Small-scale Combustion Appliances Research Articles

Additives as a fuel-oriented measure to mitigate inorganic particulate matter (PM) emissions during small-scale combustion of solid biofuels

During combustion of solid biofuels, particulate matter (PM) harmful to humans and the environment is formed and released into ambient air. These aerosols can be distinguished by their origin, as they either originate from inorganic elements (e.g., K, Na, Cl, S, Ca, Mg, Si, P, Zn, Pb) naturally contained in solid biofuels or from the organic matrix (e.g., C, H, O) of the biofuel. While inorganic particles (e.g., KCl, K2SO4, CaO, ZnO) are usually inevitably released during combustion, organic emissions (e.g., PAHs, VOC, soot) are primarily obtained due to the incomplete fuel oxidation. Within this context, this article aims to summarize the basics of inorganic PM formation during small-scale combustion of solid biofuels; this is especially realized for wood fuels. First, some basics on PM emissions are presented; this includes overall definitions, common particle size fractions (e.g., PM10, PM2.5, PM0.1), chemical composition, and physical characteristics. Subsequently, the PM formation during thermochemical conversion of solid biofuels is described for PM of inorganic origin. Then, primary measures to reduce these inorganic PM emissions in small-scale combustion appliances are discussed. This is especially true for an additivation as one option to reduce PM of inorganic origin. Hence, mechanisms and reactions of different additives (e.g., based on alumina silicate, calcium, or phosphorus) are outlined, before conclusions regarding the impact potential of the considered additives are drawn. Finally, the overall information about reactions, products and byproducts are summarized related to the most efficient additive.

Content of potassium and other aerosol forming elements in commercially available wood pellet batches

In wood fuels, potassium is by far the most abundant aerosol forming component. Given this predominant role, a major contribution for meeting the tightened total particulate matter emission thresholds on European level could thus be expected by the introduction of an upper limit for potassium content in wood pellets. To evaluate options to introduce such an upper limit for the potassium content of wood pellets, an extensive pellet screening was performed in Germany covering 22 of the 46 ENplus certified wood pellet producers representing more than 50% of the current German wood pellet production of about 2 million t/year. This screening was accompanied by the compilation of wood pellet fuel data from different sources. In total, analysis data of 249 high quality wood pellet samples has been evaluated. Almost all pellet samples originated from European production whereof the majority were pellets from certified German pellet producers. The pellet screening revealed a very large variation in the potassium content of high quality wood pellets ranging from below detection limit to well above 1000 mg/kg d.b. The results also confirmed the predominant role of potassium among the aerosol forming elements potassium, sodium, lead and zinc in wood samples. Furthermore, the screening highlighted that the ash content is not sufficient to evaluate the potassium content of wood pellets. Analysis of selected raw materials from three different pellet producers revealed a strikingly large variation in the potassium content of the raw materials. Consequently, controlled adjustment of the potassium content in wood pellets in the production process would require dedicated mixtures of different raw materials and the limitation of the share of raw materials with particularly high potassium contents. In this way, the potassium content of wood pellets could be limited and this could contribute to the compliance with strict particulate emission thresholds for small scale combustion appliances. Prerequisite for such an approach would be a fast, simple and cheap possibility to determine the potassium content of raw materials for wood pellet production at the production sites and the incorporation of the potassium control in the quality assurance of the pellet production process. Consequently, wood pellets could be produced that facilitate compliance with particulate matter emission threshold without secondary emission reduction measures.

Blended biomass pellets as fuel for small scale combustion appliances: Effect of blending on slag formation in the bottom ash and pre-evaluation options

Non-woody biomass fuels have a great potential to replace fossil fuels and reduce greenhouse gas emissions. At the same time, their application in small scale combustion appliances for heat production is often associated with increased operational problems (e.g. slagging in the bottom ash or deposit formation) as well as elevated gaseous and particulate matter emission levels. To mitigate these problems, scope and limitation of blending raw materials owing critical fuel composition with less problematic biomasses have been systematically studied during combustion experiments in a commercially available small scale combustion appliance with a nominal heat capacity of 30 kW. Three pellet batches of pure biomass (i.e. pine wood, miscanthus and wheat straw) and seven blended biomass pellet batches have been employed. Slag formation in the bottom ash as well as amount of agglomerated bottom ash particles were monitored and evaluated with respect to the determined ash melting temperatures and the measured fuel bed temperatures. Significant reduction of the slagging risk in the bottom ash during combustion of herbaceous fuels can only be achieved for high blending ratios with more than 70 wt% wood. It was furthermore studied to which extent fuel indices are applicable for the prediction of the bottom ash behavior of blended biomass fuels. Furthermore, ternary diagrams were used to rationalize the observed slagging characteristics. It was found that ternary diagrams are superior for a pre-evaluation of the slagging risk in the bottom ash to fuel indices alone. With the knowledge of the fuel ash composition and the ash melting temperatures, a well based pre-evaluation of utilization options can be performed.

Blended biomass pellets as fuel for small scale combustion appliances: Influence on gaseous and total particulate matter emissions and applicability of fuel indices

Non-woody biomass fuels have a great potential to replace fossil fuels and reduce greenhouse gas emissions. At the same time, their application in small scale combustion appliances for heat production is often associated with increased operational problems (e.g. slagging in the bottom ash or fouling) as well as elevated particulate and gaseous emission levels. To mitigate these problems, scope and limitation of blending raw materials owing critical fuel composition with less problematic biomasses have been systematically studied during combustion experiments in a commercially available small scale combustion appliance with a nominal heat capacity of 30kW. Three pellet batches of pure biomass (i.e. pine wood, miscanthus and wheat straw) as well as seven blended biomass pellet batches have been employed. Slagging, emission of total particulate matter (TPM) and gaseous emissions (i.e. CO, NOx, SO2 and HCl) were monitored. The results were evaluated with respect to the emission reduction potential of the blending strategy as well as the applicability of fuel indices which were originally developed for coal and pure biomass fuels. Based on the results, blending of herbaceous raw materials with woody biomass reduces the slagging risk in the bottom ash and leads to reduced emission levels, though significant reduction potential was observed only for blends with at least 50wt% wood. The blending of miscanthus with wood seemed to be more effective. Most of the fuel indices which were deduced from the chemical composition of the fuel seem to be applicable for a preliminary evaluation of blended biomass fuels and the prediction of critical emission levels.

A Review on Particle Emissions from Small Scale Biomass Combustion

Combustion of biomass fuels in small scale heating appliances is an important source of particle emissions. Majority of the particles is less than 1 µm (micrometer) and emitted directly to the ambient air from the combustion devices . There is a growing concern worldwide about particle emissions from biomass combustion. This review paper has been conducted based on the research articles published in the journals, conference proceedings, and technical reports from the research institutions between the periods from 2000 to 2010. Types of particles, formation mechanisms, their characteristics and available measurement techniques are discussed at the beginning . Particle emission characteristics on small scale biomass combustion from the literature have been discussed in order to provide in depth knowledge as assistance to readers interested in this research field. Finally, the literature data has been summarized into three categories to show how combustion appliances and fuel affect the mass concentrations, number concentrations, corresponding size distributions and chemical compositions. Mass concentrations of particles in the flue gas from small scale combustion appliances reported in the literature to be in the range of 7-67 mg/Nm 3 , while particle number concentrations in the range of 0.8 x 10 7 - 5.5 x 10 9 particles/cm 3 . The dominating chemical compositions of the particle emissions in the fine mode were potassium fraction followed by sulphur, chlorine and sodium and consequently dominating chemical compounds were K 2 SO 4 and KCl. From the literature it is clear that particle emissions from biomass combustion are closely related not only to the fuel properties but also to the combustion conditions and combustion appliances.