Operation and Emissions of a Hybrid Stove Fueled by Pellets and Log

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Operation and emissions of a hybrid stove fueled by pellets and log wood Heikki Lamberg, Olli Sippula, Jarkko Tissari, Annika Virén, Terhi Kaivosoja, Aki Aarinen, Vesa Salminen, and Jorma Jokiniemi Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b02717 • Publication Date (Web): 06 Jan 2017 Downloaded from http://pubs.acs.org on January 11, 2017

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Energy & Fuels

OPERATION AND EMISSIONS OF A HYBRID STOVE FUELED BY PELLETS AND LOG WOOD Heikki Lamberg*1, Olli Sippula1, Jarkko Tissari1, Annika Virén1, Terhi Kaivosoja1, Aki Aarinen2, Vesa Salminen2, Jorma Jokienimi1 1

Fine Particle and Aerosol Technology Laboratory, Department of Environmental and Biological Sciences, University of Eastern Finland, P.O. Box 1627, FI-70211, Kuopio, Finland 2

Warma-Uunit Ltd., FI-23500, Uusikaupunki, Finland

*Corresponding author: Heikki Lamberg; email: [email protected]; Address: University of Eastern Finland, Department of Environmental and Biological Sciences, Fine Particle and Aerosol Technology Laboratory, P.O. Box 1627, FI-70211 Kuopio, Finland; telephone: +358403553285; fax: +35817163098. Keywords: residential combustion, log wood, pellets, emissions, abatement strategies, hybrid stove

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Abstract

Small-scale wood combustion in wood-log fired appliances is commonly used for heat production in residential buildings, leading to significant particle and gaseous emissions and impaired air quality. Thus, there is a need for effective emission reduction methods for residential wood combustion. In this work, the operation and emissions of a novel hybrid stove, i.e., a slow heat releasing appliance capable of using both wood pellets and log wood as fuel, were studied. While log wood operation in this stove represents conventional combustion technology, the operation and emission performance of the pellet burner system is demonstrated for the first time. Particle and gaseous emissions were carefully characterized under various operating conditions, including real-time measurements of particle mass, number and size, as well as analyses of the particulate chemical composition of filter samples. The use of pellet fuel decreased the fine particle emissions by 92% and the CO emissions by 65% compared with the log wood combustion. Operational practices in the log wood combustion affected the gaseous emissions but only had a minor effect on the particle emissions. The fine particle emissions from the pellet combustion were mostly comprised of elemental carbon (soot) particles. The real time measurements showed that most of the particle emissions were formed after ignition, for both fuels, whereas the end of combustion proved to be important for carbon monoxide emissions. This study shows that the novel pellet batch burner used in a hybrid stove can be used as an effective low-cost solution to reduce the emissions from residential wood combustion. The advantage of this system is that it requires no automation and thus an electricity supply is not required.


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1. Introduction Residential combustion of solid biomasses, typically wood, is an important source of energy for domestic heating in many areas of the world, and there is a current interest to further increase the share of renewable biomass-based fuels due to their carbon dioxide (CO2) neutrality. Residential combustion appliances have been studied widely over the past years and have been identified as an important source of both gaseous and fine particle emissions,1-3 having a major effect on the air quality.4-6 Emission factors (i.e., emissions normalized to fuel usage) from residential combustion appliances vary greatly.7-9 Emissions are affected, e.g., by combustion technology, operational practices and fuel quality.10,

11

Combustion emissions at a residential

scale are far less regulated than in large units, and the emission characteristics differ considerably between large and residential scales.12, 13 Moreover, numerous studies have shown the detrimental effects of fine particles in the atmosphere, on both the environment and human health.14-16 Previous studies on combustion emissions have shown that fine particle emissions from modern pellet appliances with continuous combustion, and operated with good quality softwood pellets, are relatively low and mainly composed of inorganic ash species, such as alkali metal sulfates, chlorides and carbonates.17-20 Combustion efficiency in modern pellet boilers are ensured with precise control of fuel feeding and combustion air supplies.20, 21 In contrast, batchwise operated heaters and stoves can have very high emission factors.7,

22

Although some

automated batch combustion appliances exist on the market, the majority of these appliances are still used without any automation.23 Particle emissions from these appliances are typically clearly higher compared with automated appliances, due to soot and organic matter emissions.11,

24


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Recent improvements in the batch combustion technology, based on combustion air staging, have decreased both gaseous and particle emissions.24, 25 However, we have observed that soot emissions may not decrease in batch combustion with the current improved combustion technology.25 Excluding the most modern appliances, small-scale batch combustion appliances are often rather simple with respect to combustion technology. Combustion appliances that do not require electricity are commonly preferred for secondary heating, and thus the potential to control the combustion process is limited compared with automated pellet boilers and burners. In addition, the batch combustion process is more variable compared with continuous combustion,7 making control more challenging than in appliances with continuous fuel feeding. Secondary emission reduction methods that are used in large units, such as electrostatic precipitators, are not primarily preferred due to increasing costs and maintenance, and there are still large development needs in the secondary reduction technology for batch-combustion appliances.26,

27

In contrast, primary methods (i.e., decreasing the formation of harmful

emissions) may offer the possibility of decreasing emissions in a cost-efficient and user-friendly manner. Because residential combustion is an important source of particulate and gaseous emissions, it is essential to introduce new technologies as well as improve the stove operational practices to reduce the detrimental effects of the combustion emissions on the environment.28, 29 Manually fed pellet combustion devices are currently not widely used and almost all pellet appliances are automated boilers and stoves. These appliances are typically equipped with precise feeding of the pellet fuel and fan-assisted combustion air input, enabling better control of the combustion process and emissions than in manually operated appliances.7, 11, 24, 30 However,


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some technologies exist on the market that enable the use of pellet fuels in batch combustion appliances with special pellet burners installed in the firebox. These manually operated burners may be designed for general use or may be designed to be used in a specific appliance. However to our knowledge, there are no publications available introducing such system and its emissions. In this study, we characterized the particulate and gaseous emissions from a slow heat releasing masonry heater equipped with the specific pellet batch burner. Real-time measurements included combustion air flows and particle and gaseous emissions, which provided information on the effects of the different combustion phases and operation on the emissions. The hybrid stove type can be operated with log wood in a conventional manner, or with pellets by using a novel pellet batch burner, and is therefore termed a hybrid stove. It requires no electricity for operation, which is not common for pellet-fired appliances. The pellet batch burner allows the use of pellet fuel in a batch combustion appliance that is typically fired with logwood, widening the range of possible fuels in this appliance type. Secondly, the combustion process in the pellet batch burner is better controlled compared with logwood combustion, introducing a new potential primary emission reduction method for residential combustion. Between the experiments, improvements in the appliance as well as the operation of the hybrid stove were made to enhance the combustion quality, and the subsequent effects on the emissions were determined. Furthermore, this work presents a comparison of the emissions from pellet and log wood batch combustion in a single appliance.

2. Methods The experiments were carried out in an emission test laboratory, where the hybrid stove was placed inside a test room. A chimney was installed through the roof of the test room and the


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measurement equipment was placed on the roof of the room. The detailed experimental setup is presented in Figure 1. The novel prototype hybrid stove (Warma-Uunit Ltd., Finland), which was used in the experiments, weighted approximately 800 kg and was constructed of rock casting material. When fired with logs, the hybrid stove represents traditional Finnish slow heat releasing technology. However, the hybrid stove also includes a new feature, i.e., the potential to utilize pellets as fuel, which represents new technology in batch combustion. Slow heat releasing appliances have considerable capacity to store heat and release it to the surrounding space slowly over time. The combustion rate is high, the heat is stored in the heat storing material and is released to the surrounding air over a period of several hours after combustion is complete.31

Figure 1. Schematic drawing of the experimental setup. Plastic tubes with an inner diameter of 102 mm were installed in the combustion air inlets, and the combustion air flows were measured using Schmidt flow sensors (model SS 20.60, Germany) with negligible pressure loss in the air supply system. Only primary air was supplied in log wood


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combustion (Table 1). In pellet combustion, primary air was measured in one duct while secondary and tertiary air was measured in the second duct. Thus, secondary and tertiary air flows are later reported together as one value. Temperatures were monitored using K-type thermocouples placed in the flue gas in the stack and in the diluted sample gas. The pressure in the stack was determined using a pressure meter (Alnor Instruments, MN, USA) to monitor the draught, and was kept constant at 12±2 Pa using a flue gas fan. The average values of the combustion air flows and temperatures are presented in Table 1. The experiments were carried out using two fuels: pellets and log wood. The log wood experiments were conducted under two different operational practices (named Log 1 and Log 2). Two 4-kg batches were burned in Log 1, i.e., six birch logs per batch, following manufacturer’s recommendation. The net heating value of the birch logs was 18.5 MJ/kg and the moisture content was 13%. The logs were positioned vertically and the 1st batch was ignited from below the logs. In Log 2, the same amount of fuel (8 kg) was divided into four batches, i.e., 2 kg/batch (six logs), and the logs were placed horizontally, following our previous experiences.32 The 1st batch was ignited from the top using birch bark kindling. The grate system in the log wood combustion was a conventional grate where combustion air enters from the bottom of the fuel. Some air also entered the combustion chamber as flush air through the door, which is used in this type of stove to keep the glass door clean. The particle measurements started when the door was closed after fuel ignition, and stopped when the CO2 concentration in the flue gas dropped below 4%. The total length of the log wood combustion period was 85–110 min. Three different versions of the pellet batch burner were tested (Pellet 1, Pellet 2 and Pellet 3). The pellet fuel was commercial softwood pellet with net heating value 18.9 MJ/kg and moisture content 7%. The grate used in the log wood combustion was removed and a specially designed


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pellet batch burner for the hybrid stove was inserted. In the pellet batch burner system, pellets were placed in a metal container. Primary air was led in through the fuel bed and secondary air was led in right above the fuel bed (Figure 2). Tertiary air was inserted in the flame approximately 10 cm above the secondary air. The primary air flow was controlled separately and the secondary and tertiary flows were controlled through a combined duct. Improvements in the combustion technology were made between the experiments (Table 2). After Pellet 1, an air tight door and refractory material on the combustion chamber walls were added. The purpose of the air tight door was to prevent any excess air from entering the combustion chamber and to improve the control of combustion air flows through the designated inlets. Refractory materials were added to the combustion chamber walls to increase the combustion temperatures. Moreover, the combustion air flows were optimized for Pellet 2, according to the experiences from Pellet 1 experiments. The mixing of combustible gases and the combustion air was improved in Pellet 3 by better directing the tertiary air outlets toward the flame. Commercial softwood pellet fuel was used in the pellet experiments. A total of 6 kg of pellets were placed in the pellet batch burner and combusted in one batch without refueling. The pellet fuel was ignited from the top using birch bark kindling and the length of the combustion period varied between 105–135 min. In the pellet experiments, mainly primary air was introduced at the beginning of the combustion to ensure sufficient initiation of the fuel burning (startup). After the fuel was stably ignited and flames reached the tertiary air tube, more secondary and tertiary air was introduced while the primary air was decreased (Table 1). The primary air was decreased to prevent too rapid gasification of the fuel and under stable combustion, most of the combustion air was inserted through the secondary and tertiary air inlets (steady firing). At the end of the combustion


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at the point when the flames were fading out (burnout), primary air was added again to ensure rapid combustion of the remaining fuel and glowing embers.

Sampling and characterization of particle and gaseous emissions The gaseous and particle emission samples were collected from the chimney, approximately 1.5 m above the stove outlet. Gaseous emissions were measured using an analyzer based on Fourier transform infrared (FTIR) spectrometry (DX-4000, Gasmet Technologies Ltd., Finland). The analyzer was additionally equipped with a ZrO2 O2 sensor. The measured NOX emissions were calculated as NO2 equivalents. The particle sample was collected from the chimney through a heated (150 °C) sampling probe. The sample was diluted in two phases: First with a porous tube diluter (PRD33) and second with an ejector diluter (ED, Dekati FPS ejector, Dekati Ltd., Finland). The same system has been used in several previous studies (e.g.,

13, 25

). The dilution ratios (DRs) are presented in

Table 1. The average DR in this dilution system varied between 28 and 53, and the filter samples for fine particle characterization were sampled downstream of the dilution system (DR1). An additional ejector diluter (Dekati DI-1000, Dekati Ltd., Finland) was applied to provide additional dilution for the online instruments (DR 240–430) to decrease the concentrations such that they would be suitable for the online particle instruments (DR2). Following the dilution, the DR was controlled and calculated by measuring the CO2 concentrations in the flue gas (FTIR) and the diluted sample gas (ABB AO2040, Uras 14, Switzerland, and Vaisala GMP 343, Finland). The measured concentrations were converted to nominal emission (per MJ) in relation to energy input in the burning process.31, 34 Because three filter samplings were conducted in the pellet experiments in which the sampling duration varied,


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the mean results for the whole combustion period obtained from the filters were weighted based on the ratio of each individual sampling duration and the duration of the entire experiment. For the particle mass and chemical analyses, particles larger than 1 µm were removed using a Dekati impactor (Dekati PM10, Dekati Ltd., Finland). Particles smaller than 1 µm (PM1) were collected on 47-mm filters. Both polytetrafluoroethylene (PTFE, Pall Corporation, NY, USA) and quartz fiber filters (Pallflex, Pall Corporation, NY, USA) were used. The filters were weighed before and after sampling using a 1-µg sensitivity scale (MT5, Mettler Toledo, OH, USA). The conditioning of the filters prior to weighing was conducted in a stable-condition room (T 20 °C, RH 40%) for 24 h. In addition to weighing, ions and metals from the PTFE filters were analyzed using ion chromatography (IC, water elution) and inductively coupled plasma mass spectrometry (ICP-MS, hydrogen fluoride nitric acid dissolution), respectively. Thermal-optical analyses were performed on the quartz fiber filters to quantify elemental carbon (EC) and organic carbon (OC). The thermal-optical analysis was performed using an analyzer from Sunset Laboratories Inc.,35 according to the National Institute for Occupational Health (NIOSH 5040) procedure. The particle number size distributions were measured using an Electrical Low Pressure Impactor (ELPI, Dekati Ltd., Finland), which was equipped with sintered impaction plates and operated at a flow rate of 10 lpm. The particle mass emissions were measured online using a Tapered Element Oscillating Microbalance (TEOM, model 1405, Thermo Scientific, MA, USA), providing real time data on the emission formation in the different phases of combustion.


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Table 1. The averaged gaseous emissions and operation parameters. O2 %

DR1

DR2

T °C

Comb. air flows, lpm

CO

Flue gas

Sample

Primary

Secondary

mg/MJ

CH4

NOX

Log 1

13.4

30

260

113

24

460

-

4000

77

120

Batch 1

12.8

31

280

91

23

440

-

1100

30

53

Batch 2

13.1

29

240

140

25

490

-

6000

100

190

Log 2

15.1

53

430

128

23

650

-

1200

23

81

Batches 1-2

15.6

63

510

98

22

620

-

1400

28

74

Batches 3-4

14.7

41

340

170

23

700

-

820

14

88

Pellet 1

14.9

33

280

85

27

310

150

1430

8

48

Startup

15.9

32

310

48

26

280

210

170

3.4

80

Steady

13.3

33

270

91

27

260

230

750

13

61

Burnout

17.0

33

290

94

27

400

-

3100

2.2

13

Pellet 2

12.6

30

240

67

25

160

120

830

16

42

Startup

16.7

26

210

32

22

160

170

860

4.8

31

Steady

10.8

31

240

63

25

10

150

1100

21

38

Burnout

14.3

28

230

80

26

320

120

590

16

49

Pellet 3

14.7

28

230

62

23

130

160

420

11

60

Startup

16.1

24

310

41

22

110

140

370

3.1

39

Steady

14.0

28

240

63

23

86

180

370

1.7

68

Burnout

16.2

28

220

81

24

290

90

220

31

46

Table 2. Tested fuels and improvements between experiments Fuel Log 1

Improvement

2 x 4 kg, 6 logs/batch


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Log 2

4 x 2 kg, 6 logs/batch

Pellet 1

6 kg of pellets

Pellet 2

6 kg of pellets

Air tight door, refractory walls, optimized comb. air flows

Pellet 3

6 kg of pellets

Pellet 2 + Tertiary air inputs directed towards the flame

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Air tight door

Figure 2. Operation principle of the hybrid stove using pellets. The pellet fuel is inserted to the fuel container, which is fueled from the top. The top plate with flame outlet is place right after the ignition and the door is closed. The flames will emerge from the flame outlet. 3. Results and Discussion Gaseous emissions The sampling periods in the log wood combustion were divided into two phases. In Log 1, one sample was collected from each batch, and in Log 2, two batches were sampled together. The average emission results are presented in Table 3 for the whole combustion and the specific sampling periods. The flue gas temperatures in the log wood combustion were somewhat higher than those in the pellet combustion (113 °C in Log 1 and 128 °C in Log 2) but were below previously reported values.7, 11, 28 The low flue gas temperatures can be explained using the basic principle of a slow


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heat releasing appliance, where heat is effectively transferred and stored in the surrounding structure around the flue gas ducts. The gaseous emissions varied naturally in the batch combustion and their concentrations depended greatly on the combustion phase (Figure 3). The operational practices affected the gaseous emissions originating from incomplete combustion. The carbon monoxide (CO) emissions were reduced on average from 4000 mg/MJ (Log 1) to 1200 mg/MJ (Log 2). The improved use in Log 2 decreased the emissions, especially after the addition of wood to the hot combustion chamber due to smaller batch size, compared with Log 1. On the other hand, the CO emissions during the first half of the experiment were lower in Log 1 than in Log 2. When the hybrid stove was operated with log wood, the CO emissions from Log 1 were somewhat higher than previous results obtained using similar log wood fired appliances,25, 28 but lower than the CO emissions from sauna stoves.7, 8 This indicates that the combustion was far from optimal. Improved operation resulted in lower CO emissions in Log 2, after which the emissions were similar to previously reported results from a similar appliance type, but were higher than typical CO emissions from masonry heaters equipped with modern combustion technology.25, 28 The combustion process in the pellet combustion was divided into three phases: Startup, steady firing and burn out. Three consecutive samplings were conducted during each experiment according to the three combustion phases (Figure 4). The flue gas temperatures in the pellet combustion were on average between 62 °C (Pellet 3) and 85 °C (Pellet 1), which were lower than those measured in the log wood combustion. The O2 concentrations in the pellet experiments were on average between 12.6 and 14.9%. Although some studies have reported even higher O2 concentrations in batch combustion,36 excess air decreases the efficiency of the combustion appliance and it should be minimized.


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Similar to the log wood combustion, the highest emissions were formed at the beginning of the combustion and in the burnout phase (Figure 4). The gaseous emissions from the pellet combustion were clearly lower than the emissions from the log wood combustion (Table 1), with the exception of the burnout phase in Pellet 1, where the CO emissions were considerable. The added refractory material in the combustion chamber and the increased supply of secondary air in the burnout phase improved the CO oxidation and lowered the emissions. Although the CO emissions in the startup phase increased from Pellet 1 to Pellet 2, the above-mentioned changes reduced the total CO emissions. From Pellet 2 to Pellet 3, tertiary air tubes were bent to improve the mixing of the tertiary combustion air and flame. This improvement lowered the CO emissions in all three phases, based on a comparison between Pellet 2 and Pellet 3. To our knowledge, there are no previously reported emissions available from hybrid stoves operated with pellets such as the one used in this study. The CO emissions in Pellet 1 were similar to those from conventional log wood-fired masonry heaters,28 but in Pellet 2, the emissions decreased to a level that was previously reported for modern masonry heaters.25, 28 The CO emissions in Pellet 3 were similar to those previously measured from automated pellet stoves.17, 30, 37


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Figure 3. Flue gas emission concentrations and combustion air flow rates in different phases of the log wood combustion experiment (Log 2). Batches 1-2 and 3-4 indicate the sampling periods.

Figure 4. Flue gas emission concentrations and combustion air flow rates in different phases of the pellet combustion experiment (Pellet 3). Combustion air flow data are missing between 13 – 20 min due to a data acquisition problem. “Startup”, “steady firing” and “burnout” indicate the sampling periods connected with the specific combustion phases.

Particle emissions Hybrid stove with log wood The particle emissions varied significantly between the pellet and log wood combustion experiments. The PM1 emissions in the log wood combustion consisted mainly of products from incomplete combustion: elemental carbon and organic material (Table 3). The PM1 composition in the log wood combustion followed a previously published pattern;31 under high emissions (for


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example, 150 mg/MJ in Log 1, batch 2), the PM consisted of both EC and OC, and the contribution of OC increased during high emitting phases. Under PM emissions of several hundreds of mg/MJ, organic matter proportions as high as 60–70% have been reported.32, 38, 39 When the PM1 emissions decreased, OC was the first PM component to decrease, whereas EC represented a significant fraction of the PM emissions, until the PM emissions were as low as those in the pellet combustion. Although the CO emissions clearly decreased from Log 1 to Log 2 with improved operation, there was only a small decrease in the average PM1 emission. However, there was a change in the time period during which most of the PM1 emissions were formed. In Log 1, the PM1 emissions were 41 mg/MJ during the first half of the combustion process, and inserting the large second batch in the hot combustion appliance resulted in considerable PM1 emissions (150 mg/MJ). The highest emission peak in Log 1 was measured after the addition of the second batch, although elevated emissions were also measured after the ignition and at the ends of the batches. Meanwhile, the PM1 emissions under stable combustion were rather moderate. Because the fuel in Log 1 was inserted in two batches (4 kg each), it can be concluded that the combustion chamber was overloaded with fuel and the gasification of the fuel in the hot combustion chamber was too fast, leading to high emissions of combustible gases and particles. In addition, the logs were placed vertically in the stove, which lead to non-optimized flows of combustion air, sped up the combustion and further contributed to the elevated emissions. In Log 2, the PM1 emissions were 130 mg/MJ during the first half (two batches) but were much lower during the last two batches (46 mg/MJ). The real-time particle data in Figure 3 further show that the second batch in particular emitted considerable amounts of particle matter, which highlights emission formation in the transient phases of the batch combustion, such as


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after ignition or the addition of a new batch. Compared with Log 1 where the logs were placed vertically, the horizontally placed logs in Log 2, together with the smaller batch size, inhibited too rapid gasification of the fuel after the fuel additions. In other words, when smaller batches of wood were used in Log 2, the high emission peaks could be avoided, but the emissions were higher during the stable phases than in Log 1. Due to this, the use of smaller wood batches only decreased the PM1 emissions by 6%. Although there were no considerable differences in the PM1 emissions between the two tested operational practices, previous results indicate that PM1 emissions can be affected by user operation.28

Hybrid stove with pellets The hybrid stove using the pellet fuel produced very low PM1 emissions that were significantly lower than those from the log wood combustion (Table 3). In Pellet 1, the PM1 emissions were 31 mg/MJ and consisted mostly of EC (20 mg/MJ). The improvements between the pellet combustion experiments decreased the PM1 emissions to 18 mg/MJ and 7.3 mg/MJ in Pellet 2 and Pellet 3, respectively. The startup phase constituted most of the PM1 emissions in all of the pellet experiments, and the main decrease in the PM1 emissions from Pellet 1 to Pellet 3 can be associated with the improved startup. The chemical composition of PM1 from Pellet 1, 2 and 3 was similar: the EC emissions were highest at the startup, whereas approximately 10% of PM1 consisted of OC. The most common inorganic species were K, Na, NO3 and SO4. The steady firing phase dominated most of the experiment in terms of duration, and the emissions in this phase were as low as 2.4 mg/MJ (Pellet 3). Similar to the CO emissions, the PM1 emissions increased slightly in the burnout phase. At the burnout phase, approximately 50% of PM1 was composed of EC (except in Pellet 1).


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In more detail, the PM1 emissions from the pellet combustion were elevated only during the ignition phase, and the peak only lasted a short time (Figure 4). The second (much smaller) peak in the PM emissions occurred just after (at time 40 min) the primary air was decreased and set for the steady combustion. The adjustment of the combustion air altered the combustion process and led to the stable firing phase. The particle emissions were very low during the stable combustion phase (45 to 75 min) but increased slightly towards the end of the steady firing phase, and again in the burnout phase. The increased emissions in the burnout phase were connected to the rapid burnout of the remaining fuel due to the increase in the primary air flow. Although the PM emissions increased slightly in this phase, the adjustment was important in reducing the CO emissions that increased at the end of the stable firing phase. The PM time trends in the three pellet combustion experiments behaved very similarly, although the emission levels varied between the experiments. In addition, it was observed that the PM time trends behaved similarly to the CO emissions, with the exception that there was no considerable increase in the PM emissions at the end of the stable firing phase. The particle number emissions varied between 9.4E+6 #/cm3 (Pellet 2) and 5.4E+7 #/cm3 (Log 2) (both at 13% O2), and they appeared to increase with increasing emissions of PM1 inorganics. This indicates that the particle number emissions are mostly governed by the formation of inorganic particles, as has been described previously.30 Thus, the particle number emissions might be determined by the release of ash-forming inorganic material from the fuel bed in the pellet combustion. However, the correlation between the PM1 inorganics and particle number was too weak to draw any further conclusions. Both the PM1 and EC emissions were greatly reduced compared with the log wood combustion, and the PM1 emissions were similar to previously reported values for modern


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Energy & Fuels

automated pellet combustion appliances20, 21, 24, 40, 41 and light fuel oil boilers.13, 42 This shows that the pellet batch combustion technology has a high potential as an emission reduction method. In fact, some of the previous studies17,

30, 37

have reported higher fine particle emissions from

automated pellet stoves than average PM1 emissions from the Pellet 3 case. Similar to previous results with pellet boiler emissions,11, 21 EC was the particle component that mainly contributed to the varying PM1 emissions under non-optimal combustion conditions. There were several possible reasons for the very low particle emission formation in the pellet batch combustion in the tested hybrid stove. First, the fuel bed was burned from top to bottom and therefore the fuel bed was not disturbed as in continuously operated top-feed boilers and pellet stoves. Second, the pellet batch was ignited in a confined vessel, restricting the gasification of the batch. Third, the combustion air was distributed through three inlets, of which only one was fed through the fuel bed and it further improved the control of the gasification rate of the fuel. Fourth, the staged secondary and tertiary air feeding enhanced mixing in the combustion chamber, leading to improved oxidation, compared with the log wood combustion. As a result, this technique proved to be capable of producing very low PM1, whereas gaseous emissions are more typical for these types of appliances. The disadvantage of the present system was the lack of possibility of adding fuel after the combustion had started. On the other hand, a single pellet batch is sufficient for heating the structure of the heater with sufficient heat release for several hours. Improved adjustment of the combustion air feed would make the system easier to operate to ensure low emissions. Although the pellet batch burner was tested and designed to be used only in the present system, this technique has potential to be utilized in similar types of batch combustion appliances, including the possibility of retrofitting the technique in existing stoves. This indicates great potential for


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Energy & Fuels

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emission reduction in batch combustion appliances, which do not utilize automated combustion control.

Table 3. Emission factors of PM1, carbon compounds and number emissions. Length, min

PM1, mg/MJ

OC, mg/MJ

EC, mg/MJ

Inorganic s, mg/MJ

Unidentified, mass mg/MJ

N, ELPI #/cm3 @ 13% O2

Log 1

78

98

35

41

4.2

19

3.2 E+07

Batch 1

36

41

5.6

26

1.7

8.2

3.6 E+07

Batch 2

40

150

60

55

6.4

28

2.7 E+07

Log 2

112

92

22

50

3.5

15

5.4 E+07

Batches 1-2

60

130

39

65

3.8

21

5.6 E+07

Batches 3-4

50

46

2.3

32

3.1

7.6

5.1 E+07

Pellet 1

90

31

2.5

20

3.3

5.1

2.6 E+07

Startup

15

140

13

110

3.2

13

8.3 E+07

Steady

49

6.1

0.5

1.3

2.6

1.7

1.2 E+07

Burnout

21

13

Operation and Emissions of a Hybrid Stove Fueled by Pellets and Log

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