Prabir Basu

Combustion of coal in circulating fluidized-bed boilers: a review

Fuel cost is the single most important operating cost in a circulating fluidized bed boiler. A well-designed CFB boiler can burn coal with a fairly high efficiency and within acceptable levels of gaseous emission. However, departure from ideal operating conditions, which often occurs in operating plants, throws the plant far off the designed performance. A good understanding of the combustion and pollutant generating processes in the boiler furnace can greatly avoid such costly upsets. This paper reviews the current understanding of combustion process. It shows that in spite of its fuel flexibility a CFB boiler may need different furnace volumes to provide the optimum combustion conditions. Depending upon the circumstances the coal char could burn in one of three combustion regimes. Several performance predictive models are available for performance analysis of the furnace. Simplified equations and methods are provided for prediction of sulfur capture and nitric oxide control capability of a CFB furnace.

Heat transfer to walls of a circulating fluidized-bed furnace

A critical review of information on heat transfer between the furnace and enclosing walls of a circulating fluidized-bed boiler is presented. A good understanding of the heat transfer process was impeded for some time by a lack of detailed information about the hydrodynamics of fast fluidization. With improvement in the understanding of the furnace hydrodynamics a clearer picture of the heat transfer process is also emerging. Several mechanistic models for the heat transfer process exist and the surface renewal model explains the observed phenomenon most faithfully. Efforts to calculate heat transfer coefficients from first principles, have been frustrated by a lack of data on residence time and surface coverage of particle strands on the wall. However, a mechanistic model is still useful in scale up of data and in the assessment of the impact of changes in the design or operating variables. Unlike in bubbling fluidized beds, the particle size has a minor effect on the heat transfer, while the average bed (suspension) density shows a major effect. A large variation in reported data between laboratory and industrial scale units is noted. Uncertainty in the measurement of suspension densities in large CFB furnaces may be responsible for this difference. Empirical correlation based on measurements in large commercial units are proposed for design calculations.

Heat transfer in high temperature fast fluidized beds

Abstract Heat transfer to the wall of a fast fluidized bed has been measured for four different particle sizes, two sizes of heat transfer probes and several temperatures from 30°C to about 900°C. A critical analysis of experimental data of other investigators is presented and compared with the present data. A mechanistic model is presented to explain the interdependence of design and operating parameters. Equations are presented for prediction of bed-to-wall heat transfer in circulating fluidized bed combustors.

An investigation into heat transfer in circulating fluidized beds

Abstract A model was proposed to predict the heat transfer in a circulating fluidized bed. To verify the model, experiments were conducted in a 102 mm diameter 5.5 m high Plexiglas column, in which the heat transfer coefficient was measured for different superficial velocities and solid circulation rates and two particle sizes. Results were compared with the experimental data of Mickley and Trilling, Kiang et al ., Fraley et al , and Kobro and Brereton.

Chemical-Looping Gasification of Biomass for Hydrogen-Enriched Gas Production with In-Process Carbon Dioxide Capture

The research presents an innovative idea of developing a continuous H 2 production process employing fluidized-bed technology from agricultural biomass with in situ CO 2 capture and sorbent regeneration. Novelty of the process lies in the generation of relatively pure H 2 from biomass with CO 2 as a byproduct using steam as the gasifying agent. Another unique feature of the process is internal regeneration of the sorbent, fouled in the gasifier. Thus, the technology will serve the twin purpose of regenerating the sorbent and generation of N 2 -free H 2 and C0 2 . This work reports theoretical energy analysis and experimental investigation of the process. The system efficiency of the chemical-looping gasification process at an ideal scenario is found to be 87.49% with biomass as fuel. A sensitivity analysis for system efficiency is also conducted by varying carbon-capture and regeneration efficiencies. The experiments conducted in a batch-type fluidized-bed steam gasifier using CaO as the sorbent shows a 71 % concentration of H 2 and nearly 0% concentration of CO 2 in the product gas when sawdust was used as the feedstock. In a separate test using a circulating fluidized-bed reactor as the regenerator, a 40% regeneration of CaO is also achieved at a calcination temperature of 800 °C.

AN ANALYSIS OF LOOP SEAL OPERATIONS IN A CIRCULATING FLUIDIZED BED

The operation of a loop seal in a circulating fluidized bed is studied on the basis of pressure balance of the circulation loop. The sharp-crested theory of free surface flow is applied to analyze the solids flow rate through a loop seal. Factors, which influence the solids flow rate through the loop seal, include loop seal air velocity, initial bed inventory, standpipe size, loop seal slit size and particle size. The solids flow could occur only between two limiting values of each of those parameters. The analysis also presented pressure distributions along the loop for different circulation rates. Results from above theoretical analyses were compared with experimental results. A good agreement between the results confirmed the validity of the present analysis.

Biomass Gasification in Supercritical Water - A Review

Supercritical water possesses a number of important characteristics that make it suitable for oxidation, synthesis and gasification reactions. It is especially advantageous for very wet biomass whose gasification in this medium avoids the large expense of energy required for drying. Although the process is in laboratory scale it has a great potential for production of hydrogen and other gases from biomass. This paper reviews the present state of the art and summarizes major observations arrived at in small scale laboratory flow and batch reactors. Effects of operating parameters like, pressure, temperature, etc., on the yield and conversion are discussed. Catalysts appear to play an important role in increasing the conversion rate and decreasing the reaction temperature for gasification. Heat recovery from the product stream holds key to making the gasification process auto-thermal. Heat exchanger efficiency, therefore, plays an important role in this process. Several investigators have used the equilibrium model and exergy analysis for thermodynamic analysis of supercritical gasification plants. Energy efficiency of such a plant could be around 50%.

A Comprehensive Review on Biomass Torrefaction

Biomass is a versatile energy resource that could be used as a sustainable energy resource in solid, liquid and gaseous form of energy sources. Torrefaction is an emerging thermal biomass pretreatment method that has an ability to reduce the major limitations of biomass such as heterogeneity, lower bulk density, lower energy density, hygroscopic behavior, and fibrous nature. Torrefaction, aiming to produce high quality solid biomass products, is carried out at 200-300 °C in an inert environment at an atmospheric pressure. The removal of volatiles through different decomposition reactions is the basic principle behind the torrefaction process. Torrefaction upgrades biomass quality and alters the combustion behavior, which can be efficiently used in the co-firing power plant. This paper presents a comprehensive review on torrefaction of biomass and their characteristics. Despite of the number of advantages, torrefaction is motivated mainly for thermochemical conversion process because of its ability to increase hydrophobicity, grindability and energy density of biomass. In addition to this, torrefied biomass could be used to replace coal in the metallurgical process, and promoted as an alternative of charcoal.

Burning rate of carbon in fluidized beds

Abstract An experimental method for determining the rate of burning of a carbon sphere within a fluidized bed of sand is described. Experiments have been carried out at three different bed temperatures. Measured values were compared with theoretical values and they were found to be in reasonable agreement. The difference between the core temperature of the carbon sphere and that of the bed was found to increase with diminishing size of the sphere.

Modeling of Pyrolysis and Gasification of Biomass in Fluidized Beds: A Review

Recent surge in interest in biomass conversion in fluidized bed calls for an analysis of the information available on mathematical modeling of this attractive process. Such an analysis could provide a useful base for the development of mathematical tools for design or optimization of fluidized bed gasifiers. This paper analyzes the current reaction kinetics and its application in modeling pyrolysis and gasification of biomass in fluidized beds. This review identifies the knowledge gaps and relevance of different modeling approaches to design and operation of gasifier plants.