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New advanced combined cycle coal-fired power generation systems are dependent on improvements in gas turbine technology and the development of hot gas cleaning techniques. These techniques are not only necessary to meet environmentally accepted emission limits for SOx and NOx but also to prevent downstream equipment from corrosion and erosion. Volatile alkali vapours in the exhaust gases produced by either coal gasification or combustion lead to corrosion of the gas turbine blades resulting in reduced operating life. Consequently, alkali removal systems which can operate upstream of the gas turbine have been incorporated into the development of the clean coal technologies. A number of studies on alkali removal systems have been performed in the temperature range of 800°C - 1000°C. Solid aluminosilicates, such as emathlite, activated bauxite, kaolinite and Fuller's Earth, react with alkali vapours at high temperatures and therefore have been characterised as suitable alkali sorbents. Fuller's Earth was identified as potentially the most suitable sorbent for use in the UK at the specified operating temperatures. This material was studied in detail by McLaughlin (1990) for use in a fixed bed configuration within the British Coal Air-Blown Gasification Cycle. Recently, it has been recognised that if ceramic filters are used for the removal of fine particulates, operating temperatures for alkali sorption will have to drop to 400°C-600°C, since these filters fail mechanically at higher temperatures. Much of the alkali will condense under these conditions and be removed by the filtration stages. However, the residual alkali levels may still exceed the revised turbine inlet specification of 24 ppb wt. Hence further studies of alkali sorption are required in this lower temperature region. During this work, it proved difficult to obtain accurate results at temperatures as low as 600°C, because of the low level of vapour phase alkali. However, experiments were performed successfully at 650°C and atmospheric pressure, on the fixed bed sorption rig used previously for tests at 827°C and 927°C. Tests comparing Fuller's Earth and kaolin, showed kaolin to have a higher sorption capacity at this temperature. Fixed bed tests with sodium and potassium were performed with Fuller's Earth pellets. The runs were of 200-600 hrs duration, with 4.58 ppm wt NaCl (1.8 ppm wt Na), 5 %vol H2O and up to 160 ppmv HCl in the inlet gas stream. Alkali uptake profiles were generated from chemical analysis of precise layers of pellets removed from the bed. Extensive modifications and improvements in analytical procedures enabled a closure of the mass balance of >99% to be achieved for a 600 hr run. Alkali exit levels measured using alumina wool filter pads in the exit gas were of the order of 5-6 ppb wt. Fuller's Earth pellets which had been pre-treated in gasifier gas and which were therefore contaminated with carbon, were tested and no difference was observed in their Na characteristics. Element mapping techniques based on Scanning Electron Microscopy, confirmed that a shrinking core model for Fuller's Earth grains and kaolin pellets was appropriate. The 'two-reaction' mechanism proposed by McLaughlin (1990), was used to fit the experimental results at 650°C. Albite was identified by X-ray diffraction studies as the reaction product under high-acid conditions and nepheline under non-acid conditions. Exit gas analysis studies with an on-line monitor for HCl, showed the production of HCl to be directly connected with the presence of NaCl vapour and to increase significantly with the presence of water vapour in the system. However, the detailed reaction mechanism has not been identified yet. The theoretical model developed for the high temperature studies (McLaughlin, 1990), using the pellet-grain model and the 'tank-in-series' method of solution has been applied successfully at 650°C. Parameters were extracted by curve fitting theoretical to experimental Na uptake concentration bed profiles. To test the numerical methods and the Szekely assumptions used in the McLaughlin program, two new computer programs were developed. The first, tested the pellet-grain model for a single pellet and the second was developed to solve the model more rigorously with a variable-order, variable-time-step numerical method. The new fixed bed model also incorporates the effects of temperature and pressure on selected parameters. It was used to predict the performance of a full-scale unit operating at 650°C and 24 bara. The results indicate that a bed of Fuller's Earth pellets, 3-10 mm in diameter, 4 m long and 4 m wide can achieve exit alkali levels below 20 ppb wt in continuous operation for up to 24,000 hrs.

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Thesis (M.S.)--Ohio University, August, 2009. / Title from PDF t.p. Includes bibliographical references.

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The core of this project was the development of a column flotation dynamic model that can reasonably predict the changes in the concentrations of all solid and bubble species, along the full column height. A dynamic model of a process is normally composed of a set of partial or ordinary differential equations that describe the state of the process at any given time or position inside the system volume. Such equations can be obtained from fundamental material and/or energy balances, or from phenomenological derivations based on knowledge about the behavior of the system. A phenomenological approach referred to as population balance modeling was employed here. Initially, a two-phase model was formulated, which represents the behavior of the gas phase in a frother solution. The column was viewed as consisting of three main regions: a collection region, a stabilized froth and a draining froth. Experiments were carried out, based on conductivity techniques, for obtaining empirical data for model validation and parameter estimation. After testing the two-phase model, the equations for the solid species were derived. Consideration of the effects of bubble loading, slurry density and slurry viscosity on bubble rise velocity and, therefore, on air fraction is included in the model. Bubble coalescence in the froth is represented as a rate phenomenon characterized by a series of coalescence efficiency rate parameters. Auxiliary equations that help describe the settling of free particles, the buoyancy of air bubbles, and the processes of attachment and detachment, were also developed and incorporated into the model. The detachment of solids from the bubbles in the froth zones was attributed to coalescence, and it was assumed to be proportional to the net loss of bubble surface area. Almost all parameters needed to solve the model equations are readily available. The set of differential equations that comprise the model can be solved numerically by applying finite difference approximation techniques. An iteration has to be performed, which involves calculating the product flowrate at steady state, modifying the tailings rate and solving the model again until a mass balance is satisfied. / Ph. D.

Numerical simulations of coal gasification process inside a generic 2-stage entrainedflow gasifier are carried out using the commercial CFD solver ANSYS/FLUENT. The 3-D Navier-Stokes equations and eight species transport equations are solved with three heterogeneous global reactions, three homogeneous reactions, and one thermal cracking equation of volatiles. Finite rates are used for the heterogeneous solid-gas reactions. Both finite rate and eddy-breakup combustion models are calculated for each homogeneous gas-gas reaction, and the smaller of the two rates is used. Lagrangian-Eulerian method is employed. The Eulerian method calculates the continuous phase while the Lagrangian method tracks each coal particle. Fundamental study is carried out to investigate effects of five turbulence models (standard k-ε, k-ω, RSM, k-ω SST, and k-ε RNG) and four devolatilization models (Kobayashi, single rate, constant rate, and CPD) on gasification simulation. A study is also conducted to investigate the effects of different operation parameters on gasification process including coal mixture (dry vs. slurry), oxidant (oxygen-blown vs. air-blown), and different coal distributions between two stages. Finite-rate model and instantaneous gasification model are compared. It is revealed that the instantaneous gasification approach can provide an overall evaluation of relative changes of gasifier performance in terms of temperature, heating value, and gasification efficiency corresponding to parametric variations, but not adequately capture the local gasification process predicted by the finite rate model in most part of the gasifier. Simulations are performed to help with design modifications of a small industrial demonstration entrained-flow gasifier. It is discovered that the benefit of opening the slag tap on the quench-type gasifier wider by allowing slag to move successfully without clogging is compromised by increased heat losses, reduced gasification performance, downgraded syngas heating value, and increased unburned volatiles. The investigation of heat transfer on fuel injectors shows that blunt tip fuel injector is less likely to fail compared to conical tip fuel injector because the maximum high temperature on the injector is scattered. Two concentric fuel/oxidant injections provide better fuel-oxidant mixing and higher syngas heating value than four separate fuel and oxidant injections.

A mild gasification method has been developed to provide an innovative clean coal technology. The objectives of this study are to (a) incorporate a fixed rate devolatilization model into the existing 2D multiphase reaction model, (b) expand the 2D model to 3D and (c) utilize the improved model to investigate the mild-gasification process and guide modification of the mild-gasifier design. The Eulerain-Eulerian method is employed to calculate both the primary phase (air) and secondary phase (coal particles). The improved 3D simulation model, incorporated with a devolatilization model, has been successfully developed and employed to determine the appropriate draft tube dimensions, entrained flow residence time, The simulations also help determine the appropriate operating fluidization velocity range to sustain the fluidized bed depth without depleting the chars or blowing the char away. The results are informative, but require future experimental data for verification.

A mild gasification method has been developed to provide an innovative clean coal technology. The objective of this study is to developed a numerical model to investigate the thermal-flow and gasification process inside a specially designed fluidized-bed mild gasifier using the commercial CFD solver ANSYS/FLUENT. Eulerain-Eulerian method is employed to calculate both the primary phase (air) and secondary phase (coal particles). The Navier-Stokes equations and seven species transport equations are solved with three heterogeneous (gas-solid), two homogeneous (gas-gas) global gasification reactions. Development of the model starts from simulating single-phase turbulent flow and heat transfer to understand the thermal-flow behavior followed by five global gasification reactions, progressively with adding one equation at a time. Finally, the particles are introduced with heterogeneous reactions. The simulation model has been successfully developed. The results are reasonable but require future experimental data for verification.