A Report on the Impact of Pellet Fuel Ash 

Content on the Particulate Emission 

Performance of Five Pellet Heaters 

Prepared for the Hearth Patio and Barbecue Association 

(HPBA) 

By: Robert Ferguson 

Ferguson, Andors & Company 

February 15, 2011
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Background

Many of the EPA certified pellet heaters were tested using pellets with very low ash 

content (< 0.5%). These pellets are made from softwood feed stock and are produced in 

the Pacific Northwest. These pellets are commonly available for the testing laboratory 

that has conducted much of the pellet heater certification testing. 

Pellets that are most commonly available in other areas of the country where pellet 

heaters are popular are generally hardwood or mixed wood and have higher ash content 

(>1%). 

There was supposition that higher ash fuels would result in higher particulate emissions. 

This was based on observations made by pellet heater manufacturers when they tested 

other fuel types (corn, grains, hulls, etc.) that were known to have higher ash content than 

premium wood pellets. 

There was almost no data directly comparing the particulate emissions from low and high 

ash wood pellet fuels in the same heater using the same test method. 

There was concern over the impacts on Best Demonstrated Technology (BDT) since 

different pellets might produce different results, even when the burn settings are 

optimized. This concern expands further when other fuel types are considered (corn, 

grain, hulls, pits, switch grass, etc.) By only considering the certified pellet heater 

database, where very low ash softwood pellet fuel was used almost exclusively, the 

determination of BDT will most certainly be skewed toward a lower emission value than 

should be considered as representative of nationwide pellet heater performance, even for 

the best performing heater models. 

Data Gathering

HPBA has been working for the past six months to obtain data that will shed some light 

on this issue. Some direct comparison testing has been conducted, both in manufacturer 

labs and at one EPA accredited lab. 

The five pellet heaters that were tested to obtain this data represent main stream pellet 

room or space heater technology and cover the typical range in terms of heating capacity. 

All five heaters fell within an EPA listed heat output range from around 7000 BTU/hour 

at the low burn rate settings to around 35,000 BTU/hour at the highest setting. 

We appreciate the cooperation and support of the manufacturers and the test laboratory 

that participated in this effort to gather important data. The identities of the 

manufacturers and heater models as well as the test lab have been masked for the 

purposes of this study. 
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The data that has been gathered for this report includes test results from more than one 

test method. The testing for the results labeled “ASTM” utilized what is now ASTM 

E2779, the new pellet heater operating test protocol (that includes an integrated burn 

cycle covering the full range of operation of the heater) in conjunction with ASTM 

E2515 for measuring particulates using a dilution tunnel. It should be noted that ASTM 

E2515 and ASTM E2779 report emission values “as measured”. The results labeled 

“EPA” are the EPA certification weighted average emission rates for the heaters. If the 

emissions during certification testing were measured using EPA Method 5G-1 or 5G-3, 

the results in the “EPA” column are values adjusted to Method 5H equivalents according 

to current EPA requirements. The results labeled “PM” are individual Method 28 burn 

rate category emission results but are reported as the “as measured” EPA Method 5G-3 

values. The pellet moisture and ash values were determined using ASTM E871 and 

ASTM D1102 respectively. Ash and moisture determinations were conducted by 

qualified third party testing labs. The data are shown in the following tables. 

Table 1 shows the results of testing from three different pellet heaters using lower and 

higher ash fuels with a direct comparison of measured emission rates as well as listing the 

EPA certification emission rate for each heater. 

Table 1
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Table 2 shows the results of testing on two additional pellet heaters. These heaters were 

tested for EPA certification using low ash pellet fuel. In addition, two individual burn 

rate category tests per EPA Method 28 were repeated using higher ash fuel. 

Table 2
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In addition, the results are also presented in the following charts. 

Chart 1 shows the ASTM E2779 average emissions and the EPA certification emission 

value for each of the three pellet heaters from Table 1, plotted versus the pellet fuel ash 

content. 

Chart 1

Pellet Ash Content Effect on PM Emission Rate

3 Pellet Heaters (A, B & C)

0.0

1.0

2.0

3.0

4.0

5.0

6.0

0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00

Pellet Ash Content % (Dry Basis)

PM Emission Rate - g/hr

ASTM E2779

EPA Cert. Value

A

A

A

A

A

B

B

B

C

C
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Chart 2 shows the comparison of the individual burn rate category test runs for the two 

heaters whose data are shown in Table 2, using both low and higher ash content fuels. 

This limited data allows the trend of impacts on PM from ash content differences as well 

as showing the trend of the burn rate impacts on measured PM, at least for these two 

individual heaters. 

Chart 2

Pellet Heater Burn Rate Impact on PM Emissions

2 Pellet Heaters (D & E), Lower and Higher Ash Pellet Fuel

0.0

0.5

1.0

1.5

0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 1.80 2.00

Burn Rate - dry kg/hr

5G-3(unadjusted) PM Emission Rate - g/hr

Hardwood (1.8% Ash)

Softwood (0.22-0.27% Ash)

E

E

E

E

D

D

D

D

Discussion

Although there is inadequate data to perform a meaningful statistical test to determine if 

the low ash and higher ash results are from separate populations, it appears qualitatively

that the trend shown in Chart 1 supports the hypothesis that higher ash pellets will 

produce higher levels of particulate emissions. 
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The data plotted on Chart 2 help reinforce the ash effect trend shown on Chart 1 and 

perhaps suggests some explanation of the cause. Chart 2 shows that regardless of fuel 

ash content, PM emissions tend to increase when burn rate increases. The fact that pellet 

heaters include combustion air blowers that create fairly turbulent flows within the heater 

burn pot and adjoining firebox area and that when burn rate increases, this turbulence 

also increases suggests that some of the additional PM collected is most probably 

inorganic ash that has been entrained in the exhaust flow. It is reasonable to expect that 

when the pellet fuel ash content increases, the increase in entrained ash caused by the 

turbulent flow would be exacerbated and this effect will be observed as an increase in 

weight of particulate matter collected. Chart 2 shows that at all but the very lowest burn 

rate, the higher ash pellets produce higher particulate emissions. Although the data are 

very limited, nothing known about the range of current pellet heater designs would 

suggest that these trends are unique to the two tested units. 

Higher ash pellet fuels dominate the available pellet fuel supply in areas other than the 

west coast (primarily due to available feedstock for the pellet mills), and other potential 

biomass fuel options generally have higher ash contents as well. The impact of pellet ash 

content on emissions performance should be considered as part of the establishment of 

the testing requirements and passing grade for pellet heaters in the revised NSPS. It is 

understood that EPA will propose requirements that address this issue 

Pellet/biomass heaters can provide clean-burning heating, even on higher ash fuels. With 

careful consideration of the ash issue, alternative bio-fuel development and heater 

development for those fuels will continue and may provide additional attractive 

renewable energy options in the near future. Without that careful consideration, those 

activities could inadvertently be stifled.