Ajay Atal

Comparison of the combustion behaviour of pulverized waste tyres and coal

Abstract Comparative combustion studies were performed on particles obtained from pulverized bituminous coal and waste automobile tyres (rubber). Particle size cuts of 75–90 and 180–212 μm were burned in a thermogravimetric analyser, at low heating rates, and in an electrically heated drop-tube furnace, at high heating rates. The combustion of individual particles in the drop-tube furnace was observed with three-colour pyrometry, to obtain time-temperature histories and with high-speed cinematography to record flame particle size histories. Combustion was conducted at a gas temperature of 1450 K, in air. Upon pyrolysis, the phenomena of melting, swelling and formation of large blowholes were observed only in the case of the coal particles. The tyre particles formed chars with rough surfaces and smaller blowholes. Separate volatile and char combustion phases were detected for the coal particles studied. Tyre particles experienced an intense primary volatile combustion phase, followed by a phase of simultaneous secondary volatile combustion, of lesser intensity, and char combustion. During the initial volatile phase combustion, the peak flame temperatures were comparable for both materials, in the range 2200–2400 K. The secondary volatile/char combustion phase, observed for the type particles, was cooler, i.e. 2000–2100 K. The coal chars burned with temperatures of 1850–2000 K. Combustion was diffusionally controlled (regime III) for coal chars of both sizes and for tyre chars of the larger size cut only. Char burnout times were considerably shorter for tyre particles than coal, which can be attributed to the secondary devolatilization and the lower density of the former.

On the survivability and pyrosynthesis of PAH during combustion of pulverized coal and tire crumb

Abstract Results are presented on the emissions of semivolatile polycyclic aromatic hydrocarbons (PAH) from the combustion of a pulverized bituminous coal and ground waste automobile tires. Streams of fuel particles were injected at steady-state steady-flow conditions, and burned inside an isothermal drop-tube furnace, in air, at a gas temperature and gas residence time of 1150°C and 0.75 s, respectively. Combustion occurred under either very fuel-lean conditions (bulk equivalence ratio, φ 1.6 and, especially, under pyrolytic conditions in N 2 . These PAHs were mostly attributed to pyrosynthesis since none of the deuterated PAHs, adsorbed on the fuels, survived the combustion process. Small amounts of the labeled compounds, however, survived under purely pyrolytic conditions. These results were confirmed with separate experiments, where deuterium-labeled PAH standards were adsorbed on highly porous calcium/magnesium oxide or mullite particles. Again, small amounts of some PAHs survived in high-temperature pyrolytic conditions, but none in oxidative environments. These observations suggest that pyrosynthesis is the major contributing mechanism to the PAH emissions from the combustion of these fuels. Survivability of parent PAHs may be a minor mechanism at very high equivalence ratios. Finally, both fuels were mixed with powders of calcium magnesium acetate (CMA), calcium carbonate (CaCO 3 ), and calcium oxide (CaO), all of which are known sulfur reduction agents, at a molar Ca/S ratio of 1. Combustion of the fuels mixed with CMA or CaCO 3 generated enhanced amounts of PAHs, while combustion with CaO had no effect on the PAH emissions.

Combustion and SO2-NOx emissions of bituminous coal particles treated with calcium magnesium acetate

Abstract Experiments were conducted in an externally heated drop-tube furnace to assess the effectiveness of the chemical calcium magnesium acetate [CMA, CaMg2(CH3COO)6] as a combustion catalyst and a coal pretreatment agent for reducing SO2 emissions. Bituminous coal particles of two distinct sizes, pulverized (75–90 μm) or micronized and beneficiated (mean diameter ∼3.5 μm) were burned. To measure the coal particle temperatures and burn times, combustion traces were recorded for single pulverized coal particles and clusters of micronized coal particles using a three-colour near-infrared optical pyrometer. The volatile and the char phase combustion temperatures of untreated pulverized-grind particles, in air at a gas temperature of 1450 K, were determined to be 2200 and 1800 K, respectively. Particles treated with CMA under the same conditions burned hotter, with the temperature of volatile and char phase being 2400 and 2000 K, respectively. SO2 and NOx concentrations were measured at the exit of the furnace for both the pulverized and the micronized coals. For furnace gas temperatures between 1250 and 1450 K, in a background gas containing 10–50 ppm SO2 and equivalence ratios, π, between 0.4 to 0.7, untreated micronized and pulverized bituminous coal particles produced SO2 emissions in the range 100–200 ppm and NOx emissions in the range 200–450 ppm. In contrast, combustion of pulverized particles treated with CMA, burning under the same conditions, not only did not produce any SO2 but also eliminated the background SO2 concentration. The combustion of micronized coal treated with CMA produced less SO2 than untreated micronized coal, but complete reduction of SO2 was not achieved. Experiments with CMA-treated micronized coal in atmospheres containing 40% oxygen suggested that the primary mechanism for sulfur removal in this virtually ash-free coal was the sulfation of resulting CaO MgO fly ash/aerosols. Experiments in which the effluent of the combustion of pulverized coal was quenched immediately after the heated furnace zone suggest that a fraction of the fuel sulfur (up to 50% of the released SO2) may have been encapsulated by ash during the combustion of treated pulverized coals, with the remaining released SO2 being removed by sulfation of Ca Mg -containing ash or submicron CaO MgO aerosols in the cool-down region of the furnace.

Burning Characteristics and Gaseous/Solid Emissions of Blends of Pulverized Coal with WasteTire-Derived Fuel

Abstract This work examined the combustion behavior (flame characteristics and temperatures) and the emissions (SO2, NO, NO2, CO, CO2, polynuclear aromatic hydrocarbons (PAH), soot and ash) from blends of a pulverized bituminous coal and ground waste automobile tires. The following fuel feed compositions were examined: 100% coal, 75–25% and 50–50% coal and tire blends, as well as 100% tire. Coal and tire particles were in the size ranges of 63–75 and 180–212 μm, respectively. Combustion of cylindrical streams of particles look place under steady flow conditions, in an electrically-heated drop-tube furnace in air, at a gas temperature of 1150°C and a particle heating rate of ≈105°C/s, The bulk equivalence ratio, φ, in the furnace was varied in the range of 0.5 to 2, by varying the particle mass loading. Combustion observations on burning clouds of particles were conducted with simultaneous pyrometry and cinematography. Interparticle flame interactions were visually observed mostly in the near-stoichiometri...

Combustion of CWF agglomerates from pulverized or micronized bituminous coal, carbon black, and diesel soot☆

Abstract Fundamental studies were conducted to explore the role of coal grind size on the combustion characteristics of coal-water fuel (CWF) agglomerates. The agglomerates were prepared from bituminous coals of two different grind sizes: pulverized (40 μm mean size), and micronized and beneficiated (4 μm mean size) as well as blends thereof. Experiments involved single predried agglomerates, free-falling in a drop-tube laminar-flow furnace at high heating rates. Combustion took place in oxygen partial pressures of 10% or 21% at a furnace gas temperature of 1450 K. Additional experiments were performed with predried agglomerates from water slurries of carbon black or diesel soot (both of particulate size and aggregate sizes in the order of 0.050 and 0.5 μm, respectively). All agglomerates were in the size range of 150–900 μm and were either plain or impregnated with calcium magnesium acetate (CMA). Upon heating and devolatilization, the bituminous coal agglomerates studied were found to melt, mildly swell, and form cenospheric structures. Individual pulverized coal particles also formed small cenospheres themselves, superimposed on large agglomerate-derived cenospheres. Combustion of CWF agglomerates, in the size range examined, occurred with distinct volatile and char combustion phases with the latter burning in a diffusion-controlled mode. The combustion behavior of pulverized and micronized coal agglomerates of the same size was strikingly similar. The presence of dissolved calcium magnesium acetate (CMA) accelerated setting of the slurries and, upon water evaporation it suppressed melting and cenosphere formation of the coal particles and agglomerates alike. Also, CMA mildly influenced the agglomerate combustion behavior, causing both larger volatile flames and char temperatures that peaked at the first half of the burnout period and decreased thereafter, but did not substantially affect the char burnout time. Carbon black and diesel soot agglomerates did not form cenospheres and the latter burned a little hotter and faster but, basically, their combustion was similar to that of CWF chars. Bulk fragmentation was consistently observed only in thecase of CMA-impregnated carbon black agglomerates.

PAH and Soot Emissions from Combustion of Coal and Waste Tire-derived-fuel in Fixed Beds

This is a laboratory investigation on the emissions from batch combustion of waste tire-derived-fuel (TDF) and coal in fixed beds. The goal was to compare the magnitude of the emissions of this mode of combustion to that of continuous combustion of streams of particles of the same fuels in pulverized form, conducted previously (Levendis et al., 1996; Levendis et al., 1998). In the experiments herein waste tire chunks (in the range of 3-9 mm), tire crumb (180 212 μm) and bituminous coal (63-75 μm) were burned in a horizontal muffle furnace, at a gas temperature of 1000°C. The fuel mass loading in the furnace was varied; the residence time of the post-combustion gases was 1 s. At the exit of the furnace CO, CO 2 , NO,, polynuclear aromatic hydrocarbon (PAH) and particulate emissions were measured. Results showed that the PAH emissions from batch combustion of tire chunks were higher than those from tire crumb, which in turn were an order of magnitude higher than those from coal, under similar combustion conditions. Increasing the mass loading for both tire crumb and pulverized coal, increased the specific emissions of PAHs. Increasing the segmentation of the tire, at a fixed mass loading also increased the specific PAH emissions. Emissions of particulates closely followed the trend of PAH emissions. Batch combustion of tire chunks resulted in higher particulate yields than tire crumb, with pulverized coal a distant third. Specific CO emissions from batch combustion of TDF were an order of magnitude higher than those from coal combustion. For both fuels, CO emissions were detected during the release and combustion of the volatiles only. On the other hand, specific CO 2 , and, especially, NO x emissions from batch combustion oftire crumb were lower than those from coal. The comparison of the emissions of batch combustion of pulverized fuels in fixed beds with those of continuous combustion of streams of particles was conducted under laminar flow conditions, similar gas temperatures and similar post combustion residence times. Results showed that (i) the PAH and CO emissions of tire crumb were much lower in the latter mode of combustion, even at bulk equivalence ratios as high as 1.4; (ii) to the contrary, for coal the mode of combustion had little influence on the PAH and CO emissions.

Observations on the combustion behavior of coal water fuels and coal water fuels impregnated with calcium magnesium acetate

Abstract Combustion studies of single free-falling coal-water fuel (CWF) droplets were conducted in a drop-tube laminar-flow furnace at high heating rates. Most experiments were conducted with predried CWF agglomerates consisting of micronized (3.5 μm mean size) and beneficiated bituminous coal. Agglomerates of known initial size, in the range of 100–600 μm, were burned in air or pure oxygen at furnace temperatures between 1300 and 1500 K. Combustion of CWF agglomerates impregnated with calcium magnesium acetate (CMA), which is being considered as a sulfur capture agent, was also examined under the same conditions. Observations on the devolatilization and char combustion behavior of CWF agglomerates, formed after the evaporation of water, were conducted using pyrometric and cinematographic techniques. Complete time-temperature histories of burning agglomerates were obtained with a three-color near-infrared optical pyrometer, where distinct phases of volatile and char combustion were observed and analyzed. The volatile flame temperatures and the char combustion temperatures in air exceeded the furnace gas temperature by as much as 1000 and 600 K, respectively. The char combustion phase is the most prominent since it accounts for 75%–85% of the burnout time. The volatile combustion phase is also important because of the resulting high-temperature radiant flames. Overall burnout times were between 150 and 600 ms, for the range of agglomerate sizes tested. Char combustion for most agglomerates was controlled by boundary layer diffusion of oxygen. While most experiments involved predried agglomerates, a limited number of experiments were performed with CWF droplets to monitor the time required for water evaporation and subsequent heatup of the resulting agglomerates. Under the oxygen partial pressure and temperature conditions of this study water evaporation took approximately 25% of the total CWF furnace residence time. The swelling behavior of the agglomerates was studied and swelling factors in the neighborhood of 1.12 were determined. Although the bituminous CWF, studied herein, was found to swell and form cenospheric structures during heatup and devolatilization, the addition of CMA catalyst inhibited swelling of the agglomerates. However, the chars containing CMA exhibited occasional splitting or fragmentation during the volatile combustion stage.