Change of existing circulating fluidized bed boilers to oxy-firing conditions for CO2 capture

This work investigates a circulating fluidized bed boiler, originally designed for air-firing, retrofitted to oxy-firing with the purpose of removing the CO2 emission from coal combustion. Previous studies have shown that the heat balance on the gas-particle side can be satisfied without changes in the boiler, but then the volume flow of gas is reduced. To retain the operation like that during air-firing, the volume flow, that is the fluidization velocity, in oxy-firing should be equal to that in air-firing. It is the main purpose of this work to determine the conditions for the transition from air to oxy-firing, while the heat transfer conditions are maintained at a constant fluidization velocity. Measures to achieve this, such as adjusting the supply of additional gas and the heat transfer surface, are analysed. The fulfilment of the furnace\u27s heat balance requires extra fuel or reduction of the heat-transfer surface in the furnace. These changes affect the performance of the back pass, which must be modified to accommodate the change in gas composition and the higher sensible heat content of the flue gas. Strategies to deal with these circumstances in CFB boilers are discussed

Fluid dynamic analysis of dual fluidized bed gasifier for solar applications

A hydrodynamic model of a dual fluidized bed gasifier (DFBG) is developed and its predictions are compared with measurements of solids flux and pressure profiles from a cold flow model (CFM). Then, the performance of a DFBG gasifier is theoretically analyzed in terms of solids circulation and solids distribution under changes in riser and loop seal aeration, solids inventory and particle size, and a sensitivity analysis is made to delimit the model prediction capability. Furthermore, the model is applied to analyze the effects of key design aspects of DFBG, such as the relative size of riser and gasifier, the connection between both units, the circulation rate of solids and their distribution around the system. The model is further used to extend the DFBG operation with external solar energy carried by heated solid particles, i.e. to design solar DFBG (SDFBG). The analysis is focused on the performance with high solids inventory in the gasifier to increase the char conversion (operation with a large solar share) and the control of solids circulation to meet the heat demand of the gasifier with the availability of solar energy. The operation with large solids inventory in the gasifier requires the size of the gasifier to increase considerably compared to that of the conventional DFBG. The substitution of the connection pipe between the riser and the bubbling bed (current design in commercial DFBG) by a lower loop seal enables better control of the solids circulation, thus, benefiting the solar design

Evaluation of char reaction rate in a fluidised bed gasifier: from reactivity determination to reactor simulation

This work analyses how to use char reactivities determined in the laboratory to predict carbon conversion in large-scale fluidised-bed gasifiers (FBG). It is shown how the char reactivity is determined in the laboratory to obtain kinetic expressions for FBG simulation, i.e. at high temperature and heating rates. A simple model is used to predict the char gasification rate in an FBG. It is shown that in most practical situations the average reactivity of char in the bed differs substantially from the reactivity calculated with the average conversion, leading to inaccuracies in predicting the carbon conversion in an FBG. Therefore, the distribution of conversion has to be addressed for proper prediction. The importance of accounting for the distribution of conversion is demonstrated in an example, where the char conversion in an industrial FBG is predicted

Char gasification model for circulating fluidized beds

A model for calculating the char conversion in biomass CFB gasifiers is presented. The approach issimplified and accounts for comminution phenomena, chemical reactions and fluid-dynamics. Theoretical submodelsare developed, supported by semi-empirical relations to describe fluid-dynamics and reaction within fuelparticles. There is also a discussion on the type of experiments needed to estimate the main input to the model byexperiments conducted in lab-scale batch-operated FB. The model is useful for implementation as a submodel inexisting CFB gasification models and also as a char conversion estimator in a pseudo-equilibrium model.Keywords: gasification; circulating fluidized bed; biomass conversio

Gasification of biomass in fluidized bed: review of modelling

Modelling of biomass gasification in bubbling and circulating fluidised bed (FB) is reviewed. The focus is on comprehensive fluidisation models, where semi-empirical correlations are employed to simplify the fluid-dynamics of the FB. The conversion of single fuel particles, char and gas reaction kinetics are dealt with, outlining the key phenomena that should be included in gasification models. An assessment of published models is presented and the need of further investigation is identified

Oxy-fuel combustion in circulating fluidized bed boilers

The conditions for CO2 reduction in a circulating fluidized bed (CFB) oxy-boiler are studied, that is, operationwith pure oxygen, diluted by recirculated flue gases to moderate the combustion process. Two casesare analyzed: the ready-to-convert case, a normal air-fired CFB boiler, only slightly modified to be operatedwith oxygen instead of air for CO2 capture, and a more general option, an entirely new design,employing high oxygen concentration in the input to the oxy-fuel CFB boiler. It is found that at a givenfuel load, the relevant parameters for maintaining the CFB performance (bed temperature and fluidizationvelocity) in the ready-to-convert case cannot be kept entirely equal to those in the air-fired case,and some compromise has to be found. The new-design case results in a smaller boiler than that ofthe comparable air-fired case, depending on the oxygen concentration and the corresponding flue-gasrecirculation. This case is expected to contribute favorably to reduction of the cost of CO2 removal

Char gasification model for circulating fluidized beds

A model for calculating the char conversion in biomass CFB gasifiers is presented. The approach issimplified and accounts for comminution phenomena, chemical reactions and fluid-dynamics. Theoretical submodelsare developed, supported by semi-empirical relations to describe fluid-dynamics and reaction within fuelparticles. There is also a discussion on the type of experiments needed to estimate the main input to the model byexperiments conducted in lab-scale batch-operated FB. The model is useful for implementation as a submodel inexisting CFB gasification models and also as a char conversion estimator in a pseudo-equilibrium model.Keywords: gasification; circulating fluidized bed; biomass conversio