03313 Mathematical modeling of fluidized bed combustion 3. Simultaneous combustion of char and combustible gases

03313 Mathematical modeling of fluidized bed combustion 3. Simultaneous combustion of char and combustible gases

09 was burnt in the fluidized bed, the concentrations of CO, NO, and NzO were measured continuously in the freeboard. Chlorine was introduced to the f...

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09 was burnt in the fluidized bed, the concentrations of CO, NO, and NzO were measured continuously in the freeboard. Chlorine was introduced to the fluidized bed as HCI.

Fly ash characteristics in co-combustion of wood 99103310 with coal, oil or peat

Steenari, B.-M. and Lindqvist, 0. Fuel, 1999, 78, (4), 479-488. For environmental, technical and economic reasons, the co-firing of biofuels with fossil fuels is often conducted. The properties of the solid residues produced from such fuel combinations cannot be predicted based on the properties of the ash from each fuel. In this work, fly ash materials from combustion of wood/oil, wood/coal and wood/peat combinations were characterized. Elemental compositions, mineralogical speciation of major elements as well as leaching properties are reported. The fly ash materials from co-combustion had in many aspects similar properties to those from combustion of pure wood. The differences noted were lower calcium, potassium and chlorine contents and higher contents of aluminium, iron and sulfur when coal or peat was added. The speciation of alkali metals in the ash materials studied here is somewhat different from that of wood ash, which leads to lower salt levels in leachates. The pH of these leachates was not as high as in leachates from pure wood ash because of a lower content of calcium oxide in the ash. Trace metal levels and leaching rates were in the same range as for wood ash materials studied earlier.

Hexadecane-heptane copyrolysis 99103311 Bartekova, E. and Bajus, M. Pen&urn and Coal, 1999, 41, (I), 48-56. At 700-78o”C, two mixtures of hexadecane with heptane in the weight ratio 1:l (mixture I) and 1:3 (mixture II) were pyrolysed in stainless steel flow reactor in the presence of steam (HsO:HC = 3:l weight). Pyrolysis of pure heptane was studied in identical conditions with the aim to determine component influence on kinetics and product distribution during copyrolysis. lnvesti ations into the control of agglomeration 99103312 and defluidisation 8 uring fluidised-bed combustion of low-rank coals Fuel, 1999, 78, (4), 419-425. Vuthalurua, H. B. et al. The effectiveness of various control methodologies in alleviating ashrelated problems such as particle agglomeration and bed defluidization during fluid bed combustion of low-rank coals was studied in a laboratoryscale spouted bed combustor. The three control techniques investigated are: (i) the use of mineral additives; (ii) alternative bed materials; and (iii) pre-treatment of coal. Mineral additives, including dolomite, two clays and gibbsite, were injected into the spouted bed combustor while burning a South Australian low-rank coal at 800°C. Samples of the same coal treated with aluminium, water washing and acid washing were also tested in the spouted bed combustor. In addition, experiments were conducted with alternative bed materials including bauxite and calcined sillimanite. Experiments showed that the three techniques reported in this paper are effective to different extents in reducing particle agglomeration and defluidization. Among the mineral additives tested, gibbsite and a clay additive rich in kaolinite and sillimanite were found to be most effective. The use of calcined sillimanite and bauxite as alternative bed materials extended the combustion time before defluidization occurred by seven and ten times, respectively, compared to silica sand. While aluminium pretreatment and water-washing were found effective for control of agglomeration and defluidization, acid-washing did not improve the operation of the bed burning this particular coal. Aluminium enrichment in ash coating of bed particles which suppress the formation of sodium and sulfur rich eutectics was identified as the main mechanism for prevention of agglomeration and defluidization by these control techniques.

Mathematical modeling of fluidized bed 99103313 combustion 3. Simultaneous combustion of char and combustible gases

Kulasekaran, S. et al. Fuel, 1999, 78, (4), 403-417. In previous papers, the heterogeneous combustion of char in air-fluidized beds and the homogeneous combustion of combustible gases injected or pre-mixed with the fluidizing air were considered individually as parts of the more complex problem. The simultaneous combustion of char and propane (premixed with air used to fluidize the bed) is considered in this paper. The hydrodynamic description of the bubbling fluidized bed, as in previous work, is based on the three-phase model. The non-isothermality of the bed resulting from the gas-phase combustion reactions is taken into account through inclusion of an energy balance for the bubble phase. The effect of the variation in superficial gas velocity-on bubble properties and crossflow-is included through an overall mass balance. Calculations are compared with experimental data on the combustion of: (a) char; (b) propane; and (c) simultaneous combustion of char and propane. The agreement, in all cases, is very good. Particularly encouraging is the agreement with data on the variation of CO, CO2 and Oz concentrations in the freeboard with time resulting from the batch combustion of char in sand fluidized by air/25 ~01% propane. This agreement suggests that the model provides a good representation of the dynamics of the combustion of char as well as combustible gases. The model is then used to investigate the interaction of heterogeneous and homogeneous combustion reactions within the bed.


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99103314 Mechanism of sulfur capture during coal briquette combustion Lu, X. er al. J. Environ. Sci., 1998, 10, (4), 4.52-457. Various factors affecting sulfur removal efficiency have been studied. XRD, Moessbauer spectroscopy, SEM, energy-dispersion X-ray analysis and electron spectroscopy for chemical analysis have been used to characterize the resultant slag. No other sulfur-containing species besides CaS04 was found. Small amount of CaFes(SiO&-OH, some complexes of CaO-SiOzFeaOs and vitreous iron oxides were identified on the surface of CaSO,, grains. This is offered as an explanation of the mechanism of sulfur fixation during the combustion of coal briquettes. 99to3315

Modeling of coal combustion in a 25-MW FBC

power plant Huilin, L. et al. Energy, 1999, 24, (3), 199-208. The paper introduces a three-phase model which allows for bubble-size variations with height and yields the gas-phase species (Oa, HzO, CO2 and CO) distributions. The empirical value of fraction of the cloud-wake phase was found. Model predictions are compared with plant data and reasonable agreement is obtained.

99103316 Multimode heat transfer in cyclic flow reversal combustion in a porous medium

Jugjai, S. and Somjetlertcraroen, A. hr. 3. Energy Res., 1999, 23, (3), 183206. Studies were performed, both experimental and numerical, of combustion and multi-mode heat transfer in a porous medium, with and without a cyclic flow reversal of a mixture through a porous medium. In order to understand combustion characteristics such as maximum flame temperature and radiative heat flux parametric studies were done using a one-dimensional conduction, convection, radiation and premixed flame model. The porous medium was assumed to emit and absorb radiant energy, while scattering is ignored. Non-local thermodynamic equilibrium between the solid and gas is taken into account by introducing separate energy equations for the gas and the solid phase. As a preliminary study, the combustion regime was described by a one-step global mechanism with an internal heat source uniformly distributed along the reaction zone. The effects of the flame position, cyclic flow reversal, period of the cyclic flow reversal, the optical thickness and the flow velocity on the burner performance were clarified by a rigorous radiation analysis. The model was validated by comparing the theoretical results with the experiments.

Near-real-time measurement of trace volatile organic compounds from combustion processes using an on-line gas chromatograph


Ryan, J. V. Waste Mgr., 1998, 18, (6-S), 403-410. The US EPA’s current regulatory approach for combustion and incineration sources considers the use of real-time continuous emission monitors (CEMs) for particulate, metals and organic compounds to monitor source emissions. Currently, the CEM technologies to support this approach have not been thoroughly developed and/or demonstrated. The EPA’s Air Pollution Prevention and Control Division has developed a near-real-time volatile organic compound (VOC) CEM, using an on-line gas chromatograph (OLGC), capable of measuring over 20 VOCs at concentrations typically present in well-operated combustion systems. The OLCC system consists of a sample delivery system, a sample concentrator and a GC equipped with both flame ionization and electron capture detectors. Application of the OLOC system was initially demonstrated through participation in the 1995 US EPA/DOE CEM demonstration programme. Additional work has improved system performance, including increased automation and improved calibration technique. During pilot-scale incineration testing, measurement performance was examined in detail through comparisons to various CEM performance criteria. Specifically, calibration error, calibration drift error and system bias were examined as a function of full scale (absolute error) and gas concentration (relative error). Although OLOC measurement performance was not able to meet standard EPA CEM measurement performance criteria, measurement performance was encouraging. The system demonstrated the ability to perform hourly trace level VOC measurements (O-100 ppbv) for as many as 23 different VOCs with boiling points ranging from -23.7 to 180.5”C at a known level of measurement performance. This system is a suitable alternative to VOC reference method measurements which may be performed only intermittently. New application of the fixed bed reactor to the 99103316 measurement of coal reactivity under dynamic combustion conductions Ozdemir, M. and Zelkowski, J. Energy Convers. Manage., 1998, 39, (1618), 1891-1898. The paper presents a new laboratory-scale facility (fixed-bed reactor) for the determination of solid fuel reactivity. The reactivity is measured by burning samples of partially pyrolysed coals under dynamic conditions. Mass conversion rate is determined from heat and mass balance equations and the influence of boundary layer diffusion and chemical reaction rate on the measured reaction rate is evaluated. In the scope of a co-operative test, the reactivity of Goettelborn bituminous coal was investigated by different research institutions. In spite of the different conditions in different laboratory scale facilities, a good correspondence of results was derived. Three coals of different rank were investigated.

Fuel and Energy Abstracts

September 1999 347