Formation of Fine Particulate Matter in a Domestic Pellet-Fired Boiler. José Madeira

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1 Formation of Fine Particulate Matter in a Domestic Pellet-Fired Boiler José Madeira Abstract This article concentrates on the formation of fine particulate matter (PM) in a domestic pellet-fired boiler. To this end, the study included detailed characterization of the combustion process inside the combustion chamber of the domestic boiler, and included measurements of local gas temperatures, major gas species concentrations and PM concentration and size distribution. After collection, the PM was also morphologically and chemically characterized. The results revealed that (i) particles smaller than.1 µm are rapidly formed at the beginning of the combustion process, where the oxidation rate is also high; however, the oxidation rate decreases rapidly along the combustion chamber height due to the significant temperature drop; (ii) the diameter corresponding to the maximum amount of particles slightly increases from the burner until 4/5 of the combustion chamber height, indicating some tendency to form aggregates; in the final region of the combustion chamber is observed a decrease in the diameter corresponding to the maximum amount of particles, as consequence of some PM fragmentation; (iii) the carbon present in the particles significantly decreases along the combustion chamber and the inorganic elements tend to condense and solidify with the temperature decay across the chamber; and, finally, (iv) the small particles are wrapped by inorganic compounds that deposited on them by heterogeneous condensation. Keywords: biomass, pellets, domestic boiler, PM formation 1. Introduction In a recent work [1], the particulate matter (PM) emissions from a domestic pellet-fired boiler was quantified and characterized morphologically during steady-state operation for different boiler thermal inputs. The results revealed that a major part of the PM collected had dimensions smaller than 2.5 µm, with the SEM analysis revealing the presence of ultrafine, sub-micrometer and micrometer sized particles. The ultrafine particles were mostly composed of inorganic compounds (O, K, Cl, Na, Ca, Mg), and sub-micrometer and micrometer sized were made of carbon and inorganic compounds (C, O, K, Ca, Mg, P). In the light of these results, it is important to understand the formation process of these particles in a combustion chamber, so their emissions can be reduced. The literature reveals that studies on PM measurements inside the flame in domestic boilers are very scarce, being so far the emphasis being placed on PM emissions. Wiinikka and Gebart [2] found that with the increasing of wall temperature of the boiler less soot and coarse particles are emitted; however an increase in the emission of submicrometric particles was observed. They concluded that to minimize particle emission, char combustion temperature should be minimized and secondary reaction zone should suffer an increase in its temperature. They also concluded [3] that with primary and secondary air equally distributed, the generation of coarse particles decays to a minimum. Since it is possible to retain those coarse particles, the combustion conditions should be optimized to minimize the formation of fine particles, since these are the most difficult to capture. Johansson et al. [4] reported that high CO emissions were associated with high emissions of total organic carbon, polycyclic aromatic hydrocarbon (PAH), methane (CH 4 ) and other volatile organic compounds. They also conclude that the emission of submicrometric particles is boosted by poor combustion conditions. Bignal et al. [5] focused their investigation in the emission of PAH and found that high emissions of this type of compounds were connected to high emissions of CO. This suggests that these emissions are consequence of inefficient combustion. To minimize these emissions, the fuel should have low moisture content and the combustion facility should be operated at maximum power, since the slumber mode produces inefficient combustion. Atkins et al. [6] further developed this analysis, relating the emissions of PAH with polycyclic chlorinated biphenyl (PCB) emissions. PCB emissions are related with PAH emissions, however with a much lesser concentration, due to the low concentration of chlorine in biomass. They also restated that the fuel should have low moisture content and that the combustion facility should be operated at maximum power, to guarantee efficient combustion. 1

2 Fitzpatrick et al [7] studied the connection between the emission of oxygenated species and soot formation. The combustion gases were analyzed in a Fourier Transform Infrared Spectroscopy device, and soot particles were collected from 3 sites in the combustion chamber. They found that oxygen appears to play a significant role in soot production and that there may be two routes by which PAH can be synthesized. These routes divide themselves in a route regarding a conventional hydrocarbon mechanism and another route regarding polymerization of biomass pyrolysis fragments. They concluded that biomass particulate has greater inherent oxygen functionalities than hydrocarbons soot and adsorbs the primary, secondary and tertiary pyrolysis products such as organic acids, aldehydes and phenolics. They also propose a model in which soot can be derived from a number of routes, which vary in importance depending on the combustion conditions. This article concentrates on the formation of fine particulate matter (PM) in a domestic pelletfired boiler. To this end, the study included detailed characterization of the combustion process inside the combustion chamber of the domestic boiler, and included measurements of local gas temperatures, major gas species concentrations and PM concentration and size distribution. After collection, the PM was also morphologically and chemically characterized. 2. Materials and Methods The present tests have been performed in a domestic wood pellet-fired boiler with a maximum thermal capacity of 22 kw, with forced draught [8]. Figure 1 shows a schematic of the experimental set-up. The pellets are manually loaded into a hopper with a capacity of 45 kg and are fed to the burner through a screw feeder that works by impulses. The feeding rate of the pellets is regulated by the boiler load and the pellets consumption rate is measured with the aid of a loss-in-weight technique, for what the boiler is mounted on a weighbridge. The combustion of the pellets takes place within a hemispherical basket (brazier) with a diameter of 12 mm. The basket is top-fed with pellets by the screw. Ignition is accomplished with the aid of an electrical resistance placed close to the basket and the primary air is supplied by a dedicated fan to the basket through several small orifices located across the basket bottom. The resulting hot gases from the combustion exchange heat with water circulating in a heat exchanger located at the top of the combustion chamber. It should be pointed out that a short cleaning period of the basket is programmed to occur once every 11.5 minutes. During the cleaning process the fuel supply decreases and the air supply increases during few minutes in order to remove the ashes accumulated at the bottom of the basket (bottom ashes). The heat transferred to the water in the boiler is dissipated through a plate heat exchanger with the aid of an external water circuit. In the present study, the combustion chamber was modified to allow the introduction of probes inside the flame. Figure 2 shows schematically the location of the points were in-flame measurements of temperature, gas species concentration and PM have been conducted. The gas temperature measurements were achieved with a type R thermocouple, and the gas species concentration measurements were obtained with the aid of a stainless-steel probe and appropriated analytical instrumentation (a magnetic pressure analyzer for O 2 measurements, a nondispersive infrared gas analyzer for CO and CO 2 measurements, a flame ionization detector for HC measurements and a chemiluminescent analyzer for NO x measurements). PM sampling was accomplished with the aid of a rapid dilution probe (Figure 1) similar to those used by Strand et al. [9] and Wiinikka et al. [1]. To prevent condensation and coagulation of inorganic species in the sampling line, N 2 was injected in the probe to dilute and freeze the reactions. The suction was created by two pumps, one connected to a Dekati 13-stages low pressure cascade impactor (or DLPI), and another one connected to a Tecora total filter holder. The arrangement is sketched in Figure 1. The PM size fractions were determined by weighting the Dekati LPI substrates. The total mass concentration of PM was also determined by weighting the quartz microfiber filter used in the total filter holder. The DLPI used allowed collecting 13 particle cut sizes from.3 µm to 1 µm. Following PM weighting, the morphology and chemical composition of selected samples was examined in a SEM JEOL, model JSM-71F. This microscope allows obtaining 3-D images from a selected area through the sample irradiation by an electron beam. The microscope is also equipped with an EDS that allows quantifying the elemental composition of a sample within a resolution of about 1 µm 2. 2

3 3. Results and Discussion Table 1 presents the main characteristics of the pine pellets used in this work, which are currently manufactured and commercialized in Portugal, and Table 2 summarizes the boiler operating condition examined here. Figure 3 shows radial profiles of gas species concentrations obtained in port 4 in different days to show the degree of typical data repeatability achieved in this study. As can be seen, the maximum differences do not exceed 1%. Figure 4 shows the radial profiles of temperature and gas species concentrations in ports 1, 2, 3, 4 and 5 for the boiler operation condition in Table 2. To help the analysis, Figure 5 shows the axial profiles of temperature and gas species concentrations for the boiler operation condition in Table 2. The results show that the main reaction zone is located between ports 1 and 3, where the measured concentration gradients are more intense. After port 3, the concentrations of O 2, CO 2, CO and HC reveal the end of the main reaction zone. Near the sidewalls of the combustion chamber high concentrations of O 2 are measured due to the lack of chemical reactions in these zones and to the high excess air level associated with the operation of this type of boilers. The concentration of NO x reaches maximum values in ports 1 and 2 where the maximum temperature is around 12 ºC. This indicates that the NO fuel mechanism should be the main contributor to the NO x emissions. Figure 6 shows the axial profiles of PM total concentration measured using the Dekati LPI and the total filter for the boiler operation condition in Table 2. It is seen that the results are consistent, being the differences due to the small fraction of particles that deposits out of the substrates in the Dekati LPI and therefore are not accounted for. The total filter collects all of the PM. Figure 7 shows the PM size distribution in various measurement locations for the boiler operation condition in Table 2. It is observed that along the burner central axis, the total amount of particles decreases significantly from port 1 to port 2. This reveals that particles are rapidly formed early in the combustion process, possibly soot particles, that also tend to rapidly oxidize between ports 1 and 2. After port 2, the soot oxidation rate decreases significantly probably due to the temperature decay in there. It is interesting to observe that the diameter corresponding to the maximum amount of particles increases slightly from port 1 to port 4, indicating some tendency to form aggregates. From port 4 to port 5 it is observed a decrease of the maximum amount of particles corresponding diameter, probably due to PM fragmentation. Away from the burner axis, the soot oxidation rates are inferior to those observed in the central zone between ports 1 and 2. Figure 8 shows typical SEM images. In Figure 8a) a large number of soot aggregates are observed. In Figure 8b) it is possible to seen small particles (probably soot) covered by inorganic compounds that have condensed over them. In Figure 8c), it is possible to observe some crystalline compounds, suggesting that the temperature in this zone is not high enough to guarantee the inorganic compounds fluidity. In Figure 8d) it is observed that there is not much condensation of inorganic compounds, probably due to the relatively high temperatures found in this region. Figure 9 shows the PM chemical composition in 6 substrates for port 1 at r = for the boiler operation condition in Table 2. For each subtract, the analysis was made in 5 different zones with an area of 5 x 5 µm 2. Results show that there are no significant changes in PM chemical composition, revealing that the existing compounds tend to form different size aggregates and that the data variation registered in the impactor is mainly due to the differences in aggregates size, being its composition very similar. Similar data was obtained for other ports (not shown here). Because of this, the substrate number 3 was chosen for the remaining analyses; this substrate was the one where more PM mass was collected. Other authors gave also emphasis to this substrate, where the collected particles have a medium diameter of around.1 µm [1]. Figure 1 shows the PM chemical composition in various measurement locations for the boiler operation condition in Table 2. Results show a consistent decay in the carbon concentration from port 1 to port 5, mainly due to its oxidation, as discussed earlier. It is also possible to observe a consistent increase in the inorganic elements concentration from port 1 to port 5. This is because these compounds are in the gas phase in the main reaction zone, between port 1 and 3, and tend to condense and solidify with the temperature decay. This effect is particularly evident in ports 3, 4 and 5. Figure 1 also reveals that the chemical data obtained in port 1 at r = 5 cm is analogous to that obtained in port 1 at r =. In port 2 at r = 7.5 cm it is possible to observe a larger carbon concentration than in port at r =. This is probably because of the lack of oxygen in this zone associated with very 3

4 high temperatures, which enhances the soot formation. The differences in the inorganic elements concentrations are due to the moderate condensation occurring in port 2 at r =. In port 3 at r = 1 cm, there are some differences in the carbon and inorganic elements concentrations, as compared to port 3 at r =. Considering the results it is possible to note that the condensation of inorganic compounds is small in port 3 at r = 1 cm. Figure 11 shows the evolution of PM chemical concentration with temperature for the boiler operation condition in Table 2. It is seen that the carbon percentage is higher at higher temperatures, while the percentage of the inorganic elements percentage are higher at lower temperature mostly due to the condensation of compounds containing them. 4. Conclusions The main conclusions of this study are as follows: 1. The detailed measurements of temperature and gas species concentration in the combustion chamber revealed that the main reaction zone is located between ports 1 and 3, where the measured quantities gradients are more intense. 2. The PM form rapidly at the beginning of the combustion process. After port 2, soot oxidation rates rapidly decreased due to the significant temperature drop in these regions. 3. The diameter connected to the maximum amount of particles slightly increases from port 1 to port 4, indicating some tendency to form aggregates. Between ports 4 and 5, the diameter connected with the maximum amount of particles decreases because PM fragmentation. 4. The particle chemical analysis revealed a consistent decay of the carbon concentration from port 1 to port 5, which was accompanied by an increase in the concentration of the inorganic elements. These elements, which exist in the gas phase in the main reaction zone, tend to condensate and solidify with the temperature decay. References [1] Fernandes, U., Costa M., Particle emissions from a domestic pellets-fired boiler, Fuel Processing Technology, doi:1.116/j.fuproc [2] Wiinikka, H., Gebart, R., Critical parameters for particle emissions in small-scale fixed-bed combustion of wood pellets, Energy & Fuels, vol 18; pp (24). [3] Wiinikka, H., Gebart, R., The influence of air distribution rate on particle emissions in fixed bed combustion of biomass, Combustion Science and Technology, vol. 177; pp (25). [4] Johansson, L., Leckner, B., Gustavsson, L., Cooper, D., Tullin, C., Potter, A., Emission characteristics of modern and old-type residential boilers fired with wood logs and wood pellets, Atmospheric Environment, vol. 38; pp (24). [5] Bignal, K., Langridge, S., Zhou, J., Release of polycyclic aromatic hydrocarbons, carbon monoxide and particulate matter from biomass combustion in a wood-fired boiler under varying boiler conditions, Atmospheric Environment, vol. 42; pp (28). [6] Atkins, A., Bignal, K., Zhou, J., Cazier, F., Profiles of polycyclic aromatic hydrocarbons and polychlorinated biphenyls from the combustion of biomass pellets, Chemosphere, vol. 78; pp (21). [7] Fitzpatrick, E., Ross, A., Bates, J., Andrews, G., Jones, J., Phylaktou, H., Emission of oxygenated species from the combustion of pine wood and its relation to soot formation, Process Safety and Environmental Protection, vol. 85; pp (27). [8] Rabaçal, M., Fernandes, U., Costa, M., Influence of the pellets type on the emission characteristics of a domestic boiler Proceedings of the 18 th European Biomass Conference and Exhibition, From Research to Industry and Markets, Lyon, France, 3-7 May 21. [9] Strand, M., Bohgard, M., Swietlicki, E., Gharibi, A., Sanati, M., Laboratory and field test of a sampling method for characterization of combustion aerosols at high temperatures, Aerosol Science and Technology, vol. 38; pp (24). [1] Wiinikka, H., Gebart, R., Boman, C., Boström, D., Nordin, A., Öhman, M., High-temperature aerosol formation in wood pellets flames: Spatially resolved measurements, Combustion and Flame vol. 147; pp (26). 4

5 Parameter Table 1. Characteristics of the pine pellets. Value Proximate analysis (% wt, as received) Volatiles 8.5 Fixed Carbon 1.9 Moisture 7.3 Ash 1.3 Ultimate analysis (% wt, dry ash free) Carbon 46. Hydrogen 6.2 Nitrogen.5 Sulphur <.1 Oxygen 47.3 Ash analysis (% wt, dry basis) SiO Al 2 O Fe 2 O CaO 26.2 SO 3.3 MgO 4.3 P 2 O K 2 O 11.5 Na 2 O 2.5 Cl.4 Other oxides 2.3 Low heating value (MJ/kg) 17.1 Average dimensions (mm) Diameter 6 Length 18 Table 2. Boiler operating conditions. Pellets feed rate (kg/h) Thermal input (kw) Flue-gas O 2 (%) Flue-gas temperature (ºC)

6 Pellets Flue gas Termocouple R 12 bit A/D HC analyzer NO x analyzer O 2 analyzer CO and CO 2 analyzers T 1 Water network T 2 Condenser Water in Silica gel Water out Zero gas and span gas Cotton filter Diaphragm pump Weighbridge BOILER Filter Pump RS 232 Filter LPI Exhaust 22 V N 2 Pump Exhaust Figure 1: Schematic of the experimental set-up. Depth = 18 Port 5 z = 186 Port 4 z = Port 3 z = 111 Port 2 z = 71 Pellets Port 1 z = 35 z = Ash hopper Air 32 Dimensions in mm Figure 2: Schematic of the combustion chamber, showing the burner and the measurement locations. 6

7 Figure 3: Radial profiles of gas species concentrations obtained in port 4 in different days for the boiler operation condition in Table 2 O 2, CO 2 (dry volume %) CO (dry volume %) NO x (dry volume ppm) HC (dry volume ppm); Temperature (ºC) O 2, CO 2 (dry volume %) NO x (dry volume ppm) Figure 4: Radial profiles of temperature and gas species concentrations in ports 1, 2, 3, 4 and 5 for the boiler operation condition in Table 2. 7

8 O 2, CO 2 (dry volume %) NO x HC T O 4 2 CO CO Axial position (cm) CO (dry volume %) NO x (dry volume ppm) Axial position (cm) Figure 5: Axial profiles of temperature and gas species concentrations for the boiler operation condition in Table HC (dry volume ppm); T (ºC) PM (mg/nm 3 ) Temperature (ºC) 7 LPI Filter 5 Figure 6: Axial profiles of PM total concentration measured using the Dekati LPI and the total filter for the boiler operation condition in Table Port Port 4 Port 3 Port 2 Port 1 dm/dlog(d p ) (mg/nm 3 ) Particle diameter D p (µm) Particle diameter D p (µm) dm/dlog(d p ) (mg/nm 3 ) 8 dm/dlog(d p ) (mg/nm 3 ) Particle diameter D p (µm) Particle diameter D p (µm) 5. Radial position (cm) Figure 7: PM size distribution in various measurement locations for the boiler operation condition in Table 2. 8

9 a): Port 1 b): Port 3 c): Port 5 d): Port 3, at r = 1 cm Figure 8: Typical SEM images Substractum 2 8 Substractum Substractum 4 8 Substractum 5 9 Substractum 6 8 Substractum Area 1 Area 2 Area 3 Area 4 Area 5 Figure 9: PM chemical composition in 6 substrates for port 1 at r = for the boiler operation condition in Table 2. 9

10 Port 5 Port 4 Port 3 Port 2 Port Area 1 Area 2 Area 3 Area 4 Area Radial position (cm) Figure 1: PM chemical composition in various measurement locations for the boiler operation condition in Table Temperature (ºC) C O Na Si S Cl K 5 Figure 11: Evolution of PM chemical concentration with temperature for the boiler operation condition in Table 2. 1