Article pubs.acs.org/EF
Combustion and Slagging Behavior of Biomass Pellets Using a Burner Cup Developed for Ash-Rich Fuels Håkan Ö rberg, Stina Jansson,† Gunnar Kalén, Mikael Thyrel, and Shaojun Xiong* Unit of Biomass Technology and Chemistry, Swedish University of Agricultural Sciences, SE-901 83 Umeå, Sweden ABSTRACT: An innovative pellet burner cup (“BTC burner”) designed to enable the burning of ash-rich fuels in small-scale boilers (30−40 kW) was evaluated, with respect to slagging behavior, combustion performance, and flue gas composition. The BTC burner features an open, flat-bottomed channel with air supply tubes on top of its lateral sides. The pellets are fed in horizontally, pushing the formed ash forward until it drops down into the ash pan. Eight different pelletized biomass fuels were combusted in the BTC burner, and no unscheduled stops occurred during the combustion tests. Conversely, seven of the eight fuel assortments caused unscheduled stops, because of slagging and/or piling-up of ash when burned in a conventional underfeed burner. The level of slagging in the BTC burner was considerably lower than in the underfeed burner, and the slag and ash were transported out of the burner cup without any complications. Combustion in the BTC burner cup also produced lower CO concentrations in the flue gas for all significantly sintering wood fuels and reed canary grass, and reduced NO emissions for all but one of the studied fuels. However, the BTC burner produced higher NO2 and SO2 levels than the underfeed burner for most assortments, and higher CO concentrations when burning corn stover with or without added lime. Further optimization of the BTC design will be required, particularly in order to minimize particle emissions.
1. INTRODUCTION Small-scale biomass pellet-fired appliances such as pellet boilers and pellet stoves are often equipped with a pellet burner that is generally designed and constructed to utilize low-ash stem wood pellets in order to maintain high operational reliability and low flue gas and dust particle emissions. The incidence of ash-related problems increase when ash-rich fuels (defined in this work as any fuel with an ash content of ≥2.0) are used in small-scale pellet burners.1−4 Along with dust emissions from the burners, this slagging has been identified as a major problem with current burner designs.2−4 While conventional pellet burners may function well during startup when using ashrich fuels, their operation may be stopped or interrupted once slag forms,5,6 and there will be an inaccessibility of the heating supply and even likely a cost to remove the resulting slag and aggregated ashes. Consequently, ash-related problems have limited the use of pellets produced from raw materials other than pure stem wood. This is unfortunate because agricultural residues and energy crops could potentially be important biomass feedstocks for energy and fuel (e.g., in the form of fuel pellets) production6,7 around the world. These materials represent an important alternative to coal and other fossil fuels, particularly in developing countries and predominantly agricultural areas.8 Another reason for increasing the range of biomass fuels that can be used in small-scale burners is the price and availability of stem wood biomass. Ash-related problems such as slagging are expected to be more common when burning agricultural residues than with stem wood.2,3,7,9 This is because agricultural residues are often ash-rich and commonly contaminated with impurities such as soil and sand. Therefore, there is an urgent need to develop technologies that will enable the small-scale combustion of ash-rich biomass materials that readily form slag. There are three main pellet burner design concepts10 that are distinguished by the way in which fuel is introduced into the © 2014 American Chemical Society
burner: (a) the underfeed burner, where pellets are introduced from beneath the burner; (b) the horizontally fed burner, where pellets are introduced horizontally; and (c) the overfeed burners, where pellets drop down into the burner. Regardless of the feeding principle, the primary air supply is introduced through the bottom and/or sides of the burner cup in all conventional burners. This is optimal for cooling the burner cup and for bringing primary air into close contact with the incoming fuel. However, this approach also means that the ash generated during combustion will be in close proximity to the air inlets. When the combustion temperature exceeds the ash melting temperatures for the fuel being burned, the ash will slag and stick to the metal or ceramic material in the burner cup. Ash slagging close to the air supply inlets will increase the risk of inlet clogging. Depending on the melting characteristics of the ash, this may reduce the operating time during which the burner will have an optimal air supply. Impairment of the burner’s air supply will dramatically increase its emissions of species such as CO, significantly reduce the operational availability of the burner cup, and increase the likelihood of a process shutdown. A burner cup design that prevents the ash from coming into contact with the air supply inlets would avoid these problems and thereby enable the effective use of ash-rich and slagging biomass pellets in small-scale boilers. Ideally, the burner cup should have a smooth internal surface to minimize the extent to which the ash is obstructed while being conveyed through the burner cup. In addition, the burner should be designed with a minimal number of moving parts in order to increase its reliability when its supply of electric power is interrupted and to Received: October 29, 2013 Revised: January 15, 2014 Published: January 16, 2014 1103
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Figure 1. Experimental setup with the horizontal burner mounted in an Eryl 50kW boiler. The inset panels depicts (A) a cross section of the BTC burner cup showing the position of the air channels, reflector, and pellet auger, and (B) a cross section of the underfeed burner cup showing the supply of primary and secondary air.
Table 1. Characteristics of the Studied Fuelsa Composition (%)
assortment stem wood pine undelimbed pine delimbed mixed undelimbed/ delimbed spruce bark RCGb corn stover corn stover + calcite
calorific value (MJ/ kg)
moisture (%)
bulk density (kg/m3)
C
H
O
N
S
Cl
P
Si
K
Na
Ca
Mg
20.5 20.8 20.6 20.8
7.4 7.6 5.7 8.6
650 629 666 656
51.9 51.2 50.4 50.6
6.0 6.4 6.2 6.1
41.8 42.1 43.2 42.5
0.01 0.3 0.2 0.3
0.00 0.02 0.01 0.02
0.00 0.01 0.01 0.01
0.00 0.02 0.01 0.03
0.01 0.20 0.09 0.23
0.02 0.12 0.05 0.12
0.001 na na na
0.06 0.12 0.06 0.17
0.01 0.03 0.01 0.03
21.0 19.5 16.8
7.0 7.2 13.5c 13.8c
736 585 629 649
53.0 48.2 45.7
5.8 6.0 6.0
36.9 41.6 42.0
0.4 1.1 0.6
0.03 0.12 0.09
0.01 0.02 0.36
0.05 0.13 0.07
0.17 0.68 0.83
0.21 0.23 0.81
0.008 0.009 0.012
1.04 0.34 0.35
0.07 0.08 0.37
All values are based on dry masses (wt %), except for the moisture contents. “na” = not available. bHarvested in spring. cMeasured just before pelletizing.
a
combustion using both burner cup types. The effects of the two burner designs on particulate matter emissions were examined briefly but are not discussed at length in this work, because they will be examined in more detail in a follow-up study that will focus on the design of pellet boilers and stove configurations.
reduce the likelihood of problems relating to shutdown and startup events. Here, we describe the evaluation of an innovative small-scale pellet burner cup design11 (see Figure 1A) with respect to combustion performance, slag formation, and flue gas composition. The performance of the new design was compared to that of a conventional underfeed burner (Figure 1B). Our study had three main objectives. The first was to evaluate the performance of the two burner cups in technical and operational terms, based on the combustion of eight different pelletized biomass fuels with varying elemental compositions and ash contents: pine/spruce stem wood, undelimbed pine, delimbed pine, a mixed material consisting of undelimbed and delimbed pine, spruce bark, reed canary grass (RCG), corn stover, and corn stover with added limestone. The second objective was to assess the combustion performance and the slagging behavior of these biomass fuels in the horizontal and underfeed burner cups from a technical and operational perspective, respectively. The third was to investigate the differences in flue gas composition during
2. MATERIAL AND METHODS 2.1. Fuels. Eight different 8-mm-diameter pelletized biomass assortments were included in this study: pine/spruce stem wood, undelimbed pine, delimbed pine, a mixed material consisting of undelimbed and delimbed pine, spruce bark, RCG, corn stover, and corn stover with added limestone. Some of the tested fuels have been tested in previous studies using an underfeed burner cup, and they were observed to cause severe slagging disturbances.4,12 The stem wood and bark pellets were purchased from commercial pellet mills, while the remaining pellets were produced in-house using an SPC PP300 Compact pelletizer (Sweden Power Chippers, Borås, Sweden) with a maximum capacity of 300 kg h−1. The biomass pellets were produced under similar conditions in order to minimize the variation in physical characteristics between the pellet assortments. An exception was the moisture content, which was optimized with regard 1104
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burner is named after Biofuel Technological Centre (BTC), Swedish University of Agricultural Sciences, where it was first constructed. The BTC burner cup was designed to avoid contact between the formed ash and the air inlets. For ash-rich fuels, a rather long burner cup is required to achieve a residence time that is sufficient for maximum burnout. Initial tests showed that a shorter burner cup resulted in more unburnt fuel pellets, since the ash on top of the fuel bed might prevent air from penetrating into the fuel bed. This issue was addressed by performing a series pilot experiments with different versions of the burner cup, in which a geometrical optimization of the burner cup’s length and width was conducted. In order to maintain an appropriate combustion temperature in the burner cup, an insulated radiant heat reflector was mounted on top of the open flat-bottomed channel (Figure 1A). The information on the setup of the underfeed burner (Figure 1B) has been described in detail by Xiong et al.12 2.3. Combustion and Online Monitoring. The combustion experiments were conducted at a nominal load of ∼12−35 kW (see Table 2), corresponding to a total pellet feeding rate of 8−10 kg/h. The deposits from the burner cup and the combustion chamber were collected and sieved to separate ash from slag and collected for analysis. All melted particles with a diameter of more than 3 mm were removed from the ash and defined as slag. The amounts of deposited ash and slag were quantified by weighing after every experiment. The slag deposits were also classified in four categories by visual inspection, using the four criteria2−4 described below:
to pelletizing properties on a case-by-case basis. The additive-doped corn stover material was prepared by mixing the pure corn residue material with 3% calcite (dry mass). The undelimbed/delimbed pine assortments are more thoroughly described in Lindström et al.,4 and the corn stover assortments are described in detail in the work of Xiong et al.12 All fuels were characterized with respect to heating value, moisture and ash content, elemental composition, and ash melting temperature (see Table 1, as well as Figures 2 and 3). The ash melting behavior was
• Category 1: nonsintered ash residue, i.e., nonfused ash (clear grain structure). • Category 2a: partly sintered ash, i.e., particles containing clearly fused ash that break at a light touch (distinguishable grain structure); Category 2b: partly sintered ash, i.e., particles containing clearly fused ash that hold together at a light touch but are easily broken apart by hand (distinguishable grain structure). • Category 3: totally sintered ash, i.e., deposited ash fused to smaller blocks that are still breakable by hand (slightly distinguishable grain structure). • Category 4: totally sintered ash, i.e., deposited ash totally fused to larger blocks that are not possible to break by hand (no distinguishable grain structure). At least two batches of each fuel was tested for each burner setting. To complete each batch of the test, fuel feeding was first stopped, and then the fan was turned off when no flame could be observed through a peephole in the boiler door. Sampling of ash and slag was conducted when the boiler had cooled. Before and after each test, the combustion
Figure 2. Ash content of the pelletized biomass materials used in the experiments. defined using the ASTM D1857-68 method,13 based on a mixed sample of each pelletized fuel. The upper limit of ash fusion was set to 1550 °C. Fuels were analyzed according to standard methods for moisture and ash content (SS187171), calorific value (SS-ISO 1928:1), and K, Si, Ca etc. (EN 13656 and EPA methods 200.7 and 200.8).14−18 2.2. Experimental Setup. The combustion experiments were performed in an Eryl 50 kW boiler (Figure 1) equipped with a horizontally fed pellet burner cup featuring an open, flat-bottomed stainless steel channel with air supply tubes on top of its open sides (inset panel of Figure 1A, not to scale). The air supply was controlled by a regulated fan and distributed through area-optimized holes. The fuel input was controlled by an intermittently activated pellet screw to achieve the desired power level. A detailed description of the burner’s construction is presented in patent WO/2007/139475 A1.11 The BTC
Figure 3. Ash melting temperatures of the pelletized biomass materials, with the limit value as the ash fusion test set to 1550 °C. 1105
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Table 2. Burner Performance and Slagging Behavior Power (kW)
Test Time (min)a
Excess O2 (%)
Temp (°C)
Unscheduled Stopb
assortment
BTC
underfeed
BTC
underfeed
BTC
underfeed
BTC
underfeed
BTC underfeed
stem wood pine undelimbed pine delimbed mixed undelimbed/ delimbed spruce bark RCGc corn stover corn stover with calcite
34 37 34 35
14 15 17 15
960 340 410 405
840 810 840 840
7.4 na na na
10.8 8.6 7.2 9.3
1290 1295 1160 na
1161 1033 983 1178
− − − −
32 31 22 28
10−15 12 39d 42d
470 420 136 125
360 960 39 43
6.9 7.9 7.6 7.6
9.5 9.5 8.1 7.6
na na na na
na na 890 860
− − − −
Slag % of Total Ash
Slag Category
BTC
underfeed
BTC
underfeed
− + + +
0 20 10 50
0 22 11 77
1 2 1 3
1 3 3 3−4
+ + + +
25 65
