Subbituminous Coal Combustion Research Articles

Influence of operating conditions and coal properties on NO x and N 2O emissions in pressurized fluidized bed combustion of subbituminous coals

This experimental study is aimed at finding effects of operating conditions in PFBC on nitrogen oxide emissions for subbituminous coals differing in ash content/composition, combustion/gasification reactivities and in particle size distribution. The experiments have been done on a smaller laboratory apparatus with ID=8 cm. The effects of operating pressure (0.1–1 MPa), temperature of the fluidized bed (800–900 °C), freeboard temperature and oxygen concentration (3–10 vol.%) on the nitrogen oxides emissions are relatively complex, coupled with temperature of burning coal particles. The coal ash content/composition (esp. CaO and Fe 2O 3) and fly ash freeboard concentration play an important role in formation/destruction chemistry of both NO and N 2O. The NO x emissions decrease with increasing operating pressure at the same volumetric oxygen concentration and temperature. Temperature, volatile content, reactivities of coals and ash composition are the most important factors for N 2O emissions. The N 2O emissions are either almost constant or can exhibit a maximum at increasing operating pressure. Influence of increasing oxygen concentration on NO x and N 2O emissions is more pronounced at lower operating pressures, esp. for the less reactive, medium ash coal. The particle size distribution of the coal (influence of coal dust) can cause characteristic changes in NO x and N 2O emissions in PFBC, esp. at lower combustion temperatures (800–840 °C). The emission changes are dependent on ash properties/composition.

Emission characteristics of NO x and unburned carbon in fly ash during combustion of blends of bituminous/sub-bituminous coals ☆

Because of large recoverable reserves, sub-bituminous coal is anticipated to become a substitute for the bituminous coal used presently as fuel in Japan. Most sub-bituminous coal contains more than 20% moisture. In the case of the utilization of sub-bituminous coal in Japan, it is considered that, in the near future, sub-bituminous coal will be applied to conventional boilers designed for bituminous coal combustion. We have already clarified that moisture in coal causes low combustion efficiency and a high conversion ratio of fuel nitrogen to NO x compared with a coal having the same fuel ratio with little moisture, in pulverized coal combustion. In particular, oxygen consumption during combustion of sub-bituminous coal with high moisture is more delayed than that during combustion of bituminous coal. Therefore, sub-bituminous coal is utilized in blended combustion with bituminous coal. In this paper, the emission characteristics of NO x and unburned carbon in fly ash in blended combustion of sub-bituminous coal with bituminous coal is investigated using a pulverized coal combustion test furnace with a single burner (coal combustion rate: 0.1 t/h). NO x concentration in blended combustion approaches the NO x concentration in sub-bituminous coal combustion with the increase of the amounts of sub-bituminous coal. As the remaining moisture content in sub-bituminous coal increases, NO x concentration becomes higher in blended combustion. On the other hand, the unburned carbon concentration in fly ash in blended combustion is higher than that in non-blended combustion of each coal. Furthermore, unburned carbon concentration in fly ash becomes much higher as the moisture in sub-bituminous coal increases. This tendency is caused by the influence of the moisture in the sub-bituminous coal on the decrease of the combustion efficiency of bituminous coal, since moisture in sub-bituminous coal delays oxygen consumption at the burner exit and the shape of the combustion flame is diffused. When the swirl vane angle of the burner is controlled in order to suppress diffusion of the combustion flame, oxygen consumption progresses and emissions of both NO x and unburned carbon in fly ash are reduced.

E212 INFLUENCE OF TWO-STAGE AIR INJECTION CONDITIONS ON NOx AND UNBURNED CARBON OF SUB-BITUMINOUS COAL COMBUSTION

Because of large recoverable reserves, sub-bituminous coal is an important energy resources. The disadvantage of sub-bituminous coal is that it contains more than 20% moisture. This makes ignition less efficient. And combustion flame became diffused. Both emissions of NOx and unburned carbon in fly ash become high. We have already shown that these emissions can be reduced by adjusting the air injection conditions from the burner to better suit sub-bituminous coal combustion. However, the reduction is insufficient compared with bituminous coal combustion. In this investigation, influence of two-stage air injection conditions on NOx and unburned carbon concentration in fly ash was studied and the optimum air injection conditions necessary in order to reduce those emissions were clarified.

Emission Characteristics of NOx and Unburned Carbon in Fly Ash of Sub-bituminous Coal Combustion.

Sub-bituminous coal is considered to be one substitute fuel for bituminous coal. Since sub-bituminous coal contains more than 20% moisture, there are some problems with its utilization, such as a decrease in combustibility, high NOx emission and so on. This report describes the emission characteristics of NOx and unburned carbon in fly ash of sub-bituminous coal combustion through the use of a pulverized coal combustion test furnace. On the sub-bituminous coal combustion, ignition at the burner exit worsened and the combustion flame became diffused. Then, both NOx emission and the unburned carbon concentration in fly ash became high. In order to keep stable combustion and to form an effective NOx reduction flame, the swirl vane angle of secondary air was reduced and the Air/Coal ratio was lowered. As a result, the combustion flame became moderate, and both NOx emission and unburned carbon concentration in fly ash could be reduced.

Evaluating mercury transformation mechanisms in a laboratory - scale combustion system

Mercury speciation measurements during injections of 10 μg/m 3 Hg 0(g) into a 42-MJ/h combustion system containing gaseous O 2–Ar- and O 2–N 2-rich mixtures indicate that 43 and 55% of the Hg 0(g) spike was transformed rapidly (<0.1 s) to Hg 2+ X(g) within a refractory-lined heat exchanger where gas temperatures decrease from ≈620 to 200°C. O 2(g) is the probable Hg 0(g) oxidant (i.e. X=O 2−). The apparent formation of HgO(g) involves a heterogeneous reaction with adsorbed Hg 0 or O 2 on refractory surfaces or a Hg 0(g)–O 2(g) reaction catalyzed by corundum (Al 2O 3) and/or rutile (TiO 2) components of the refractory. The potential catalytic effects of Al 2O 3 and TiO 2 on Hg 0(g) oxidation were investigated by injecting Al 2O 3 and TiO 2 powders into ≈650°C subbituminous coal (Powder River Basin, Montana, USA) combustion flue gas. On-line Hg 0(g) and total mercury measurements indicate, however, that Al 2O 3 and TiO 2 injections were ineffective in promoting the formation of additional Hg 2+ X(g). Apparently, either the chemically complex flue gas hindered the catalytic effects of Al 2O 3 and TiO 2, or these compounds are simply not Hg 0(g) oxidation catalysts.

Mercury transformations in coal combustion flue gas

Mercury chlorination [i.e., formation of HgCl 2(g)] is generally assumed to be the dominant mercury-transformation mechanism in coal combustion flue gas. Other potential mechanisms involve mercury interactions with ash particle surfaces where reactive chemical species, oxidation catalysts, and active sorption sites are available to transform Hg 0(g) to Hg 2+X(g) (e.g., where X is Cl 2 or O) as well as Hg 0(g) and HgCl 2(g) to particulate mercury, Hg(p). Results from an investigation of Hg 0(g)–O 2(g)–HCl(g) and Hg 0,2+(g)–HCl(g)–CaO(s)-fly ash interactions in a 42-MJ/h combustion system are consistent with the following mechanisms: mercury chlorination, catalysis of mercury oxidation by Al 2O 3(s) and/or TiO 2(s), and mercury sorption on a calcium-rich (25.0 wt.% CaO) subbituminous coal fly ash. Additions of 50 and 100 ppmv of HCl(g) and ≈12.6 wt.% of CaO(s) to the subbituminous coal combustion environment inhibited Hg(p) formation, primarily via a change in ash surface chemistry and a decrease in particle surface area, respectively.

00/00025 Paritioning behaviour of some toxic volatile elements during stoker and fluidised bed combustion of alkaline sub-bituminous coal

00/00025 Paritioning behaviour of some toxic volatile elements during stoker and fluidised bed combustion of alkaline sub-bituminous coal

Partitioning behaviour of some toxic volatile elements during stoker and fluidised bed combustion of alkaline sub-bituminous coal

During coal combustion, the partitioning behaviour of the volatile toxic trace elements, boron, arsenic and selenium, is influenced by the highly alkaline nature of a typical New Zealand industrial sub-bituminous coal, and the combustion regime (underfeed stoker or fluidised bed) used to burn it. A significant amount of boron is retained in the bottom ash of the underfeed stoker, and some arsenic and selenium retention is also encountered. Mercury partitioning is not influenced by the combustion regime or alkalinity, although the fly ash generated from the stoker combustion appears to possess a significant mercury capture capability. Over 60% sulphur retention may be achieved, without additive, for fluidised bed combustion. Some of the chemical and physical factors responsible for the above partitioning behaviours are identified.

97/02094 Development of a stack plume opacity index for subbituminous coal-fired utility boilers

Powder River Basin subbituminous coals were burned using conventional and low-NO{sub x} combustion conditions in a drop-tube furnace equipped with a multicyclone ash collection device. Fine ash fractions (< 2 {micro}m in diameter) collected during the tests were analyzed using computer-controlled scanning electron microscopy (CCSEM). Advances in particulate sample preparation methods enabled the CCSEM analysis of individual ash particles with submicron diameters as small as 0.1 {micro}m. The fine ash samples produced from the conventional combustion of coal consisted of discrete spherical particles, whereas particle agglomerates were characteristic of the low-NO{sub x} ash samples. Particle-size distributions of the low-NO{sub x} fine ash fractions were coarser because of the agglomeration. Theoretical light-scattering calculations indicate that for a given coal, the ash produced in low-NO{sub x} conditions causes less opacity as compared to conventional combustion conditions. The following phases were abundant in the ashes: Ca aluminosilicate, Ca aluminate, aluminosilicate, silica, (Ca, Mg)O, CaSO{sub 4}, Na{sub 2} SO{sub 4}, and (Na, K)Cl. Primary mechanisms that produced the fine ash include the thermal metamorphism of small (0.1 to 5 {micro}m) mineral grains and the vaporization and subsequent condensation of organically bound Na, Mg, and Ca, Empirical equations for estimating the concentration of fine ashmore » produced from burning subbituminous coals were formulated into an opacity index based on CCSEM coal mineral and fine ash analyses and on drop-tube furnace testing results. The effects of ash electrical resistivity on electrostatic precipitator collection efficiency are also considered in the index.« less

96/00076 Pilot study to determine potential combustion and heat transfer benefit of using finer than standard pulverized subbituminous coal

In response to a potential shortage of oil and gas, a number of studies were completed in the 1980`s to investigate using very finely ground coal in place of these fuels. Consensus of the findings was that finely ground coal looked promising as a replacement for oil and gas due to an increased combustion rate, and decreased slagging and ash accumulation as coal particle size decreases. Detroit Edison recognized these as potential benefits for their coal-fired plants that have switched to subbituminous coal to lower sulfur emissions. Earlier studies used bituminous coal; the authors testing indicated they could expect similar benefits when burning sub-bituminous coal.

The behaviour of mineral matter during combustion of Spanish subbituminous and brown coals

AbstractCombustion experiments up to 1400°C were carried out with subbituminous coals from the Teruel power station, the Teruel Mining District, the Santa Eulalia coal deposit and with lignite from the As Pontes power station in Spain. The characterisations of the occurrence and distribution of inorganic matter and its transformation during combustion of these coals were carried out by means of X-ray diffraction and optical and electron microscopy. The combustion experiments show that the incorgainc transformations during the combustion of all the coals studied vary depending on the sulphur and calcium contents. The sulphur, iron and calcium contents govern the quality of anhydrite crystallisation (which takes place between 600 and 900°C Furthermore, the high calcium oxide content produces the fouling of the combustion wastes at relatively low temperatures (1200°C), prevents the occurrence of mullite and magnetite in the ashes and leads to the crystallistation of anorthite and esseneite during the colling. The comparison of the inorganic phases of fly ashes and slags from the Teruel power station with those of the experimental wastes shows that the inorganic transformations during coal combustion in the power station can be predicted by means of laboratory furnace experiments provided that the residence time in the flame and the effect of the cooling and evacuation controls of gases and particles from the power station are taken in consideration.

Ash formation during pulverised subbituminous coal combustion. 2. Inorganic transformations during middle and late stages of burnout

Two subbituminous coals were burned in a down-fired combustor of 745 MJ/h.m 3 capacity. Transmission electron microscopy, computer-controlled scanning electron microscopy, and X-ray diffraction were used to study changes of inorganic matter in aerodynamically sized char samples collected at different degrees of burnout. The previous paper in this series showed that the coals underwent 33%-50% burnout of combustible matter between introduction to the combustor and the first sampling port. During this stage, inorganic interactions consisted primarily of vaporization and condensation of inorganics that were originally associated with the organic portion of the coals. The present paper deals with subsequent changes that occur in two stages up to 99.9% combustible burnout

Ash formation during pulverized subbituminous coal combustion. 1. Characterization of coals, and inorganic transformations during early stages of burnout

As part of a broad effort to develop the capability to predict ash behavior based on detailled analyses of the inorganic matter in coals, two subbituminous coals were tested in a new down-dired combustor of 745 MJ/(h.m 3 ) capacity. This paper deals with changes in the inorganic matter during the first 30-50% burnout of combustibles, corresponding to 0.07 s residence time. The inorganic components reached temperatures of 1210-1220 o C. Detailed analyses of coals and chars were performed by transmission and computer-controlled scanning electron microscopy and X-ray diffraction. Sodium, calcium, and magnesium decreased in concentration in the larger char particles, but increased in the smaller size particles, indicating volatilization of these elements at a rate more rapid than combustible burnout

Ignition mechanism of subbituminous coal combustion in the stretched-flow field

The relationship between the ignition mechanism and the stretched-flow characteristics of subbituminous coal particles was experimentally studied. The results indicate that two ignition mechanisms exist, namely heterogeneous char ignition and homogeneous volatile ignition in the vicinity of the stagnation point, and that the determining factor is the global flow stretch rate. In general, the global Damkohler number decreases steadily with Reynolds number, in agreement with the scale analysis prediction. Moreover, the ignition delay time increases with the global flow stretch rate at low Damkohler number. However, when the Damkohler number is high, the reduction in the stretch rate increases the ignition delay time.

NO x generation from the combustion of Australian brown and subbituminous coals

The formation of NO x during the combustion of pulverized brown and subbituminous coals from Victoria and Queensland respectively was investigated in an entrainment reactor. As no NO 2 was detected, all the NO x was present in the form of NO. The brown coals exhibited a significantly greater potential for NO emission under fuel-lean conditions than did the subbituminous coal, even though the latter coal had a higher nitrogen content. However, under fuel-rich conditions the conversion of coal nitrogen to NO for the subbituminous coal was higher than for the brown coals. The differences in conversion efficiency may have been related in part to the nature and reactivity of the volatile nitrogen species. Reactivity differences between the chars produced from the brown and subbituminous coals may also have accounted for different extents of removal of NO. There was a significant reduction in the amount of NO emitted when brown coal was added to a combustion gas stream containing an appreciable quantity of NO before coal injection.

Speciation in Size and Density Fractionated Fly Ash

ABSTRACTMorphological, chemical and mineralogical speciation of fly ash from a power plant burning sub-bituminous coal has been investigated by examination of size and density fractions. It was found that whereas, fractionation by size revealed little information as to speciation among particle types, separation of the ash into six density fractions showed major differences in properties associated with true particle density. In particular it was found that at least two types of glass co-exist in the ash: “Glass I” – a predominantly silico-aluminous glass associated with particles of low density (cenospheres); “Glass II” – a calcium alumino-silicate glass associated with high-density particles. These glasses were found to differ greatly in composition and to be characterized by shifts in the position of the 2-theta of the XRD-halo. In addition, it was shown that cryptocrystalline mullite is associated only with the low-density particles. It is proposed that particles comprising low-density fractions can be considered as glassceramics with low degrees of crystallization. Particles of high-density are better described as the products of internal lime-sinter reactions.

Vaporization of refractory oxides during pulverized coal combustion

A laboratory coal combustor was used to study the vaporization behavior of ash duringpulverized coal combustion. The variation in the amount and composition of the ash vaporized with coal type was investigated. The effect of coal size and combustion temperature on the vaporization rate of Si, Mg and Ca was also investigated. At a moderate combustion temperature of 2000° K, the amount of ash vaporized was onthe order of a few percent of the total ash content of the coals tested. The ash vaporized from combustion of subbituminous coals and lignites is composed primarily of MgO or of Na 2 O if the coal contains high concentrations of sodium. The ash vaporized from combustion or bituminous coals is composed primarily of SiO 2 and iron oxide. This effect of rank is a result of the appreciable vaporization of the organically bound magnesium present in the low rank coals. The vaporization rates of Si and Mg are dependent on the coal size and vary by over four orders of magnitude in the combustion temperature range of 1600–3000° K. A model presented to interpret the experimental results considers the degree of dispersion of mineral matter in coal, chemical reduction of refractory oxides (SiO 2 , MgO, CaO) to the corresponding volatile suboxide or metal vapor, and the internal and boundary layer transport processes. The model adequately describes the vaporization rate of both metals associated with mineral inclusions and those organically bound in the parent coal, and the changes in vaporization rates with changes in temperature and in coal particle size.

Trace-Element Content of Northern Great Plains Subbituminous Coal and Gulf Coast Lignite, 1979: ABSTRACT

A total of 453 coal samples from Northern Great Plains subbituminous coal beds, and 92 samples from the Gulf Coast lignite beds were chemically analyzed and studied from 1975 to 1979. These samples were analyzed for major, minor, and trace elements by the U.S. Geological Survey. In addition, ultimate, proximate, and Btu analyses were performed by the Coal Analysis Section of the U.S. Department of Energy. Of the 18 elements reported in the summary, the geometric means for F, Zn, and S are as much as two-fold higher in ligite, compared with subbituminous coal. The elements Mn, Cr, and Se are more than four-fold higher in lignite than in subbituminous coal. All other reported elements are two to four-fold higher in lignite than in subbituminous. About 8 metric tons (8 Mg) of Gulf Coast lignite are required to produce 1 billion Btu; in contrast, approximately 6 Mg of Northern Great Plains subbituminous coal are required to produce the same quantity of Btu. According to these comparative tonnages, 1.7 Mg of ash will be produced by combusting 8 Mg of lignite and 0.5 Mg of ash by 6 Mg of subbituminous coal. During the production of 1 billion Btu from lignite, 27,000 g of sulfur will be mobilized and a similar Btu production from subbituminous coal will mobilize 76,000 g of sulfur. Approximately 2,700 g of the other elements will be mobilized during the combustion of lignite and about 730 g will be mobilized from the combustion of subbituminous coal. End_of_Article - Last_Page 767------------

Composition and variation of pulverized fuel ash obtained from the combustion of sub-bituminous coals, New Zealand

Twenty daily samples of pulverized fuel ash (PFA), obtained from the combustion of sub-bituminous Waikato coals, have been analysed. The contents of MgO, SO 3, B 2O 3 and total and uncombined CaO in the New Zealand material were high when compared with most PFA produced in Europe and the USA. The PFA was separated into three major components, namely a magnetic fraction, and nonmagnetic acid-insoluble and acid-soluble fractions. These three fractions were related to three major mineral suites found in the coal: respectively hydrated iron carbonates and oxides, quartz and aluminosilicates, and calcium minerals. The variable composition of the PFA arises from differences in the proportions of these three mineral suites in the original coal. The composition was also found to vary with particle size. It is suggested that this was due to differences in the particle sizes of minerals in the coal, and to increased thermal decomposition of minerals in the smaller particles of PFA. A low-density fraction was separated and found to be similar in composition to cenospheres found in British and American PFA. The decomposition and fusion of a mineral common to many coals may be the source of these cenospheres.