Study of Performance of an Atmospheric Bubbling Fluidized Bed Combuster Using Rice Waste

Abstract

Rice is cultivated in all the main regions of world. The worldwide annual rice production is 666 million tons for year 2008. Rice husk, rice straw and rice bran are three types of rice crop waste materials available today. Of these, rice husk and rice straw are the only types of rice waste which could be incinerated in fluidized bed combustors. The total rice waste generated is comprised of 20% rice husk, 70-80% rice straw and 2-3% rice bran. Rice straw is burned in open fields, ploughed in fields, used in compost, cattle house flooring and about 10-15% of total rice straw is available for direct combustion in commercial fluidized bed combustors in Punjab state. The available 10-15% rice straw for combustion in fluidized bed combustors is also dependent on region/area. Due to higher collection cost of rice straw it was not used in many of commercial fluidized bed combustors in the state. Rice husk being easily bought from rice mill owners is easily available for most of the commercial fluidized bed combustors. The study presented here is mainly focused on rice husk, which is abundantly available in this region. The combustion of rice husk/rice straw in fluidized bed combustors is an attractive possibility of future for power generation, the solution of waste disposal problems and the reduction of greenhouse gases. As paddy plants consume carbon dioxide emissions during photosynthesis process, the overall carbon dioxide emissions during combustion of rice husk/rice straw can be offset, which is fundamental advantage of extracting energy from rice husk/rice straw through combustion and gasification process in the perspective of sustainability. Rice husk characterized by having high ash content, low bulk density and poor flow characteristics makes it difficult for energy conversion. Where as rice straw is having low ash content as compared to rice husk, but when available for combustion it creates feeding and agglomeration problems more predominantly as compared to rice husk. The fluidized bed combustor is ideally suited to burn such fuels since the uniform mixing condition ensures efficient combustion, even at temperature as low as 873 to 923 K. The work presented in this thesis is mainly divided into three main parts. In the first part a mathematical model for exit gas composition and solid population balance for a 10 MW bubbling fluidized bed combustor at Jalkheri, Distt. Fatehgarh Sahib, Punjab, India based on rice husk has been developed. The model is based on three phase theory of fluidization and material balance for shrinking rice husk particles. The burning of rice husk is assumed to take place according to single film theory. The model has been used to predict the exit gas composition particularly oxygen, carbon dioxide and nitrogen. Then solid population model has been developed to calculate bed carbon load and carbon utilization efficiency. The model predictions are compared with the data collected from the power plant at Jalkheri which uses rice husk as a fuel input (at the time of study). All the results from the two models for rice husk are coming within permissible limits. In the second part, problem of agglomeration in fluidized bed combustors (FBC) using rice husk and rice straw has been discussed. The problem of agglomeration with respect to rice husk/straw has also been discussed as during study rice straw is available only in combination with rice husk. The agglomeration problem of 10 MW power plant at Jalkheri, which is main reason for defluidization of bed has been discussed. Scanning electron microscope (SEM) of ash agglomerates has been done. Ash samples taken from the Jalkheri power plant and Ludhiana based power plant has been analyzed and discussed. The problem of agglomeration at 3.5 MW cogeneration plant at Ludhiana with rice husk has also been studied. In the third part computational fluid dynamics (CFD) analysis of an atmospheric bubbling fluidized bed combustors based on rice husk has been discussed. For CFD analysis mainly the fluidized bed has been considered. Due to less computational space the two dimensional fluidized bed has been considered. The Eulerian multiphase model has been used for the fluidized bed. Fluidized Bed is assumed to be in isothermal conditions. Modeling of fluidized bed has been done in Fluent 6.2 commercial code. The CFD modeling has been done to visualize the phases present in fluidized bed with respect to rice husk. The fluidized bed has been modeled with and without bed super heater tubes.