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CO

NOx

SO2

PM2.5 

Gaseous and Particle Emission Rates from a High Efficiency Wood 

Boiler Using Two Types of Fuel: Wood Chips and Wood Pellets 

J. Laing, S. Raja, T.M. Holsen, P.K. Hopke

Introduction

There has been mounting concern over the health risks posed by pollutants emitted from biomass 

combustion, particularly fine PM. However, newer designs of fully automated, high efficiency wood 

boiler systems have been presented as a solution due to their low emissions. Continuous measurements of 

carbon monoxide, nitrogen oxides, sulfur dioxide, and PM2.5 from a Hamont CATfire (Hamont Consulting 

and Engineering, Austria) gasification wood boiler (Figure 1) running at steady state were determined 

using two types of fuels, wood pellets and wood chips. The combustion of both fuel types resulted in low 

emissions of carbon monoxide and high combustion efficiencies. Higher fuel nitrogen and sulfur content 

resulted in higher emissions of nitrogen oxides and sulfur dioxide, respectively. It was also seen that 

higher ash content lead to higher PM2.5 emissions.

Results

Summary of Findings

Selected two and a half hours of continuous gaseous and particle emission measurements using wood pellets and 

wood chips are displayed in Figures 3 and 4, respectively. The temporal variations in emissions are seen 

collectively among all gaseous species, indicating that they are a function of small differences in boiler output. 

Emissions factors of gaseous emissions and combustion efficiencies from the combustion of wood pellets and wood 

chips are provided in Table 2. Carbon Monoxide emissions for both fuel sources were low and comparable with 

previous studies of pellet burner/boiler emissions. Combustion efficiencies (CO2/(CO2 + CO)*100) demonstrate 

highly efficient combustion in both boilers. From the limited samples examine here, the higher fuel nitrogen and 

sulfur content (Table 1) resulted in higher NOxand SO2emissions (Table 2), verifying the fact that the gaseous 

emissions are formed through the oxidation of their fuel bound constituents. Higher ash content resulted in higher 

PM2.5 emissions. With nearly complete combustion achieved, the particulate matter is predominately derived from 

inorganic ash-forming components in the fuel vaporizing and subsequently nucleating in the flue gas. 

Acknowledgements

This study was funded by the New York State Energy Research and Development Authority (NYSERDA) which is supported by the U.S. Environmental Protection Agency. This work has not been subjected to the Agency’s required peer and policy review and

therefore does not necessarily reflect the views of the agency and no official endorsement should be inferred. Some of equipment used in this study was provided by the New York State Foundation for Science, Technology and Innovation (NYSTAR). We thank

Sean McMahon and Yang Qi for their help in the field work. We also thank the volunteers for their participation in this study.

Experimental Methods

CO, NOx, SO2, and PM2.5 emission rates from the gasification wood boiler running at steady state were 

determined using two types of fuels, wood pellets and wood chips. The boiler has a thermal capacity of 

150kW and was run at 76% and 60% capacity for wood pellets and wood chips, respectively. A dilution 

stack sampling system (Figure 2) conforming to EPA’s conditional test method (CTM) 039 was used to 

provide sample gas to ambient gas monitors and a TEOM with FDMS (Models 42i, 43i, 48i, and 8500b , 

Thermo Scientific, Franklin, MA). A switch valve prior to the gas monitors provided background 

ambient concentrations ten minutes out of every hour. Direct measurements of carbon dioxide were 

made using an in-stack Bacharach PCA®2 . 

Figure 2. Dilution Sampling System.

Table 1. Characteristics of the Fuel.

Fuel Analysis

The wood pellets used were manufactured by New England Wood Pellet, LLC and the wood chips were waste wood

scraps with a considerable amount of fines. Samples were collected for every testing period from the boiler feed bin.

Gross heating value, ash content, moisture content, sulfur content, and nitrogen content were determined (Table 1).

Wood 

Pellets

Wood 

Chips

heat content 

(MJ/lbs) 8.49 6.72

moisture (%) 4.80 26.6

ash (d.w. %) 0.60 1.79

carbon (d.w. %) 51.5 45.0

nitrogen (d.w. %) 0.13 0.37

sulfur (d.w. ppm) 70.1 175 Figure 1. Hamont CATfire Wood Boiler.

Table 2. Gaseous and particle emission factors and combustion efficiency for wood pellets and wood chips.

Emission Factors (mg/MJ) combustion

Fuel Source CO NOx

SO2 PM2.5 CO2(%) efficiency

Wood Pellets 115 18.4 0.26 25.1 7.86 99.89

Wood Chips 125 127 1.42 41.3 10.1 99.94

Figure 3. Gaseous and particle emission concentrations 

during steady state operation for wood pellets.

Figure 4. Gaseous and particle emission 

concentrations during steady state operation for wood 

chips.

Acknowledgements

This study was funded by the New York State Energy Research and Development Authority (NYSERDA) which is supported by the U.S. Environmental Protection Agency. This work has not been subjected to the Agency’s required peer and policy review and

therefore does not necessarily reflect the views of the agency and no official endorsement should be inferred.