Article pubs.acs.org/EF
Influence of Peat Ash Composition on Particle Emissions and Slag Formation in Biomass Grate Co-combustion Jonathan Fagerström,*,† Ida-Linn Naz̈ elius,‡ Carl Gilbe,‡ Dan Boström,† Marcus Ö hman,‡ and Christoffer Boman† †
Thermochemical Energy Conversion Laboratory, Department of Applied Physics and Electronics, Umeå University, SE-901 87 Umeå, Sweden ‡ Energy Engineering, Division of Energy Science, Luleå University of Technology, SE-971 87 Luleå, Sweden ABSTRACT: Co-combustion by fuel blending of peat and biomass has shown positive effects on operational problems. However, peat ash compositions vary considerably, and this has been shown to affect the potential for operational problems in different fuel-blending situations. The present work used three different peat types with the objective to elucidate how the variation in peat ash composition influences both particle emissions and slag formation during co-combustion with three different biomasses in a small-scale pellet boiler. Estimations of potassium release and slag formation were performed and discussed in relation to fuel composition in the (K2O + Na2O)−(CaO + MgO)−(SiO2) system. All tested peat types reduced the fine particle emissions by capturing potassium into the bottom ash as one or several of the following forms: slag, sulfates, chlorides, and alumina silicates. However, there were considerable differences between the peat types, presumably depending upon both their content and mineral composition of silicon, calcium, aluminum, and sulfur. Some general important and beneficial properties of peat type in co-combustion situations with biomass are defined here, but the specific blending proportion of peat should be decided on an individual basis for each scenario based on the relative contents in the fuel mixture of the most relevant ashforming elements.
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INTRODUCTION The biomass sources used for heat and power production have for a long time relied on wood fuels with low ash contents. The demand for such clean wood fuels is rising because of an increased usage of biomass in heat and power production and the emergence of other value chains, e.g., biofuels for transportation. The competition for clean wood fuels has led to the introduction of new ash-rich fuels for heat and power production. The introduction of such ash-rich biomass fuels in combustion processes increases the risk of particle emissions and operational problems, such as corrosion and deposit and slag formation.1−5 Particle emissions might be a threat to the environment and human health, and operational ash-related problems can lead to, e.g., inefficient combustion, higher maintenance costs, shorter component life, operation instability, and eventually boiler shutdowns. These issues must therefore be controlled if the fuel resource base for combustion processes is to be extended with ash-rich biomasses. Cocombustion by fuel blending of peat and biomass has shown positive effects on operational problems, such as reduced deposit formation and high-temperature corrosion,6−9 and bed agglomeration in fluidized beds.10,11 The effect of peat cocombustion on particle emissions is, however, less covered, but the reduction potential can be expected to follow the reported outcomes on deposit formation.6−9 The positive effects of peat co-combustion with ash-rich biomasses are accordingly linked to the ash composition of the peat. However, the ash-forming elements can vary considerably between different peat types, in both total content and elemental composition.12 Accordingly, some peat types are expected to be better suited for cocombustion than others. In a previous work by Pommer et al.,12 © 2014 American Chemical Society
a classification of 83 different peat samples was made using multivariate data analysis. The classification was performed on the basis of the occurrence of main ash-forming elements, where the variations of silicon (Si), aluminum (Al), calcium (Ca), and sulfur (S) formed the basis for the subsequently identified eight different subgroups. All subgroups were shown to have relatively low concentrations of potassium (K), with some of them not more than 10% of the levels found in, e.g., forest residues. Blending peat and biomass in different proportions therefore results in wide ranges of ratios for potassium, silicon, calcium, aluminum, and sulfur. These elements are highly important in ash transformations in biomass combustion.13,14 Potassium is one of the major ashforming elements in biomass fuels and plays a key role during combustion with respect to ash transformation processes. Potassium and also sodium (Na), although often present in much lower concentrations, can be distributed between three ash fractions, non-sintered bottom ash, slag, and aerosol, depending upon the chemical environment and temperature. Physical aspects related to combustion equipment design and operational conditions additionally affect the potential entrainment of coarse (>1 μm) aerosol particles. The distribution of potassium between the formed non-sintered bottom ash/slag and aerosol phase is vital for the overall ash chemical behavior, in literature sometimes referred to as potassium release.15−17 The released fraction of potassium in biomass grate combustion processes generally constitutes to volatilized (i.e., gaseous) Received: November 29, 2013 Revised: April 3, 2014 Published: April 11, 2014 3403
dx.doi.org/10.1021/ef4023543 | Energy Fuels 2014, 28, 3403−3411
Energy & Fuels
Article
species that subsequently form condensed fine aerosol particles (i.e., particles with an aerodynamic diameter of
