Article pubs.acs.org/EF
Residential-Scale Biomass Boiler Emissions and Efficiency Characterization for Several Fuels Sriraam R. Chandrasekaran,† Philip K. Hopke,*,† Michael Newtown,‡ and Arthur Hurlbut‡ †
Center for Air Resources Engineering and Science, Clarkson University, Post Office Box 5708, Potsdam, New York 13699, United States ‡ State University of New York (SUNY) at Canton, Canton, New York 13617, United States S Supporting Information *
ABSTRACT: Detailed particulate and gaseous emission characterizations were conducted on six commercially available residential-scale wood pellet boilers. The objective of the study was to define emission factors for these six different appliances burning wood pellets, grass pellets, and a blend of grass pellets and corn as fuels under low and high loads. Continuous monitoring of criteria pollutants, including PM2.5, NOx, SO2, and CO, was conducted using an EPA CTM-039 dilution sampling system. The PM10 emissions with wood as the fuel ranged between 14 and 17 mg/MJ and between 16 and 21 mg/MJ at low and high loads, respectively. The PM10 emissions from grass were found to be higher for all of the appliances compared to wood pellets at both low and high loads (28−33 and 37−44 mg/MJ, respectively). CO emissions, an indication of combustion efficiency, were found to be higher for the grass pellets, indicating less complete combustion. NOx and SO2 emissions were also higher for grass and grass/corn blends, attributable to the higher fuel N and S. PM samples collected on Teflon and quartz substrates were analyzed for ions and trace elements. Semi-volatile organic compounds collected on quartz and polyurethane foam (PUF) plugs were also analyzed. Levoglucosan, a molecular marker for wood combustion, was the predominant organic compound found in the grass combustion PM2.5 and ranged between 6 and 100 μg/MJ for grass and between 9 and 130 μg/MJ for wood. Polycyclic aromatic hydrocarbon (PAH) emissions were relatively higher for grass combustion ranging from 10 to 700 μg/MJ than for wood combustion ranging from about 5 to 200 μg/MJ. Dioxin and dibenzofuran emissions were found to be substantially higher for grass pellet emissions compared to wood pellet emissions at both high and low loads for all of the appliances.
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INTRODUCTION Biomass fuels have gained attention in recent years because they provide environmental benefits, such as reduction of greenhouse gas emission, reduction of acid rain, and soil improvements, as well as local employment.1 Biomass combustion in residentialscale appliances can be a major source of energy for space heating. Nearly 23% of the total U.S. energy was consumed by households primarily for heating purposes.2 Wood accounts for a total of 28% of the renewable energy used in the U.S. in 2008. While research is being conducted on wood as a fuel, there is a growing interest in alternative, fast growing plants, such as grasses, that have several advantages over other feedstocks for biomass-based fuel production. Grass pellets have an energy content that is similar to wood but is rapidly renewable, with multiple harvests per year. “Grasses can be grown on marginal farmlands that would otherwise not be used. In New York State, there are about 1.5 million acres of underutilized agricultural land that could be used to grow grass.”3 However, high ash and chlorine contents create problems for combustion. Grass fuels can be improved using soil management techniques or using better, advanced staged combustion units. Both may be needed if grass is to be routinely used as fuels.1,3 Currently, a variety of biomass conversion technologies are available. The choice of an appropriate technology depends upon the fuel type, fuel availability, desired energy carrier, end use, environmental standards, and economic conditions.4 There are several available methods for biomass conversion, including © 2013 American Chemical Society
combustion, gasification, pyrolysis, or some combination of these. The problem associated with direct biomass combustion is that it can be a substantial source of air pollution. The most important operational parameters for complete combustion are a high combustion temperature, sufficient combustion air supply, adequate mixing of combustion air and fuel gas evolved from the solid fuel, and residence time in the combustion zone.1,5 Dependent upon these operating parameters, the efficiency and emissions from biomass boilers vary substantially. The effects of these parameters are discussed elsewhere.1,6 The common types of combustion appliances can be categorized into batch and continuous appliances. Masonry heaters, wood stoves, and wood log boilers are batch appliances, while pellet burners, boilers, and stoker burners are continuous appliances. The current study focused on six different continuous residential-scale combustion appliances that could handle both wood and grass pellets as fuels. The first objective of the study was to define emission factors for these six different appliances using wood pellets, grass pellets, and a blend of grass pellets with corn as fuels at low and high loads. The second objective was to estimate the efficiency for the six appliances that use different technologies, including direct combustion, gasification, λ control, Received: May 13, 2013 Revised: July 21, 2013 Published: July 22, 2013 4840
dx.doi.org/10.1021/ef400891r | Energy Fuels 2013, 27, 4840−4849
Energy & Fuels
Article
Table 1. Characteristics of the Appliances appliance
type
capacity (kW)
origin
A
boiler
30
Poland (Europe)
B
boiler
32
Austrian (Europe)
C D
boiler boiler
33.4 15
U.S.A. Austrian (Europe)
E
boiler
25
U.S.A.
F
furnace
29
United Kingdom (Europe)
air flow
feed variable underfeed variable underfeed underfeed fixed drop feed in variable drop feed in fixed
fixed and continuous but manually variable fixed and continuous λ controlled
ash removal auger three pass heat exchanger three pass heat exchanger, storage tank
manually adjustable
no self- cleaning sliding grate ash removal auger
four air blowers
no
modified burn pot to burn wood and grass pellets
biomass
units
grass pellets
wood pellets
corn kernels
heating value moisture ash C H N O Li Na Mg Al K Ca V Cr Mn Fe Co Ni Cu Zn As Se Cd Sb Ba Pb
MJ/kg % % dw
19 ± 0.13 5.7 ± 0.6 4.80 ± 0.02 47.2 ± 0.2 5.7 ± 0.8 1.1 ± 0.1 46.05 167 14396 65462 16670 322032 39919 30 180 4532 15724 6.38 96.30 249.10 1864.00 5.38 5.27
