Article pubs.acs.org/EF
Emissions from Residential Wood Pellet Boilers and Stove Characterized into Start-up, Steady Operation, and Stop Emissions Kaung Myat Win* and Tomas Persson Solar Energy Research Center (SERC), School of Technology and Business Studies, Dalarna University, 791 88 Falun, Sweden ABSTRACT: Gaseous and particulate emissions from three residential wood pellet boilers and a stove were characterized and quantified at start-up, steady (full, medium and low combustion power), and stop phases. The aim was to characterize the emissions during the different phases of boiler operation and to identify when the major part of the emissions occur to enable actions for emission reduction where the savings can be highest. The investigated emissions comprised carbon monoxide (CO), nitrogen oxide (NO), total organic carbon (TOC), and particulate matter (PM 2.5). In this study, particle emissions were characterized by both number and mass concentration. The emission characteristics at high combustion power were relatively similar for all tested devices while significant differences in CO and TOC were observed at lower combustion power. Highest CO and TOC emissions are produced by the bottom fed boiler at low combustion power. The accumulated start-up emissions of the tested devices varied in the ranges of 0.5−12 g CO, 0.1−0.7 g NO, 0.1−2 g TOC, 0.12−2.9 g PM2.5, and 2.4 × 1013 to 3.1 × 1014 particles PM2.5. The accumulated stop emissions varied in the ranges 4−15.5 g CO, 0.01−0.11 g NO, 0.02−1.6 g TOC, 0.1−1.3 g PM2.5, and 3.3 × 1013 to 1.4 × 1014 particles PM2.5. The bottom fed boiler B1 had higher start-up and stop emissions than the tested top fed boilers and more particle emissions were accumulated in start-up phase than in stop phases of boiler B1, B3, and stove S1. Number of particles emitted from residential wood pellet combustion is dominated by fine particles smaller than 1 μm and similar particle distribution both in number and mass were observed for the tested devices. The start-up phase generated higher accumulated particle mass than the stop phase.
1. INTRODUCTION Residential wood combustion has been considered as a major contributor to the urban air pollution and the gaseous and particulate emissions from residential wood combustion have been related to negative health effects in recent studies.1−5 The emission data presented from inventories or national emission factors for residential pellet heating systems are generated mainly from measurements during steady operation.6 Similarly, the test methods for the quality labels for heating devices primarily focus on the emissions during operation on full load and part load7 and the emissions from transient operations (start-up and stop phase) of the heating devices are generally not taken into consideration. Residential wood pellet boilers in real life operation often start and stop more than thousand times annually.8−11 Although the duration of start-up and stop phases in most cases are shorter than the steady operation, the accumulated annual emissions from these phases can be significantly high due to that non-optimal combustion conditions occur during these periods. Laboratory measurements of residential heating systems operated under realistic operation conditions have shown that a dominating part of the annual CO and TOC emissions and about 30% of the particle emissions are produced from these transient phases in the operation cycle.11 Considering that a large part of the total annual emissions from residential boilers and stoves can be linked to the start-up and stop phases,9,11 a greater focus has to be addressed to these emissions. Start-up and stop emissions should be taken into account when optimizing the combustion efficiency of boilers, when the boiler controllers are developed in order to avoid unnecessary boiler cycling and when estimating annual emission factors. With the knowledge of the fuel consumption and the number of starts and stop of a © 2014 American Chemical Society
residential heating system in a year, the annual emissions can be estimated from the emission factors of steady operation and accumulated emissions from a start and a stop sequence. Emissions characteristics of residential pellet combustion devices vary significantly depending on the fuel composition and a number of parameters related to the design of the devices for combustion conditions such as air−fuel ratio, combustion temperature, and mixing in the combustion chamber. There are several studies of the particles and gaseous emission characteristics from steady operation for different residential pellet heating devices using varying fuels, 12−16 for different devices13,15,16 and different loads.13,16,17 However, the emission characteristics related to start-up and stop phases of the residential pellet devices are scarce and only few emission data have been reported. Accurate measurements of the quantities of emissions during non-steady periods require a flow sensor that can measure the low flue gas flow rates during these phases.18 Accurate quantification of transient emissions can also be achieved by using a constant flow dilution system, for example, constant volume sampling system (CVS) as extensively used in engine exhaust measurements. A constant volume sampling system was designed and evaluated for residential biomass combustion appliances during normal operation by Boman et al.19 In CVS methodology, total amount of particles over specific duration is measured and dynamic emission profile during transient time is unknown. In this study, the flue gas flow during start-up and stop phases was measured with a Received: August 23, 2013 Revised: March 14, 2014 Published: March 17, 2014 2496
dx.doi.org/10.1021/ef4016894 | Energy Fuels 2014, 28, 2496−2505
Energy & Fuels
Article
customized flow sensor and used to calculate instantaneous emissions flow rates. Streicher et al.20 reported the start-up and stop emissions of CO and TOC for one pellet boiler. The studies by Persson10,21 and Fiedler22,23 present measured CO emissions for start-up, steady operation and stop phases of several wood pellet devices and system simulations show that the CO emissions that arise during start-up and stop phases are the dominating source of annual CO emissions from the residential heating boilers operated under realistic conditions. Higher concentration of particles are shown to be generated during the start-up phase and load change phase due to incomplete gas phase burn out.24,25 In the study by Schmidl et al.,16 both particle and gaseous emissions are also presented for the start-up phase. Good and Nussbaumer26 have reported the emission characteristics during three operation phases for two pellet boilers and presented the emissions for both start-up and stop phases as well as for steady operation. Win et al.11 presented annual emissions of NO, CO, TOC, and PM 2.5 based on laboratory measurements under realistic operating conditions. This study aims to present the emissions characteristics from different operating phases from three residential pellet boilers and a pellet stove. The gaseous and particle emissions were measured in a laboratory and characterized into start-up, steady operation and stop emissions separately.
phase to shut down the boiler. The pellet stove S1 has a nominal power of 12 kW and the fuel bed is situated in a ceramic cup. The stove operates at a constant power and has a cleaning routine at every 1.5 h of operation during which the ashes are removed by opening the bottom of the combustion cup. All tested pellet combustion devices are coupled with automatic pellet feeding and use an electric element to ignite the fuel with hot air. The elemental composition, moisture content, and lower heating values of the soft wood pellets used in the tests was analyzed by an accredited laboratory and are presented in Table 2. The pellets are made from soft wood and certified with SS Table 2. Elemental Compositions and Lower Heating Value (LHV) of the Used Wood Pellets
fuel feeding principle ignition type
B1
B2
B3
20
25
12
50 modulating/ standby/off bottom fed
150 high/low/ off top fed
130 on/off
20 on/off
top fed
top fed
hot air
hot air
hot air
hot air
batch II
carbon oxygen hydrogen nitrogen ash moisturea LHVa
wt % dry wt % dry wt % dry wt % dry wt % dry wt % wet MJ/kg
51.20 42.00 6.30 0.20 0.30 8.20 17.68
50.74 42.52 6.23 0.10 0.41 6.80 17.56
18712027 for the Swedish market. Two batches of purchases were made for all the tests and the pellets from batch I was used in the measurement of boilers B1 and B2. Pellets from batch II were used in the measurements of boiler B3 and stove S1. Samples of the fuel from each test day were collected in plastic bags and the moisture ratio was analyzed according to the standard EN 14774−1.28 The actual humidity ratio was used to recalculate the heating value of the fuels depending on the humidity level. 2.2. Experimental Procedure. A schematic figure of the experimental set up is shown in Figure 1. The type of measurements performed and the number of repetitions are described in Table 3. Steady state measurements were conducted at high, medium, and low combustion power as presented in Table 4 in the result section. The air supply and fuel flow rate of the boilers were adjusted prior to the measurements at each combustion power except for the stove S1, which was preadjusted by the retailer. The adjustment criteria for each combustion power was to minimize the CO concentration by adjusting the combustion air flow rate and avoiding high NO emissions and frequent CO peaks, which may occur at very low air to fuel flow ratios. However, the adjustment criteria was not applied to low and medium combustion power of B2. In all tested combustion appliances, the settings of air and fuel supply for start-up and stop phases are separated from that of normal operation. The O2 level was checked for start-up and the fuel dose and air supply for startup was adjusted if O2 concentration during the start-up was too high or too low according to the instructions in the operating manual. As each device has different operating procedures and control strategies, the strategy used to be able to operate the boiler at the desired combustion power for a long time period was to control the boiler temperature and heat load. The duration when the emissions was counted for the steady state measurements, lasted from the point where steady operation
S1
12
batch I
Measured at the accredited test institute. Sample from each test day were analyzed according to EN 14774-128 and the LHW was adjusted.
Table 1. Summary of Tested Boilers and Stove Characteristics
nominal power (kW) water vol. (L) control principle
unit
a
2. MATERIAL AND METHODS 2.1. Combustion Devices and Fuel. Emission measurements were performed on three residential wood pellet boilers and a pellet stove and their main characteristics are presented in Table 1. Boiler B1 is a bottom fed boiler with a nominal power
boiler characteristics
element
of 12 kW, which can be operated on a fixed combustion power or in a mode where it is modulating between three different combustion powers depending on the water temperature. B2 is a top fed boiler with a nominal combustion power of 20 kW with an integrated hot water preparation unit. It has a tube type burner and operates with two operating settings; high and low power. The boiler B2 starts on a lower water temperature setting and operates at high combustion power until a slightly higher water temperature is reached and then on low combustion power until the water temperature reach to the maximum set temperature. This boiler uses a cleaning routine with compressed air, which is activated during the stop sequence. The remaining uncombusted fuel and ash are discharged with compressed air into the ash box. Boiler B3 is also a top fed boiler with a tube type burner similar to boiler B2, and it has a nominal combustion power of 25 kW. The boiler operates at a constant combustion power until the desired water temperature is reached, and it begins the stop 2497
dx.doi.org/10.1021/ef4016894 | Energy Fuels 2014, 28, 2496−2505
Energy & Fuels
Article
Figure 1. Schematic of the measurement set up. (TS: temperature sensor).
2.3. Fuel and Flue Gas Flow Measurements. The boiler and the fuel storage were installed on a scale (WS1 in Figure 1) which has a measurement resolution of 200−300 g to measure the fuel consumption continuously. The fuel consumption during the start-up and stop phases were small relative to the scale’s measurement resolution and consequently the uncertainty of the weight measurement for the total fuel consumed during the start-up and stop phases are high. Therefore, the fuel consumption was calculated from the measured flue gas flow rate, O2 concentration, and fuel composition using combustion calculations according to Wester29 and Persson et al.30 The transient flue gas flow rate during the start-up and stop phases are lower than that most commercially available flow sensors can measure with acceptable precision. Therefore, a set of multiport averaging Pitottubes calibrated in combination with the pressure transducer specifically to the installed chimney was used in order to be able to measure the transient flue gas flow rate during start-up and stop phases. The sensors including the pressure transducers were calibrated using hot and cold air taking into account both density and viscosity effects. The flue gas flow sensor is sensitive to changes in temperature and vibrations and sometimes the zero point drifts away with no obvious reason and in these cases the data was excluded. Regular zeroing of the sensor was performed and a continuous check of the sensor was carried out for each measurement by comparing the total fuel consumption calculated by the measured flue gas flow rate and the fuel consumption registered by the weight scale. The humidity and the temperature of the room air are continuously monitored as shown in Figure 1. The moisture content in the flue gas was calculated based on the humidity in the room air and the fuel moisture content according to the calculation procedure described by Persson et al.30 The gaseous emission flow rates were calculated from measured gaseous concentration based on dry flue gas flow.
Table 3. Overview of Performed Measurements and Number of Repetitions operation
B1
B2
B3
S1
steady high power (HPb) steady medium power (MPb) steady low power (LPb) cold start-up warm start-upa stop
3 3 2 9 3 12
3 4 3 13 14 12
4 3 3 9 3 12
3 3 3 9 3 12
Start-up with the initial boiler water temperature above 40 °C. bThe combustion power of HP, MP, and LP of the devices are given in Table 4. a
was reached (CO-emissions and temperatures are stable and constant) until the stop phase was introduced. The length of the steady combustion power measurements in the tests varies between two and four hours. The start-up phase was defined to last from the rise of CO emission from ignition until CO concentration, flue gas temperature, and operating power have reached the level same as in a steady operation. The levels of these three parameters differ according to the operation settings of a steady operation and the end of the start-up phase is identified by changes in combustion condition from start-up to steady operation. Among three parameters for defining the end of start-up phase, the main parameter is the CO emission which is a good indicator of changes in combustion condition from start-up phase to steady state. The determination of start-up and stop phases is illustrated in Figure 2 for the boiler B1. The fall of higher CO emissions to a steady level marks the end of the start-up phase. The stop phase was determined from the abrupt fall of combustion power, flue gas temperature, and the electricity consumption of the boiler as the fuel feeding screw stop operating. The emissions from the stop phase were accounted for until the CO-emissions reached a level of less than 10 ppm. 2498
dx.doi.org/10.1021/ef4016894 | Energy Fuels 2014, 28, 2496−2505
Energy & Fuels
Article
Table 4. Average Combustion Power, Oxygen Concentration, Emission Factors, and Dilution Ratio (DR) during Steady State Operation of the Pellet Combustion Devices B1, HP B1, MP B1, LP B2, HP B2, MP B2, LP B3, HP B3, MP B3, LP S1, HP S1, MP S1, LP
Pcomba (kW)
O2 (%)
CO (mg/MJ)
12 9 6 17 14 10 22 19 14 11 9 6
9 11 14 8 11 12 7 8 9 12 13 15
54 ± 3 217 ± 19 485 ± 8 12 ± 3 17 ± 5 19 ± 7 15 ± 3 16 ± 1 14 ± 1 16 ± 2 23 ± 2 48 ± 5
NO (mg/MJ) 67 65 62 64 65 64 62 61 59 63 63 60
± ± ± ± ± ± ± ± ± ± ± ±
4 3 2 1 1 1 1 1 1 1 1 1
TOC (mg/MJ) 1.6 2.2 7.1 1.8 1.9 1.3 0.5 0.6 0.7 0.8 1.3 1.9
± ± ± ± ± ± ± ± ± ± ± ±
0.1 0.1 0.2 0.7 0.6 0.2 0.1 0.1 0.1 0.1 0.3 0.1
PM2.5b (mg/MJ) 56 51 62 57 65 53 50 56 65 55 43 55
± ± ± ± ± ± ± ± ± ± ± ±
8 2 2 11 4 14 1 1 2 3 6 5
PM2.5 (particles/MJ) 1.5 1.2 1.4 1.2 1.3 1.2 1.2 1.3 1.3 1.3 1.2 1.6
× × × × × × × × × × × ×
1013 1013 1013 1013 1013 1013 1013 1013 1013 1013 1013 1013
± ± ± ± ± ± ± ± ± ± ± ±
2 3 1 2 3 3 1 1 1 1 3 1
× × × × × × × × × × × ×
1012 1012 1012 1012 1012 1012 1012 1012 1012 1012 1012 1012
DR 72 ± 1 104 ± 8 102 ± 5 52 ± 2 54 ± 6 71 ± 3 122 ± 8 138 ± 1 184 ± 10 189 ± 19 79 ± 4 223 ± 10
a
Pcomb: Combustion power, HP: High power. MP: Medium power. LP: Low power. bPM2.5 mass calculated from ELPI which has an uncertainty of 20% REF.
2.4. Gaseous Emission Measurements. Gaseous components are sampled from the chimney through a heated filter and transported via a heated tube to the gas analysers to avoid the condensation of hydrocarbon as shown in Figure 1. Carbon dioxide (CO2), carbon monoxide (CO), and nitrogen oxide (NO) are measured with a non- dispersive infrared gas analyzer, oxygen (O2) with a paramagnetic gas analyzer, and total organic carbon (TOC) with a flame ionization detector (FID). The emissions of TOC are presented in methane equivalents. The gaseous emission measurement equipment was calibrated using calibration gases. The mass flow rates of the emission are calculated for each time step and summed up as accumulated emissions. 2.5. Particle Measurements. The sampling of particulate matter was performed in the dilution tunnel at a near ambient condition according to the Norwegian standard (NS3058-2, 1994)31 and continuously measured with an electrical low pressure impactor (ELPI). ELPI measures the particles in aerodynamic diameter and the effective density of particles is assumed to be unity in ELPI data conversion. Particle mass can be overestimated in measurement of fine particles with ELPI due to both particle diffusion and density. The estimated uncertainty of particle mass measurements due to the assumed unit particle density in ELPI is about 20%.32 Flue gas from the chimney is first diluted with the room air, and a constant volume of flue gas sample is extracted through a sampling tube with a diameter of 10 mm. A hatted-probe was used to remove the coarse particles as it has been commonly used in the measurement of diesel particles.33 The temperature of diluted flue gas at the sampling point is maintained around 20 to 35 °C to get a near ambient temperature condition. A second dilution in the diluter (Figure 1) with pressurized clean dry air is applied to the extracted sample flue gas in order to reduce the concentration and extend the cleaning interval. The average total dilution ratios during steady operation of the boilers are presented in Table 4. Particulate matters are measured in number concentration and size distribution in the range from 7 nm to 10 μm and are calculated for PM 2.5 in this study.
Figure 2. Parameters for determination of start-up and stop phases in boiler B1.
presented in the table are the average of the repeated measurements at the respective combustion power presented together with the standard deviation, calculated from the variations in the data. The numbers of repetitions are given in Table 3. The results show that the emission characteristics at high combustion power were relatively comparable for all tested devices while significant difference were observed at lower combustion power. In boiler B1 and stove S1, CO, and TOC emissions increase with decreasing combustion power while only small changes of CO and TOC emissions were observed for B2 and B3 with varying combustion power. The oxygen concentration increases with lower combustion power and that is valid for all boilers. The highest emission factors for CO and TOC, 485 mg/MJ and 7.1 mg/MJ, respectively, were found for the boiler B1 operating at low power. The increased emissions at low operating power are likely to be generated due to less turbulence (mixing) and lower combustion flame temperatures caused by the lower flow rates and higher relative amounts of surplus oxygen. As the boiler B1 is a bottom fed boiler, the ash removal is accomplished by the new pellets inserted from below and displacing the ash. Regular short cleaning sequences with high amount of surplus air assist the ash to fall over the edge of the combustion pot into the ash tray. Frequent CO and TOC emission peaks were observed during the steady operation, and one reason could be due to the varying amount of ashes accumulated over the fuel bed,
3. RESULTS AND DISCUSSION 3.1. Steady State Emissions. Emission factors of CO, NO, TOC, particle mass, number of particles, and the operating combustion power from steady state measurements of the tested devices are presented in Table 4. The emission data 2499
dx.doi.org/10.1021/ef4016894 | Energy Fuels 2014, 28, 2496−2505
Energy & Fuels
Article
Figure 3. Particle emissions and TOC versus combustion power during steady operations.
which decrease the mixing with secondary air and cause poor combustion. In the boiler B3, the oxygen concentrations was adjusted to 7−9% range for all operating power, and as a result, the CO and TOC emissions for the boiler B3 were not significantly increased when reducing the combustion power, as it is shown in Figure 3. NO emission factors for all tested devices are in the range 59−67 mg/MJ. Nitrogen oxide (NO) emissions from the small scale combustion of wood pellet mainly originates from the fuel bound nitrogen and combustion temperature; therefore, only small variations are measured between different boilers and combustion powers. However, a small decrease with decreased combustion power is measured as a result of slightly decreased combustion temperature. The gaseous emission factors from steady operations are within the ranges reported by the previous studies.12,13,16,17,34 Particle emission factors during steady combustion operations were between 37 and 67 mg/MJ for particle mass and between 9 × 1012 and 1.7 × 1013 for particle number. Emission factors for particle mass from measured data versus combustion power are plotted in Figure 3. The emissions of particles shows a linear correlation with combustion power in boiler B3, but it appears to be random in relation with combustion power for boiler B1, B2, and stove S1. Boiler B1 and stove S1 have minimum particle emission operating at medium power while boiler B2 have the maximum particle emissions operating at medium power. The random nature of particle emissions may be influenced by the change in fuel load as oxygen level and combustor temperature vary with fuel load and excess air. In boiler B3 where excess air was adjusted at each operating power to optimize the combustion, the emission of particles shows an almost linear correlation with combustion power. The particle emission is found minimum operating at high power and maximum at low power. The particle mass emission factors from this study at nominal operation (37−67 mg/MJ) are higher than the range 13−22 mg/MJ reported by Johansson et al.,13 15−26 mg/MJ at high load reported by Boman et al.,17 3.5−24 mg/MJ reported by Schmidl et al.16 and within the range 37−78 mg/MJ reported by Sippula et al.12 Increase of PM at reduced combustion power is also seen in two pellet stoves in the study by Boman et al.17 and the boiler in the study by Schmidl et al.16; however, a decrease in particle emission is also observed at reduced power for the stove in the study by Schmidl et al.16 The emission factors for particle number from steady operation in this study are lower than the ranges 4−25 × 1013 reported by Boman et al.,17 3.3−6.6 × 1013 reported by Sippula et al.12 and are within the range 0.8−1.4 × 1013 reported by Johansson et el.13
In comparison with the reported particle emission data, the differences in the measurement procedures and sampling conditions such as dilution ratio, temperature, and measurement equipments must be taken into consideration.35 The particle emissions in the studies by Johansson et al.13 and Boman et al.17 are sampled in a dilution tunnel with constant volume sampling where the flue gas flow was diluted with hot air in the dilution tunnel. In the studies of Sippula et al.12 and Schmidl et al.,16 a partial sample flow extracted from the chimney is diluted with filtered room air in the dilution tunnel where the particles were measured. Moreover, particle mass in reported studies are sampled on a filter and measured with a gravimetric method. Particle emissions in this study are measured in diluted flue gas in a full flow dilution tunnel with a high dilution ratio at a temperature close to room temperature. Condensation of organic particles occurs during the dilution with room temperature air and generally higher particle mass are resulted in this study and particle mass is calculated from ELPI data which has about 20% uncertainty in particle mass calculation.32 The uncertainty in PM mass due to fuel rate calculation from flue gas flow measurement is estimated to be around 5−10%. The higher PM mass measured in this study can be partly contributed by the above-mentioned uncertainties, and moreover, it was found that the DAD diluter was generating signal output about 20% higher in dilution ratio than the actual dilution was. Therefore, overall measurement uncertainty for PM mass can be up to 20−50% in this study. 3.2. Start-up and Stop Emissions. In Figure 4, the carbon monoxide and the particle emission characteristics of different phases of the tested devices are shown. The time in the figure is counted from the beginning of the start-up phase and the duration before the CO emission’s increase represents the heating and ignition of the pellet fuel. The duration and the amount of fuel consumed for the start-up and stop phases of the tested devices are presented in Table 5, and it can be seen that there are large variations between the tested devices as their operation procedure and combustion settings are different in each phase. More particle emissions were accumulated during the start-up phase than during the stop phase. In Figure 5, the measured accumulated particle emissions and gaseous emissions during start-up and stop phases are presented in box plots with minimum, first quartile, mean, third quartile, and maximum values. In the bottom fed boiler B1, the combustion air is heated and fed through primary air holes to the fuel bed to ignite the pellets. The fan remains in nominal power for about 5 min after ignition to support the flame and then stops for 5 min for flame propagation. The fan 2500
dx.doi.org/10.1021/ef4016894 | Energy Fuels 2014, 28, 2496−2505
Energy & Fuels
Article
Figure 4. Start-up and stop CO and particle emissions profiles of tested pellet boilers and stove.
resumes with the regular loading of the fuel and several peaks of CO emissions are found until the CO emission and combustion reach the steady operation condition. Details of the start-up of boiler B1are also illustrated in Figure 2. The duration of the start-up phase of B1 varies from 19 to 26 minutes depending on the combustion condition after flame propagation causing a wider range of accumulated emissions for cold start-up in Figure 5. During an automatic warm start-up of B1, electrical ignition does not occur, since there is still glowing fuel in the combustion cup in contradiction to tested top fed boilers. The duration of the warm start-up of B1 is therefore shorter than for the cold start-up and hence a smaller amount of emissions are produced. When the boiler water temperature exceeds the set temperature, the fuel feeding, and the fan stop. The typical stop phase of B1 takes 105−120 minutes. During the stop phase of B1, the glowing of fuel occurs in the combustion cup with the pellet fuel below and the consequent CO emissions continues up to 3 hours to reach less than 10 ppm during a complete stop sequence. Therefore, the accumulated emissions during the stop phase of B1 appear in a wide range (Figure 5). In boiler B2, B3, and stove S1 the pellet fuel is fed from the top and the duration of the start-up phase of B2 is 7.5 minutes, B3 is 13 minutes, and S1 is 21 minutes. The emission levels during the warm start-up of the boiler B2, B3, and S1 are similar to that of the cold start-up except for particle emissions which are reduced during the warm start-up of B3. Boiler B2 performs a cleaning sequence with compressed air during the stop phase and this causes accumulation of uncombusted glowing pellet in the ash box which leads to longer stop phases and higher emissions. Both boiler B1 and B2 have high TOC emissions during the stop sequence caused by the long glowing
Table 5. Duration and Amount of Fuel Consumed for Startup and Stop Phases of the Tested Appliances B1 cold start-up B1 warm start-up B1 stop B2 cold start-up B2 warm start-up B2 stop B3 cold start-up B3 warm start-up B3 stop S1 cold start-up S1 warm start-up S1 stop
duration (min)
fuel (MJ)
19−26 11−13 105−180 7.5 7.5 20−180 13 13 27−37 21 21 24−27
9.1−11.6 3.5−4.7 0.5−2.4 1.5−5.4 1.9−3.6 0.7−1.8 5.9−8.8 6.1−7.8 1.0−1.9 7.1−8.5 8.3−8.4 0.8−1.4
period. The typical duration of the stop phase in B2 is 20 to 40 minutes, but it can last up to 180 minutes with large amount of glowing fuel in the ash box (Table 5). This wide variation in duration of stop phase of B2 leads to a wide range of accumulated CO and TOC emissions during the stop phases (Figure 5). The long glowing period of uncombusted pellet in the ash box may be, to some extent, affected by the forced draft through the chimney by the hood after the burners had stopped. The draft through a 5 m high chimney at a temperature that is 40 degrees above ambient, which corresponds to the water temperature, generates a draft with an under pressure of about 7.5 Pa, and the forced draft in the study is set about 10 to 12 Pa. The increase in natural draft flow at an under pressure of 10 Pa instead of 7.5 Pa is about 15%. 2501
dx.doi.org/10.1021/ef4016894 | Energy Fuels 2014, 28, 2496−2505
Energy & Fuels
Article
The stove S1also had as low start-up emissions as B2 despite a longer duration of the start-up phase. The variation in accumulated NO emissions depends mainly on the fuel consumed during the phases as NO emission from wood pellet combustion comes from fuel bound nitrogen. Start-up and stop emission characteristics for residential pellet boilers and stoves are scarce in the literature and the reported data are mainly for CO emissions. Emission characteristics reported by Good and Nussbaumer26 for two pellet boilers are presented in Table 6. All emission data in Table 6 are found to be within the ranges of the start-up and stop emissions presented in this study (Figure 5). The CO and TOC emissions of B2, B3, and S1 are similar to that of boiler II in Table 6, but their particle emissions are higher than measured for boiler II. It should be noted that the particle emissions in the study by Good and Nussbaumer (Table 6) were sampled in the chimney and the particle emissions in this study were sampled in the dilution tunnel which generally produce a higher total particle mass due to condensation of organic compounds in the diluted flue gas.35 Schmidl et al.16 reported emissions for start-up phases for an automatically fed pellet stove (135 mg·MJ−1 CO, 95 mg·MJ−1 NO, 13 mg·MJ−1 TOC, 10 mg·MJ−1 PM10; values converted from mgcm−3 at 13% O2) and for an automatically fed pellet boiler (373 mgMJ−1 CO, 90 mgMJ−1 NO, 14 mgMJ−1 TOC, 40 mg·MJ−1 PM10; values converted from mgcm−3 at 13% O2). In comparison with the stove S1, the start-up emissions of the pellet stove are within the ranges of CO and TOC emissions of S1 (106−312 mg·MJ−1 CO, 12−36 mg·MJ−1 TOC), above the range of NO emission (50−62 mg·MJ−1 NO) and below the range of PM emission (68−160 mg·MJ−1 PM2.5). Similarly, the start-up emissions of the pellet boiler are within the ranges of CO and TOC emissions of the tested top fed boilers (124−750 m·gMJ−1 CO, 11−139 mg·MJ−1 TOC), above the range of NO emission (37−60 mg·MJ−1 NO) and below the range of PM emission (35−265 mg·MJ−1 PM2.5). Streicher et al. report accumulated start-up and stop emissions of a boiler to 8.6 g of CO and 0.45 g of TOC, which are within the ranges of the top fed boilers in this study. Persson10 reported accumulated start CO-emissions for three pellet boilers and two pellet stoves between 0.5 and 2.2 g and their stop CO emissions ranged between 1.2 and 21 g, which is a higher range than reported in this study. 3.3. Particle Number and Mass Distributions. The number size distributions of particle emissions from different operating phases are presented in Figure 6, and mass size distributions of particle emissions are presented in Figure 7. The concentration in the figures were not normalized to a particular oxygen concentration (commonly 10% or 13% O2) to be able to compare the distribution between different phases as well as different boilers. Normalization to 10% O2 increases
Figure 5. Accumulated emissions during start-up and stop phases of the tested boilers and the stove. (B1.c: B1 cold start-up. B1.w: B1 warm start-up, same abbreviations applied to B2, B3, and S1. TOC for B1 warm start-up was not measured.).
Thus, only a minor increase in the natural draft flow, and therefore, the effect of forced draft on longer stop phase are estimated to be a small extent. During a realistic operation, the burner of B1 and B2 may start again while unburned pellet is still glowing in the ash box and this may then reduce the stop emissions. The stop phase of B3 takes 27−37 minutes and for S1 it lasts about 24−27 minutes. The accumulated TOC emissions during the stop phases are much higher for B1 and B2 than for B3 and S1 due to the long glowing time that occurs in the bottom fed boiler B1 and in boiler B2 with the cleaning sequence. Boiler B2 had the shortest start-up time and only a small emission peak, and therefore, fewer emissions are accumulated during the start-up.
Table 6. Accumulated Emissions for Start-up and Stop Phases of Pellet Boilers and Stoves Reported by Good and Nussbaumer
boiler I
boiler II
a
phase
duration (min)
CO (g)a
NO (g)b
TOC (g)a
PM2.5 (g)a
PM2.5 (no.)c
fuel (MJ)
cold start-up warm start-up stop cold start-up warm start-up stop
19 15 10 14 8 16
7.64 6.68 0.43 2.63 2.98 7.43
0.34 0.46
