• Energy Research

In developing a mathematical model for a CFB boiler we use earlier work. Our model includes mathematical descriptions of the underlying physical and chemical processes. It has been applied to simulation of a 12 MW CFB boiler. The calculations agree well with test results.

Abstract A novel model is presented that incorporates the Arrhenius integral into one of the generalized extreme value (GEV) distributions to describe the pyrolysis of complex feedstocks. The model differs in structure from distributed activation energy models (DAEM), and its self-variable differs from Weibull mixture models. It is found that all first order reactions, regardless of their activation energies and pre-exponential factors, can be best modeled by a unified Frechet (inverse Weibull) minima distribution versus a reaction progress factor, followed by the Gumbel maxima and classical Weibull minima distributions. When applied to thermogravimetric analysis (TGA), the peak temperature is linked to a characteristic reaction progress factor, which largely accounts for the increase of peak temperatures with increasing heating rate. The model is able to deconvolute the contributions of multiple components in the sample and to estimate kinetic parameters for individual components by least squares regression. A correlation derived from the model can be used to evaluate apparent activation energy based on the shift in peak temperature as the heating rate varies, similar to the Kissinger method and its variants. The present work provides not only a different model framework, but also new perspective on the interactions between the statistical distribution of material properties and reactivity.

Abstract A non-stoichiometric equilibrium model based on free energy minimization is developed to predict the performance of gasifiers. The model considers five elements and 44 species in both the gas and solid phases. The gas composition and heating values vary primarily with temperature and the relative abundance of key elements, especially carbon, hydrogen and oxygen. Pressure only influences the result significantly over a limited temperature range. The model predicts the onset of formation of solid carbon where the gas composition becomes insensitive to additional carbon. The carbon formation boundary is plotted in C–H–O ternary diagrams as a function of temperature and pressure. When the experimental carbon conversion is introduced, the kinetically modified equilibrium model gives good predictions of the gas composition from an air-blown pressurized circulating fluidized bed gasifier for two coals. The role of water, including both fuel moisture and steam injection, is examined based on a water balance on the feed and product gas to evaluate the steam demand.

Abstract A new system has been developed for clean and highly efficient utilization of coal. The coal is first gasified and the fuel gas is then used for industrial purposes in town gas or as a fuel for gas turbines. The char residue from the gasifier is burned in a circulating fluidized-bed combustor to generate steam for power generation, process heating, refrigeration, air conditioning, etc. The ash is used to produce construction materials. Metals such as vanadium and uranium can be extracted if the metal contents of coal are sufficiently high. An important system component is the combined gasifier–combustor. Experiments show that the system can produce gas and steam simultaneously. The gas heating value is 10–14 MJ/Nm 3 and the fuel conversion is over 90%. The system has low emissions and low cost. A demonstration system has been constructed at the Yangzhong Thermal Power Plant in Jiangshu Province of China. It produces 3 500 Nm 3 /h of dry gas and 75 mt/h steam. The CFB boiler has been in commercial operation since April 1995. It has high efficiency, good fuel adaptability, and a high load turndown ratio.

Abstract This paper presents the results from biomass gasification tests in a pilot-scale (6.5-m tall × 0.1-m diameter) air-blown circulating fluidized bed gasifier, and compares them with model predictions. The operating temperature was maintained in the range 700–850°C, while the sawdust feed rate varied from 16 to 45 kg/h . Temperature, air ratio, suspension density, fly ash re-injection and steam injection were found to influence the composition and heating value of the product gas. Tar yield from the biomass gasification decreased exponentially with increasing operating temperature for the range studied. A non-stoichiometric equilibrium model based on direct minimization of Gibbs free energy was developed to predict the performance of the gasifier. Experimental evidence indicated that the pilot gasifier deviated from chemical equilibrium due to kinetic limitations. A phenomenological model adapted from the pure equilibrium model, incorporating experimental results regarding unconverted carbon and methane to account for non-equilibrium factors, predicts product gas compositions, heating value and cold gas efficiency in good agreement with the experimental data.