Packed-bed Combustion Research Articles

Packed bed combustion of char particles: experiments and an ash model

This paper presents the results of a series of experiments on the packed bed combustion of coke particles, including temperature and gas concentration profiles in the bed and measurements of particle size, void fraction and sphericity on both fuel and ash. An earlier numerical model is extended to include the effects of ash on the bed void fraction and other properties as well as on heat conduction within the bed and heat and mass transfer between particulate and gas. The model is shown to give good agreement with experimental data. It is demonstrated that the physical properties of ash—particle size, void, and sphericity—can have a considerable influence on other bed processes.

Measurements of mass flux and stoichiometry of conversion gas from three different wood fuels as function of volume flux of primary air in packed-bed combustion

This paper presents the first experimental series applying a new measurement method to determine mass flux and stoichiometry of the conversion gas as well as the air factor from packed-bed combustion of biofuels. The conversion system employed in this experimental study is characterized by the following concepts: overfired, updraft, fixed horizontal grate, and batch reactor. Three types of wood fuels are studied, namely wood pellets ( 6 mm ), wood chips (4– 50 mm ), and fuel wood ( 300 mm ). The above-mentioned quantities are measured as function of six levels of standard volume flux of primary air in a range of 0.06– 0.46 m 3 n/ m 2 s for all the wood fuels. Altogether, 18 (3×6) single tests are carried out. In spite of the fact that single tests are carried out and that the course of a batch run is highly stochastic, it is possible to draw some general conclusions with respect to the particular conversion concept studied. Firstly, the course of a batch conversion of wood fuels is proven to be very dynamic. For example, the dynamic ranges for the air factor of the conversion system is 10:1 and for the stoichiometric coefficients is CH 3.1 O: CH 0 O 0 during a batch for a constant volume flux of primary air. Secondly, despite the fact that the fuel wood was exceptionally dry, only 8 wt% water content on wet basis, the fuel wood studied displayed significantly lower time-integrated mean of mass flux of conversion gas (12– 31 g/ m 2 s ) relative to the wood pellets (37– 62 g/ m 2 s ) and the wood chips (50– 90 g/ m 2 s ). Thirdly, based on the fact that the conversion gas stoichiometry is unsteady it is confirmed that the molecular composition of the conversion gas varies during batch conversion of wood fuels.

A numerical model for packed-bed combustion of char particles

This paper presents a numerical model for the combustion of char particles in an overfed packed bed. The processes included in the model comprise: heterogeneous reactions for the production of CO, the oxidation of CO in the gas phase, heat transfer in the gas and solid phases as well as between phases, particle motion and shrinkage within the bed, heat transfer in the grate, and the behaviour of ash. The bed is assumed to be one-dimensional — radial variations of temperature and composition are neglected. The model is shown to agree well with the published data. Typical results are presented for steady-state operation, including the conditions for extinction. Back-diffusion, grate design, and ash layer buildup, hitherto neglected, are shown to have significant effects.

A mathematical model of simultaneous formation of volatile-NO and char-NO during the packed-bed combustion of coal-char particles under an NH 3O 2Ar gas stream

A mathematical model for the simultaneous formation of volatile-NO and char-NO in coal combustion has been developed from the proposed kinetic mechanism involving 36 heterogeneous and homogeneous reactions, taking account of the effect of heat and mass transfer. The model can describe not only the rate process of NO formation but also those of the main reactions of combustion: C + 1 2 O 2 → CO , CO + OH → CO 2 + H, etc. To check the applicability of the model, the packed-bed combustion of coal-char particles was carried out in an NH 3O 2Ar gas stream. Sample coal-char particles were from Taisei coal (a Chinese anthracite) and an NH 3 gas simulated the volatile nitrogen compounds (volatile-N) evolved during coal combustion. The predictions of the present model compared favourably with the experimental data for the transient formation and reduction of CO 2, CO, N 2 and NH 3, as well as of NO. The relative contributions of volatile-NO and char-NO to the total fuel-NO emissions can be separated theoretically by the present model. In addition, char-N 2 from char-N and volatile-N 2 from volatile-N (NH 3) are distinguished both theoretically and experimentally.

Heat Transfer to Oil Shale in a Closely Spaced Bundle as an Approximation to a Packed Bed

The heat transfer to a single cylindrical sample of oil shale in a staggered tube bundle was studied both numerically and experimentally in order to evaluate the thermal and chemical processes associated with the retorting of oil shale in packed beds particular to in-situ processing. The cylinders were subjected to constant gas temperatures and to gas temperature histories experienced in an actual combustion retort. The results of the numerical modeling were compared with the experimental data in order to evaluate the model’s performance. It was found that the model satisfactorily described the thermal processes experienced during the combustion retorting of oil shale within the limits of the accuracy of published data on oil shale thermal properties and chemical kinetics. Net heat transfer to cylindrical oil shale samples in a staggered bundle configuration was also calculated and was shown to nearly duplicate published data related to gas-solid heat transfer in a packed bed combustion retort.

In-Situ Coal Gasification: Model Calculations and Laboratory Experiments

Abstract One concept for in-situ coal gasification involves fracturing thick, deep, coal seams using chemical explosives. The resultant high-permeability zone then would be ignited and reacted with a steam/ oxygen mixture to produce medium-Btu gas suitable for upgrading to pipeline quality in a surface plant. This paper discusses the calculational modeling and supporting laboratory experiments relating to the gasification process. The primary aim of this preliminary work is to predict and correlate reaction preliminary work is to predict and correlate reaction and thermal-front propagation rates and product gas composition as a function of bed properties and process operating conditions. process operating conditions. Our initial efforts are restricted to onedimensional, transient Darcy flow in a permeable packed bed. The numerical calculations include a packed bed. The numerical calculations include a detailed description of the reacting system chemistry (13 species) with appropriate reaction rates and over-all heat and mass transport in the system. Comparison of calculated results with experimental data from a packed-bed combustion tube shows good agreement for reaction-zone propagation rates and produced-gas compositions. propagation rates and produced-gas compositions. However, the sensitivity of the calculations to other reaction-rate and transport-coefficient models should be investigated. Introduction In-situ coal gasification has received renewed interest recently. It offers four potential advantages over conventional mining and subsequent surface processing of coal: (1) the product gas may be processing of coal:the product gas may be cheaper because of lower capital investment requirements;environmental damage is likely to be lower;hazards to miners are avoided; andit may make possible the exploitation of coal resources too deeply buried for economical recovery by conventional strip or deep mining operations. The Lawrence Livermore Laboratory (LLL) packed-bed concept for coal gasification was packed-bed concept for coal gasification was originated in 1972. Major program funding by the U.S. ERDA began in 1974. The LLL concept is designed to recover medium-Btu gas from the thick, deeply buried, subbituminous coal deposits prevalent in the western U.S. After upgrading in a prevalent in the western U.S. After upgrading in a surface facility the product gas would have sufficiently high energy density to make pipeline distribution attractive economically. The packed-bed concept calls for creating a permeable zone of coal by detonating chemical permeable zone of coal by detonating chemical explosives in an array of drilled boreholes. The top of the resulting permeable zone is supplied and a steam/oxygen reactant mixture is supplied. The oxidation reactions produce a high-temperature zone that propagates through the bed as a slowmoving thermal wave. The thermal wave first dries the coal downstream from the reaction zone and then pyrolyzes (devolatilizes) it, forming a char. The char undergoes further reactions with the steam present. The major products of the over-all process include H2, CO, CH4, and CO2 as gases, process include H2, CO, CH4, and CO2 as gases, and water and tar as liquids. Mathematical modeling and laboratory experimentation have been carried out to increase understanding of the important parameters of the in-situ gasification process. The purpose of this paper is to present a mathematical description of paper is to present a mathematical description of the gasification process, together with results obtained from calculations and laboratory-scale gasification reactor experiments. The long-range goal of our modeling effort is to acquire the ability to predict resource recovery for a variety of different field geometries and operating conditions. This is a multidimensional, multiphase flow problem. The preliminary model described here is a transient, one-dimensional model of the gasification process in a packed bed. The primary reason for its development is to provide a framework in which to test the importance of accurate specification of the large number of physical and chemical processes involved in gasification. This will be accomplished primarily through comparisons with carefully controlled experiments performed in the 1.6-m reactor. SPEJ P. 105