Emissions and Particle-Size Distribution of Some Metallic Elements of Two Peat/Oil-Fired Boilers Ahti 0. Itkonen*+ and Matti J. Jantunent Department of Environmental Hygiene, University of Kuopio, SF-702 11 Kuopio, Finland, and Department of Environmental Hygiene and Toxicology, National Public Health Institute, Box 95, SF-7070 1 Kuopio, Finland
Two peat/oil-fired power plants-plant A, lOO-MW, pulverized peat fired, electrostatic precipitator (ESP) equipped, and plant B, 7-MWt fluidized bed fired, multicyclone (MC) equipped-were studied to determine emissions of particles and 11elements. Particulate samples were collected and fractionated before and after the fly ash collectors by the University of Kuopio modified Bird & Tole five-stage cascade centripeters, weighed, and analyzed by use of atomic absorption spectrometry. In pulverized peat firing the aerodynamic size distributions of particles were bimodal both before and after the ESP. The mass median aerodynamic diameter (MMAD) of the particulate matter was reduced in the l3SP from 17 to 9.6 pm. In fluidized bed peat firing the particle size distribution showed only one mode. After the MC the MMAD was 2.3 pm. The studied elements were nonvolatile (Fe, Ca, Mg, and Co), slightly volatile (Cu and Ni), and volatile (As, Cd, Pb, and Zn). The concentrations of these elements in the fly ash, as well as the enrichment of the volatile elements on the smallest particles, showed no difference from pulverized coal combustion results. The volatile elements were further enriched in the ESP, because it collects large particles better than smaller ones and spherical molten ash particles better than complex, nonmolten ones. The latter resulted in concentration enrichment even within each particulate size class.
Introduction During the last decade the objects of the Finnish energy policy have been to conserve and replace imported fuels with domestic ones, most importantly peat. Since 1970 the use of peat for fuel has increased rapidly. In 1980 about two million tons of peat was used in 11 municipal and industrial boilers. Most of these plants are fired with pulverized milled peat together with 5-60% oil. The quality of peat varies considerably due to its moisture content (40-60 % ), sulfur content (0.1-0.4 % ), organic consistency, and degree of humification. As a fuel it is comparable to lignite. In the carbonizing chain peat is a prestage of brown coal. Because of the low heat value (6-12 MJ/kg) and density (300-400 kg/m3) of milled peat, its use is economical only in the vicinity of the sources. Department of Environmental Hygiene, University of Kuopio, SF-70211 Kuopio, Finland. Department of Environmental Hygiene and Toxicology,National Public Health Institute, Neulaniementie 4, Box 95, SF-70701 Finland.
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Peat contains 2-10% ash. This incombustible mineral matter results from decayed plants, airborne dust, and waterborne matter that is washed or dissolved from the nearby mineral soils. Peat ash contains most of the known elements at least in trace quantities ( I ) . Their concentrations vary significantly between bogs and even within the same bog (2-4). The use of fossil fuels has attained widespread interest in environmental research. Environmental consequences of large-scale peat combustion have been studied only since 1978, and only a few publications of their results have appeared so far (5-7). Peat combustion is known to produce plenty of particulate emissions, but increased efficiencies in solid fuel combustion and modern fly ash collectors have reduced the emissions of soot and fly ash. The results presented in this publication are a part of a larger study made for the Finnish Ministry of Trade and Industry (7).
Materials and Methods Plant Description. Plant A is a municipal power plant in Kuopio, Central Finland. Its operation began in 1972, and it is the first large peat-fired boiler plant in Finland. At capacity it produces 30 MW of electric power and 62 MW of district heat. The pulverized peat- and oil-fired boiler is equipped with two beater mills and four front-wall oil and peat burners. During our sampling the plant was run at 52-92% of the maximum load, and on the average 67% (3944%) of the heat input was produced by peat. The combustion temperature was 1500-1600 K, and the flue gas temperature before the electrostatic precipitator was 430-450 K. Plant B is located in Suonenjoki, 50 km from Kuopio. It produces district heat by burning milled peat and oil in a fluidized bed combustor (FBC). Its operation began in 1979. In a pyroflow FBC most of the bed material follows the hot flue gases. The hot cyclone adjancent to the boiler removes most of this material and returns it into the bed. The final fly ash collector is a multicyclone (MC) (8). During our sampling the plant was run at 33-53% of the maximum load, and on the average 66% (52-75%) of the heat was produced by peat. The combustion temperature was 970-1070 K, and the flue gas temperature was 390-410 K. Sampling and Analysis. Particulates were collected before and after the ESP and the MC by modified Bird & Tole, five-stage cascade centripeter samplers. The cut
0 1986 American Chemical Society
Environ. Sci. Technol., Vol. 20, No. 4, 1986
335
Table I. Calibrated and Repaired Cut Sizes of Cascade Centripeter sampler stage 1 2 3 4 5
particle diam, bum calibration plant A, dry air, 297 K 430 K >45 13-45 3.8-15 1.4-3.8
>52 15-52 4.6-15 1.6-4.6
dA > 14.5 pm
TOTAL EMISSION
I
I
Particulate emission (pg/J)
Figure 2. Frequency distribution of particulate emissions before the ESP at plant A. Soot blowing samples are marked darker (61 samples during 96 h included 9 soot blowing samples).
DA < 1.6 pm is more pronounced than before the ESP. The frequency distribution of the total fly ash emission before the ESP is presented in Figure 2. The mean emission was 2600 ng/J before, and 160 ng/J after, the ESP. The average penetration through the ESP was 6% with a range of 1-17%. The frequency distributions of particulate emissions was log-normal, with a median of 1500 ng/J and an S, of 1.8 before, and a median of 90 ng/J and an S, of 1.7 after, the ESP. Morphologically, see Figure 3; the particulates can be categorized as follows: dense spherical molten ash particles, 2-15 pm in diameter, typical for coal combustion; typical sponge-like spherical particulates from oil combustion, 5-80 pm in diameter; remnants of botanical cell structures, very irregular in shape, and 8-40 pm in length; and tarlike condensed material on the backup filter. In plant B the average size distributions of fly ash during soot blowing and normal operation are presented in Figure 1. This particle size distribution is unimodal. The mode
Table 11. Comparison of Fly Ash Particle Size Distributions Measured from Solid-Fuel-Fired Bailer P l a n t s
combustion method
fly ash collector, penetration. %
Ondov et al. (1979)" A B Carpenter et al. (1980) before collectorb after collectorb Sbendrikar et al. (1983) before collector after collector Jantunen et al. (1980)A B
pulverized oulverized kuidized bed
Coal ESP 0.3 ESP 2.5 CYC + BGH 0.8-1.5
chain grate pulverized
MC 11 ESP 0.4
Carpenter et al. (1980) before collectorb after collector'
fluidized bed
Lignite CYC + BGH' 2
Plant A normal, before collector, normal, after collector soot blowing, before collector soot blowing, after collector TWA? before collectoP TWA, after collector Plant B normal. after collector S w t blowing, after collector TWA
pulverized
pulverized
particle size distribution parametera measured MMAD. range (DJ, modes wm SS Pm (Dd, Pm 1.6 8.6
1.57 2.2
3.943.1 2.3-2.4
0.610 0.6-10
1.1 2.9
0.02-10 0.02-10 1-50 1-50
1, 4 7
BGH kd, loss of A will be zero order in A. For determinations of [lo2], (see below) it is important to limit the trapping agent to low enough concentration ([A] = O.lkd/kA) to avoid repressing [‘O,],,and to keep the loss of A first order. For furfuryl alcohol, the upper concentration limit therefore is about 2 X M, since kd = 2.5 X lo6 s-’ (14) and k A = k, = 1.2 X lo8 M-l s-I (7). This is also often the practical concentration limit for compounds
0 1986 American Chemical Society
Environ. Sci. Technol., Vol. 20, No. 4, 1986
341
