Article pubs.acs.org/EF
Emissions of Heavy Metals during Fixed-Bed Combustion of Six Biomass Fuels Henrik Wiinikka,*,†,‡ Carola Grönberg,† and Christoffer Boman§ †
Energy Technology Centre (ETC), Box 726, S-941 28, Piteå, Sweden Division of Energy Science, Luleå University of Technology, S-971 87, Luleå, Sweden § Energy Technology and Thermal Process Chemistry, Umeå University, S-901 87, Umeå, Sweden ‡
ABSTRACT: Few studies examine heavy metal emissions during the small-scale combustion of various solid biofuels. This issue may become more important, as one can expect new regulations governing such emissions from biomass combustion similar to those governing waste incineration. This paper investigates the emissions of particulate-associated heavy metals (i.e., Sb, As, Cd, Co, Cr, Cu, Pb, Mn, Ni, Tl, V, Hg, and Zn) during the fixed-bed combustion of six solid biofuels (i.e., stemwood from birch and pine/spruce, bark from birch and pine, salix, and oat grains) and of peat and bituminous coal for comparison. The results indicate that the flue gas concentration (normalized to 11% O2) of the sum of all measured metals (Zn excluded) during the biomass combustion tests ranged from 57 μg Nm−3 for birch stemwood to 198 μg Nm−3 for birch bark. The concentration of Zn in the flue gas was generally considerably higher than those of the other metals, ranging from 646 μg Nm−3 for spruce/pine stemwood to 7948 μg Nm−3 for birch bark. Compared with coal and peat, the biomass fuels produced higher Zn emissions, but lower or similar emissions of the sum of the other metals. The volatile behavior and concentration of the metal in the flue gases as a function of the heavy metal in the fuel are also presented for selected heavy metals. Lind et al.16 investigated the volatilization of the heavy metals Zn, Cd, Pb, and Cu during the circulating fluidized-bed combustion of forest residue.16 Their results indicate that the major fractions of all investigated metals are found in the coarse fly ash particles and that none of the investigated heavy metals is enriched in the fine particles. The dominant gas-to-particle conversion route for Cd, Pb, and Cu was found to be chemical surface reactions, probably with silicates. Jimenez et al.17 investigated the vaporization of trace elements (i.e., Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, As, and Pb) during the pulverized combustion of olive residue (orujillo) in an entrained flow reactor. Their results indicate that As, Pb, Cr, Cu, Ni, and Zn are enriched in the fine fly ash particle fraction and Co, Mn, and Fe in the bulk fly ash. Furthermore, the concentration of trace elements in the fine fly ash increased with increasing combustion temperature. Interest in heavy metal emissions from solid fuel combustion has increased in the EU since regulations for heavy metal emissions from waste incineration were introduced (see Table 1).18 Few studies have examined heavy metal emissions from
1. INTRODUCTION Measurements of the flue gas from various small- and mediumsized biomass-fired boilers provide a good understanding of the formation mechanisms and emission levels of the combustiongenerated particles emitted by such devices.1−13 Under good burn-out combustion conditions, particle emissions are dominated by inorganic particles produced by two different mechanisms, i.e., coarse particles (>1 μm) produced by the fusion of nonvolatile ash elements in the burning char particles, and fine particles ( 4.5 μm from the sampling flow. The temperature of the precyclone and the filter was always >105 °C to prevent condensation of moisture on the collected particles. A gravimetric impactor (GI; Dekati Ltd., Tampere, Finland) was used to fractionate the particles in four size classes according to aerodynamic diameter. Prior to the GI, the sampling flow was diluted approximately seven times with pure air. The GI classifies particles into size fractions with stage cut points of 2.5, 1.0, and 0.5 μm. Teflon plates were used as substrates in the GI to collect particles. A backup filter collects particles smaller than 0.5 μm. The impactor substrates and backup filter from the GI will be used in a parallel study of the in vitro toxicological properties of various kinds of biomass combustion particulate emissions. The smallest particles collected on the backup filter are of special interest from a toxicological perspective, since they can penetrate deep into the lungs. Therefore, particles in this size fraction were chemically analyzed. To obtain particles for chemical analysis of the same size as were captured in the GI backup filter, a low-pressure impactor (LPI; Dekati Ltd.) was used to collect particles < 0.5 μm. Prior to the LPI, the sampling flow was also diluted approximately seven times with pure air. In its original form, the LPI separates aerosols according to their aerodynamic diameters in 13 size intervals from 0.03 to 10.7 μm. However, the last six plates from the bottom where removed for this work, so the cut point of the last impactor stage that remained was 0.507 μm. Smaller particles were collected on a backup filter. The LPI backup filter was analyzed for heavy metals (i.e., Sb, As, Cd, Co, Cr, Cu, Pb, Mn, Ni, Tl, V, Hg, and Zn) by ALS Scandinavia AB according to the SS-EN 14385:2004 standard. The burner operating conditions used in the controlled combustion experiments with the eight fuels are summarized in Table 4. The mass flow of fuel was controlled to obtain a thermal power of ∼8 kW. In all experiments, the reactor walls in the combustion zone were slightly insulated and the wall temperature was ∼450 °C. The total amount of combustion air distributed between the primary, secondary, and tertiary air registers corresponds to an excess air level of ∼6% O2 in the flue gas. For practical reasons, it was impossible to run the burner using the same amount of combustion air in each of the three air registers, because the differing properties (e.g., ash and char contents) of the studied fuels affected each fuel’s combustion behavior. Instead, before the particle measurements started, the experimenter redistributed the air flow between the three air registers to optimize the
small- and medium-scaled biomass-fired appliances used for heating. This information may be crucial in the future, with anticipated new emission regulations governing biomass combustion and with increased environmental health concerns related to combustion-derived fine particulate and heavy metal emissions. The study investigates the emissions of heavy metals (i.e., Sb, As, Cd, Co, Cr, Cu, Pb, Mn, Ni, Tl, V, Hg, and Zn) during the small-scale fixed-bed combustion of six biomass fuels, and of peat and coal (for comparison). The specific objectives are (i) to learn about the heavy metal emission levels for certain woody and nonwoody biomass fuels relevant to present and future markets and (ii) to experimentally elucidate the behavior of heavy metals during the fixed-bed combustion of selected biomass fuels with respect to distribution between ash fractions. This knowledge is especially important for the smallest-scale combustion equipment, for example, residential pellet burners (
