Technologies, Agilent GC3000). Chromatography calibration was performed with the standard gas, and the standard deviation curve of the typical component was drawn. Argon was used as the carrier gas at a flow rate of 40 mL/min. The temperature of the chromatography column was 80 °C, and the temperature of the thermal conductivity detector (TCD) was 120 °C. 2.5. Methods of Data Processing. The HHV of fuel gas is defined as follows: HHV ¼ ðXCO 3018 þ XH2 3052 þ XCH4 9500Þ 0:01 4:1868 ðkJ N1 m3 Þ
ð1Þ
where XCO, XH2, and XCH4 are the volumetric percentages of CO, H2, and CH4 in fuel gas, respectively. The dry gas yield, Y, is figured out from the material balance of nitrogen Y ¼
Qa 79% Wc XN2 %
ð2Þ
where Qa is the flow rate of air (N m3 h1), Wc is the coal feed rate (kg/h), and XN2 is the volumetric percentage of N2 in fuel gas. The carbon conversion is calculated by Xc ¼
12Y ðCO % þ CO2 % þ CH4 %Þ 100% 22:4Wc C %
ð3Þ
where Y is the dry gas yield (N m3 h1), C % is the mass percentage of carbon in coal ultimate analysis, and the other symbols are the volumetric percentage of fuel gas compositions.
3. RESULTS AND DISCUSSION 3.1. Effect of the O/C Ratio. The effect of the O/C ratio on the gas composition and the HHV is plotted in Figure 2. It can be seen that, with the rise of the O/C ratio, the concentrations of 1965
dx.doi.org/10.1021/ef101754v |Energy Fuels 2011, 25, 1964–1969
Energy & Fuels
ARTICLE
Table 1. Proximate and Ultimate Analyses and Major Oxides of Coal and Biomass Samples dry basis, by weight moisture
ash
volatile
sulfur
British thermal
carbon
hydrogen
nitrogen
oxygen
chlorine
mercury
fluorine
bromine
(%)
(%)
matter (%)
(%)
unit (btu/lb)
(%)
(%)
(%)
(%)
(ppm)
(ppm)
(ppm)
(ppm)
PRB coal
15.01
7.64
43.32
0.54
11818
69.02
4.72
0.80
17.28
82
0.12
54
ND
biomass
6.57
0.56
80.27
0.01
8488
46.65
5.90
ND
46.89
132
