The characteristics of air pollutants from the combustion of biomass pellets

Biomass is renewable clean energy. The aim of this study is to explore the combustion properties and emission characteristics of NO_X, SO₂, PM, and HCl in the combustion process of biomass pellet fuels. In this study, three kinds of fuels (pine sawdust, mixed wood, and corn straw) were selected to be studied by using a tube furnace to simulate industrial boiler. Experiments were conducted under different combustion conditions (combustion temperature and air flow). The results show that pollutant emissions were related to fuel type, combustion temperature, and air flow. The emissions of NO_X were contingent on N content in the fuel and the peak emissions of NO_X appeared in the range of 50~600 mg/m³ at 4 L/min and 700℃. The emissions of SO₂ were related to combustion condition and close to zero under the condition of sufficient combustion. The emissions of HCl and particulate matter (PM) increase with the rise of temperature, but the emission of PM was minimal at 800℃. Average HCl emission was 0.2~0.5 mg/g under steady-state conditions (4 L/min and 700℃). All in all, the pollutant emissions of biomass pellet fuels during combustion are lower than those of the traditional fuel, and the combustion efficiency is relatively higher.     

Mineral transformation during combustion of coal blends

Mineral behaviour for two individual coals (I, J) and their two‐component coal blends and 800°C ash blends heating were studied. Ash samples were heated progressively from 800°C to IT (initial deformation temperature) at 100°C intervals under different conditions. Coal samples were heated from room temperature to the corresponding temperature. Mineral transformation at each temperature was determined by X‐ray diffraction and SEM measurements. The results show that Si, Al, Fe and Ca compounds have a great form variation during heating. Their forms at different temperatures depend on the chemical composition of the ash, the blending ratio and the atmosphere. For different coal ashes, the main mineral matters at 800°C were quartz, anhydrite, hematite, calcite and feldspar. As the temperature increased, oxidation, thermal decomposition, transformation and reaction occurred between the components. Comparing a 40% I+60% J ash blend with individual ashes, fayalite was formed at 1100°C for the blend; the reaction product existed in a glassy phase at 1300°C. For a coal blend having the same ash ratio as the ash blend, FeO reacted with amorphous SiO₂ or Al₂O₃ to form fayalite and hercynite at 1000°C. As the temperature increased to 1100°C, fayalite and hercynite increased obviously. At 1200°C, some iron inclusion compounds melted to become glassy phase matter. Compared with the ash blend, iron species undergo a different change during coal blend heating: fayalite and hercynite formed earlier, iron compounds melted to form a glassy phase at lower temperature. This may be caused by early combustion of the more reactive coal (J coal) in the blend inducing local variation in oxygen concentration gradients around the less reactive coal and consequently affecting the reaction atmosphere and Fe mineral behaviour and interaction. That is to say, for coal blends, the mineral transformation was affected by both the mineral species interaction and the combustion behaviour. The calculations were performed to examine the fate of mineral matter under different combustion conditions using a thermodynamic chemical equilibrium calculation program. Calculations from coal blends were comparable with experiments from ash blends, this is because the calculation program only considers the interaction among the mineral species but does not consider the combustion reaction. It indicates that combustion and the relative volatiles also affected the mineral behaviour and slagging during coal blend combustion. Meanwhile, the mineral species evaporations were measured at high temperature: the main evaporated species were Na, K pure species and compounds, Fe, FeO, SiO and SiO₂. The evaporation of Fe has an important effect on initial deposition. Calculations were comparable with the experiments. Copyright © 1999 John Wiley & Sons, Ltd.     

Behavior of basic elements during coal combustion

X-ray absorption fine structure (XAFS) spectroscopy, Mossbauer spectroscopy and computer-controlled scanning electron microscopy (CCSEM) have been used to investigate the reactions of Ca, Fe and alkalies in combustion systems. Ca may either transform to a CaO fume that reacts with SO₂ to form CaSO₄, or may react with clays, quartz and other minerals to form slag droplets, or flyash. Similarly, pyrite may devolatilize and oxidize exothermically to form molten or partially molten iron sulfide-iron oxide mixtures, or may react with other minerals to become part of the slag. Alkalies in lignites (principally Na) volatize and may react with either SO₂ to form sulfates or with clay minerals (principally kaolinite) to form aluminosilicate slag droplets. K in bituminous coal is contained in illite which melts and becomes part of the slag phase. The calcium and alkali sulfates and the iron-rich species are observed to be concentrated in the initial layers of deposits, while the complex aluminosilicate slag droplets collect to form an outer glassy layer.     

竹园污泥焚烧污染物排放特性的试验研究

选取上海竹园污泥,将原始污泥样品分为全干污泥、均匀干化污泥(含水率10%)和干湿混合污泥(含水率20%,干湿污泥质量比例为10∶3)三种样品,分别送入小型流化床焚烧炉中焚烧。试验研究了全干污泥在750℃、850℃、950℃三个工况下的燃烧特性,并研究了均匀干化污泥和干湿混合污泥在850℃工况下的燃烧特性。研究发现,上海竹园污泥在小型流化床焚烧炉中燃烧时,排放的主要常规污染物包括CO、SO2、NOx、HCl等以及二噁英和Cd、Hg、Pb等重金属。不同燃烧温度和含水率对污染物的排放有一定影响,提高燃烧温度,CO、SO2、NOx、HCl等的排放基本呈现出下降趋势,而烟气中Pb的排放随着温度上升而升高,相同燃烧温度下含水率升高能降低二噁英的排放总量,随着含水率的提高飞灰中重金属含量有所降低,而底渣中重金属含量呈上升趋势。     

有机钙废弃物与煤混合燃烧进行脱硫的应用研究

利用含有机钙的工业废弃物对煤在燃烧过程进行脱硫效果明显,脱硫效率达65％以上,并且操作运行简单方便、廉价易得、不造成二次污染,达到、废物利用的目的,对治理大气污染和保护环境具有一定的实用和推广价值。     

Emissions of organic air toxics from open burning: a comprehensive review

Emissions from open burning, on a mass pollutant per mass fuel (emission factor) basis, are greater than those from well-controlled combustion sources. Some types of open burning (e.g. biomass) are large sources on a global scale in comparison to other broad classes of sources (e.g. mobile and industrial sources). A detailed literature search was performed to collect and collate available data reporting emissions of organic air toxics from open burning sources. The sources that were included in this paper are: Accidental Fires, Agricultural Burning of Crop Residue, Agricultural Plastic Film, Animal Carcasses, Automobile Shredder Fluff Fires, Camp Fires, Car–Boat–Train (the vehicle not cargo) Fires, Construction Debris Fires, Copper Wire Reclamation, Crude Oil and Oil Spill Fires, Electronics Waste, Fiberglass, Fireworks, Grain Silo Fires, Household Waste, Land Clearing Debris (biomass), Landfills/Dumps, Prescribed Burning and Savanna/Forest Fires, Structural Fires, Tire Fires, and Yard Waste Fires. Availability of data varied according to the source and the class of air toxics of interest. Volatile organic compound (VOC) and polycyclic aromatic hydrocarbon (PAH) data were available for many of the sources. Non-PAH semi-volatile organic compound (SVOC) data were available for several sources. Carbonyl and polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofuran (PCDD/F) data were available for only a few sources. There were several known sources for which no emissions data were available at all. It is desirable that emissions from those sources be tested so that the relative degree of hazard they pose can be assessed. Several observations were made including:Biomass open burning sources typically emitted less VOCs than open burning sources with anthropogenic fuels on a mass emitted per mass burned basis, particularly those where polymers were concerned. Biomass open burning sources typically emitted less SVOCs and PAHs than anthropogenic sources on a mass emitted per mass burned basis. Burning pools of crude oil and diesel fuel produced significant amounts of PAHs relative to other types of open burning. PAH emissions were highest when combustion of polymers was taking place. Based on very limited data, biomass open burning sources typically produced higher levels of carbonyls than anthropogenic sources on a mass emitted per mass burned basis, probably due to oxygenated structures resulting from thermal decomposition of cellulose.It must be noted that local burn conditions could significantly change these relative levels. Based on very limited data, PCDD/F and other persistent bioaccumulative toxic (PBT) emissions varied greatly from source to source and exhibited significant variations within source categories. This high degree of variation is likely due to a combination of factors, including fuel composition, fuel heating value, bulk density, oxygen transport, and combustion conditions. This highlights the importance of having acceptable test data for PCDD/F and PBT emissions from open burning so that contributions of sources to the overall PCDD/F and PBT emissions inventory can be better quantified.     

Thermal analysis and kinetics of coal during oxy-fuel combustion

The pyrolysis and oxy-fuel combustion characteristics of Polish bituminous coal were studied using non-isothermal thermogravimetric analysis. Pyrolysis tests showed that the mass loss profiles were almost similar up to 870℃ in both N_2 and CO_2 atmospheres, while further mass loss occurred in CO_2 atmosphere at higher temperatures due to char-CO_2 gasification. Replacement of N_2 in the combustion environment by CO_2 delayed the combustion of bituminous coal. At elevated oxygen levels, TG/DTG profiles shifted through lower temperature zone, ignition and burnout temperatures decreased and mass loss rate significantly increased and complete combustion was achieved at lower temperatures and shorter times. Kinetic analysis for the tested coal was performed using Kissinger-Akahira-Sunose(KAS) method. The activation energies of bituminous coal combustion at the similar oxygen content in oxy-fuel with that of air were higher than that in air atmosphere. The results indicated that, with O_2 concentration increasing, the activation energies decreased.     

The effect of charcoal combustion on iron-ore sintering performance and emission of persistent organic pollutants

A study was carried out into the use of hardwood charcoal as a supplementary fuel in the iron-ore sintering process. The primary fuel was coke breeze with 0%, 20%, 50% and 100% replacement of the energy input with charcoal to produce raw blends with the same heat output as 4.0 wt.% coke breeze. Experimental results indicate that fuel blends where 20% of the heat input was provided by charcoal may improve both the sinter yield and sintering productivity by up to 8%, under normal sintering conditions. In addition, the 20% replacement of coke energy with charcoal would mean that part of the carbon dioxide emitted from the process would be from a renewable source and could be used to offset carbon dioxide emissions from non-renewable fossil fuels. At higher rates of coke breeze energy substitution with charcoal, the lower sintering performance observed was mainly attributed to the lower fixed carbon content and higher volatile matter content of the fuel mix.At the optimum rate of 20% substitution of coke breeze energy input with charcoal, the emission of dioxins were similar to those observed with coke breeze alone as the fuel. However, sintering with 20% energy input from charcoal resulted in a slight increase in middle molecular weight and lower molecular weight PAHs, contributing to a minor increase in B[a]P-eq from 0.15 μg/m³ to 0.17 μg/m³. Overall the results from the laboratory scale tests suggest that it is feasible to substitute 20% of the coke breeze energy input with an equivalent amount of energy from charcoal in the iron-ore sintering process.     

Mineral behavior during coal combustion 1. Pyrite transformations

The transformations and deposition characteristics of pyrite (FeS₂) under pulverized coal combustion conditions were examined in an entrained flow reactor. Mössbauer spectroscopy, scanning electron microscopy, and energy dispersive X-ray analysis, were used to monitor the evolution of particle composition and morphology. Pyrite initially decomposed to form pyrrhotite (Fe_0.877S). Fissures resulting within the particle during this step led to limited bulk fragmentation. Subsequently, the pyrrhotite particles melted to form an iron oxysulfide droplet. Magnetite (Fe₃O₄) crystallized out of the melt once the melt oxide content exceeded 85%. Hematite (Fe₂O₃) was formed at longer residence times. A comparison of kinetic model calculations with experimental data revealed the controlling resistances for each of the stages during the transformation process. Deposition experiments established the iron oxysulfide phase to be responsible for particle adhesion, and the duration of this melt phase was determined to be a function of pyrite particle size, gas temperature, oxygen concentration, and the extent of fragmentation. Combustion experiments were also conducted with two coals, one with predominantly included pyrite grains and the other with a significant fraction of excluded pyrite, to discern the limiting behavior of pyrite in forming the final ash.     

Mineral behavior during coal combustion 2. Illite transformations

The physical and chemical transformation of excluded crystalline illite particles and of illite grains included within a carbon matrix were examined in a laboratory scale reactor. Scanning electron microscopy was used to determine the particle morphology, and energy dispersive X-ray analysis, Mössbauer spectroscopy, and XAFS expectroscopy were used to monitor the chemical changes. At temperatures above 1400 K, illite lost its crystalline structure and was transformed to a glass. Melting, pore generation, and cenosphere formation were observed. For both included and excluded illite particles, neither segregation of volatile components at the particle surface, nor vaporization of potassium species, was observed during combustion. Combustion of synthetic chars containing illite inclusions demonstrated coalescence of these inclusions to form larger ash agglomerates. Comparison of these results with ash particle compositional data obtained from the combustion of a bituminous coal containing illite showed intermediate compositions indicating interaction between the molten illite and quartz, kaolinite, and pyrite. Deposition experiments revealed a distinct temperature range above which the transformed illite particles had sufficiently low viscosity to deform and stick upon impaction.     

Investigation on ash deposition characteristics during Zhundong coal combustion

The reserves of Zhundong (ZD) coal in China are huge. However, the high content of Na and Ca induces serious slagging and fouling problems. In this study, the ZD coal was burned in a DTF (drop tube furnace), and the ashes collected at different gas temperature with non-cooling probe were analyzed to obtain the ash particle properties and their combination mode. The results showed that Na, Ca and Fe are the main elements leading to slagging when the gas temperature is about 1000 °C during ZD coal combustion, but their mechanisms are quite different. Some sodium silicates and aluminosilicates and calcium sulfate keep molten state in the ashes collected at 1000 °C. These molten ash particles may impact and adhere on the bare tube surface, and then solidified quickly. With the growth of slag thickness, the depositing surface temperature is increased. The molten ash particles might form a layer of molten film, which could capture the other high fusion temperature particles. The Fe₂O₃ sphere were captured by the formed molten slag and then they blended together to form a new molten slag with lower melting temperature.     

Fundamentals of coal combustion during injection into a blast furnace

The efficiency of coal combustion is important for the blast furnace process. Incomplete combustion of coal does not reduce coke consumption as can be expected and decreases burden permeability which results in improper gas flow and temperature distribution. Consequently, this reduces the throughput of the blast furnace.

This paper describes combustion conditions and mechanisms of coal combustion in the blast furnace, and discusses factors affecting coal combustion such as injector location, coal type, injection rate, maceral composition, and air blast parameters. Also, mathematical models of coal and coal/coke combustion in the blast furnace are considered.     

Factors influencing the transformation of minerals during pulverized coal combustion

The transformation of minerals and dispersed inorganic constitutents during pulverized coal combustion has been examined by burning utility sized coals (70% less than 200 mesh) in a laboratory-scale combustor. Experiments were conducted with four U.S. coals possessing different mineralogies. Size and composition of the initial minerals and the resulting ash were measured by a variety of techniques, including computer controlled SEM, low temperature ashing, deposition on a cascade impactor, and optical (Malvern) particle size analysis. Results for a Kentucky # 11 coal with a large amount of fine, included silicate minerals suggest that coalescence amongst illite, kaolinite, and quartz minerals was the dominant process, with occasional iron incorporation into the resulting glass. Pyrite was found to fragment to a limited extent. Illinois # 6 bituminous coal, possessing a similar mineralogy, yielded similar results. For a Beulah lignite coal containing large pyrite grains, mineral fragmentation was inferred from the data, increasing with increasing oxygen level. A high ash content San Miguel Texas lignite containing zeolite minerals demonstrated little mineral interaction during combustion. Differences in results obtained for the different coals highlighted the importance of understanding individual mineral transformations in predicting the formation and behavior of ash.     

煤粉燃烧过程中富钙生物油脱硫脱硝特性

富钙生物油充分结合了生物质的可再生性及有机酸钙盐的高脱硫脱硝性,是一种优质的环保型脱硫脱硝剂,对煤燃烧过程中污染物的脱除有重要的意义。文章利用连续燃烧实验装置,研究了温度(T)、过量空气系数(a)、钙硫比(Ca/S)等因素对富钙生物油在煤燃烧过程中脱硫、脱硝特性以及联合脱除特性的影响,并与醋酸钙、石灰石的脱硫脱硝性能进行了比较。研究表明:富钙生物油脱硫效率在900℃左右达到最大值,高达90%左右;脱硝效率在1 200℃左右达到最大值,为55%左右;富钙生物油同时脱硫脱硝较适宜的Ca/S为2~3;为保证较高的脱硫脱硝效率,过量空气系数不宜太高;3种钙基中,富钙生物油脱硫脱硝性能最好,具有很好的应用前景。     

Mercury emission control from coal combustion systems: A modified air preheater solution

Results are presented of pilot-plant testing of a new approach to resolve the mercury control problem primarily from coal combustors. Performed at the Western Research Institute’s facility in Laramie, Wyoming, the recorded data reflect the previous laboratory results that proved that untreated steel or platinum surfaces in a specific downstream temperature range can significantly convert gaseous atomic mercury to its water-soluble gaseous dichloride without a need for catalysis or amalgamation. This was confirmed by utilizing various steel inserts in a pilot-plant scale combustor fueled by a low-chlorine Powder River Basin coal. The results and mechanism help to explain why it is in full-scale combustors that large excesses of chlorine generally are necessary to convert their very low concentrations of mercury. These efforts have further validated the underlying predominance of mercury’s heterogeneous chemistry. Most importantly it has illustrated that the efficient laboratory proven mechanism extrapolates to the low mercury parts per billion by volume (ppbv) concentrations relevant to full-scale coal combustion. Depending on conditions, chlorine can become less controlling, removing the need for halogen addition or coal blending approaches as possible methods for mercury control. Additional testing now is establishing the exact surface area of the inserted thin metallic surfaces and the residence time required to attain specific conversion efficiencies. Optimal surface temperatures are about 230 ± 30 °C (450 ± 50 °F) for the conversion mechanism. As a result, this temperature region generally will occur in the location of air preheaters in many coal combustions. This also explains previous reports of mercury oxidation across these and other lower temperature devices. Suggestions now are made for modifying or retrofitting air preheater designs. This enhancement of what is mercury’s natural chemical mechanism now raises the possibility for a one-time modification of the combustion train to resolve this otherwise rather expensive problem.     

Numerical simulation of particle size distribution evolution during pulverized coal combustion

Two numerical simulations of particle size distribution (PSD) evolution during ash-free char combustion are presented to help determine the sensitivity of measured coal PSD evolution to fragmentation. The first simulation is based on percolation theory, and it builds the PSD evolution from an ensemble of individual particle size time histories. The second simulation is population balance that operates on the entire distribution as a unit. Inputs to the simulations come from experimental data available in the literature, and results of the simulations are discussed in conjunction with these data.     

关于影响煤燃烧固硫反应的主要因素及其机理的研究进展

燃烧过程中的脱硫是锅炉脱硫工艺的重要组成部分之一 ,已被广泛应用于各种流化床锅炉和煤粉炉中。为开发低成本、高效率的燃煤固硫技术 ,世界各国学者进行了大量的实验和机理性的研究。本文对这方面的研究进展做了总体回顾 ,并在前人研究的基础上提出了关于燃烧过程中固硫化学反应机理研究的发展趋势。     

Preliminary study of trace element emissions and control during coal combustion

Hazardous trace element emissions have caused serious harm to human health in China. Several typical high-toxic trace element coals were collected from different districts and were used to investigate the emission characteristics of toxic trace elements (As, Se, Cr, Hg) and to explore preliminary control methods. Coal combustion tests were conducted in several bench-scale furnaces including drop tube furnace (DTF), circulating fluidized bed (CFB) combustion furnace, and fixed-bed combustion furnace. Calcium oxide was used to control the emission of arsenic and selenium. The granular activated carbons (AC) and activated-carbon fibers (ACF) were used to remove mercury in the flue gas from coal combustion. The chemical composition and trace element contents of ash and particulate matter (PM) were determined by X-ray fluorescence (XRF) spectrometry and inductively coupled plasma-atomic emission spectrometry (ICP-AES), respectively. The speciation and concentration of mercury were investigated using the Ontario-Hydro method. X-ray diffraction spectrometry (XRD) was used to determine the mineral composition of production during combustion experiments. With the addition of a calcium-based sorbent, arsenic concentration in PM₁ sharply decreased from 0.25–0.11 mg/m³. In fixed-bed combustion of coal, the retention rates of selenium volatiles were between 11.6% and 50.7% using lime. In the circulating fluidized-bed combustion of coal, the content of selenium in ash from the chimney was reduced to one-fourth of its original value and that in leaching water from the chimney decreased by two orders of magnitude using lime. Calcium-based sorbent is an effective additive to control the emission of As and Se during coal combustion. The emission of chromium is influenced by the occurrence mode of Cr in coal. Chromium emission in PM_2.5 during coal combustion is 55.5 and 34.7 μg/m³ for Shenbei coal and mixed Pingdingshan coal, respectively. The adsorptive capacity of granular activated carbon for Hg⁰ is significantly enhanced through ZnCl₂-impregnation. The activated carbon fibers showed decent efficiency in mercury adsorption, on which surface oxygen complex showed positive effects on mercury adsorption.     

Emissions of nitrous oxide from combustion sources

Nitrous oxide (N₂0) has recently become the subject of intense research and debate, because of its increasing concentrations in the atmosphere and its known ability to deplete the ozone layer and also to contribute to the greenhouse effect. There are both natural and anthropogenic sources for N₂O; however, the man-made sources are increasing at a much higher rate than natural ones. Until very recently it was believed that the combustion of fossil fuels, especially coal, was the major contributing factor to these anthropogenic sources. For example, 30% of all N₂0 released into the atmosphere was once attributed to combustion sources, with 83% of the combustion sources coming from coal combustion. Correction of a recently discovered sampling artifact, whereby SO₂, H₂O and NO in combustion gases react in a sampling vessel to produce N₂O, has revealed that, in fact, less than 5 ppm of N₂0 are found in most product gases from combustion systems. Fluidized bed coal combustors are the exception, though, yielding N₂O levels of ca. 50ppm in their off-gases.

The gas-phase reactions of N₂0 in flames are reviewed first. It is clear that in most cases N₂0 is a very reactive intermediate, which is quickly destroyed before being emitted from a flame. The important homogeneous reactions removing N₂0 are thermal decomposition to N₂ and O₂ and also radical attack in e.g. N₂O + H → N₂ + OH. Nitrous oxide is formed from nitrogen-containing species by NO reacting with a radical derived from either HCN or NH₃; the reactions are NCO + NO → N₂0 + CO and NH + NO → N₂0 + H. The levels of N₂O observed are a balance betwen its rates of formation and destruction. It turns out that HCN is a more efficient precursor than NH₃ at producing N₂0. The removal of N₂O is fastest at high temperatures and in fuel-rich systems, where free hydrogen atoms are present in relatively large amounts.

When coal burns in a fluidized bed, most of the N₂O detected is produced during devolatilization, rather than in the subsequent stage of char combustion. It is clear that HCN and NH₃ are produced from nitrogenous material released during devolatilization; these two compounds give N₂0 when the volatiles burn. The burning of char, on the other hand, involves the chemi-sorption of O₂ on to sites containing carbon or nitrogen atoms, followed by surface reaction, with one of the products being N₂0, in addition to CO, CO₂ and NO. Fluidized coal combustors have temperatures around 900°C, which is low enough for the thermal decomposition of N₂O to be relatively slow. In addition, the presence of the solid phase provides a large area for radical recombination, which in turn reduces the rate of removal of N₂O by free radicals. Parametric studies of fluidized bed combustors have shown that factors such as: temperature, amount of excess air, carbon content and O/N ratio of the coal, all have a significant effect on N₂O emissions. It is important to note that heterogeneous reactions with solids, such as CaO and char, can cause large decreases in the amount of N₂O produced during the combustion of coal in a fluidized bed. In fact, there are several methods available for lowering the yields of N₂O from fluidized bed combustors generally. Areas of uncertainty in the factors affecting N₂O emissions from fluidized bed combustors are identified.