Production, Characterization and Cleaning of Producer Gas from Gasification of Refuse Derived Fuels (RDF) for Application in IC Engines

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Date

2017-10

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Addis Ababa University

Abstract

A well-planned research activity conducted in Addis Ababa University, Addis Ababa Institute of Technology, School of Mechanical and Industrial Engineering, under Thermal and Energy Conversion Chair is utilizing a thematic research grant from AAU – Vice President for Research and Technology for this thesis and future works out side this scope, for its practical importance in the energy sector of Ethiopia, most importantly in the off-grid part of the country. Most of the rural areas of Ethiopia are known to have no available electric power supply from the centralized main grid. This huge energy problem has retarded the economic growth of the areas due to the daily, backward and labor intensive activities of the society to meet their daily energy needs. Beyond this, Ethiopia is one of vast and most populous countries in Africa with remarkably high daily waste production rate. All these problems call for a sound measures to be implemented in terms of energy integrated waste management and this paper is aimed at solving the electric shortage problem in off grid parts by preparing high calorific waste fractions in to RDF for good output in internal combustion engine for electricity generation. The research project is divided in to four manageable phases. Phase 1: Fuel preparation, studying theoretical gasification, modeling and solving of the theoretical gasification using computational methods, process design and sizing of the gasifier, studying gas contaminants and gas cleaning and conditioning techniques, design of the gas cleaning and condition system to meet IC engine requirements for its utilization, which are all paper and computer based tasks. Phase 2: Manufacturing a pilot gasifier – engine set up with the gas cleaning and conditioning system for test run in AAU – AAiT School of Mechanical and Industrial Engineering work shop. Phase 3: Conducting experiment/test on the pilot gasifier – engine coupled setup to carry out validation, experimental investigation and performance analysis. Phase 4: Publication of important scientific outputs from the research in high rated open access journals. Gasification is an attractive alternative for thermo – chemical conversion of wastes for its flexibility in terms of various materials as feedstock, relatively higher conversion efficiency and wide range of applications in relatively small area per unit output energy. MSW, separated and screened from non-combustibles (soil, metals, glass …) and biodegradable components was prepared to produce RDF fuel and characterized. The LHV value of the prepared RDF is 16.63MJ/kg, which is well accepted good RDF as its LHV is greater than 15MJ/kg as recommended by studies. Based on the good results from RDF characterization, a brief mathematical modeling of the gasification of RDF using non – stoichiometric equilibrium model was presented. The code formulated for the model was implemented on MATLAB to estimate concentration of product gas species and the corresponding heating values for wide range of gasification temperatures and equivalent ratios. The model gives optimal values of temperature and equivalent ratio to be 850 C and 0.2 respectively for best operating conditions. The gas heating value was 8.164MJ/Nm 0 , which is well accepted yield from downdraft gasifier to be used for applications such as utilization of the gas in IC engines for electricity generation. 3 Downdraft gasifier is found to be technically good for engine applications and the process design of the gasifier determines the capacity of the gasifier to be 147kW at feed rate of 46 kg/hr and product gas flow rate of 65.14Nm /hr to meet engine requirements. The cold gas efficiency of the gasifier is 70% and carbon conversion efficiency and energy balance calculations give results that support the fact that basic assumptions underlying the equilibrium model employed in the computational tool give an over estimated theoretical maximum yield. Gasifier sizing was performed based on internationally accepted used sizing method and the size calculation results were validated with recommended ranges of sizes for same product gas flow rate. 3 Major gas contaminants for this specific application are well identified and studied for proper cleaning and mitigation. Based on the gas quality demanded by the application, a sound gas cleaning and conditioning system is designed. The paper based designs of the gasifier and gas conditioning system is brought to the ground by manufacturing and testing a pilot gasifier-engine setup with gas cooling and conditioning system for a test run to validate the modeling output with the experimental results and to perform performance evaluation of the gas cleaning. A good agreement was observed between experimental and simulation results for gas characterization at the gasifier outlet. Catalytic gasification and the gas cleaning system give promising results for future advanced investigations on the use of Dolomite and the secondary gas cleaning. Even though gas cleaning performance experimental tests require more investigation with laboratory facility that measure all possible product gases and tar, the results obtained so far give a satisfactory motive to do more improvements and conduct advanced tests. Both experimental and simulation results make electricity generation using a specific producer gas engine generator set based on the proven characterization of the gas (heating value and inflammability), provided that gas suction fun or blower or gas compressor is installed before it to overcome the unexpected high pressure drop observed as the gas pass through the gas cleanup system to meet the engine pressure requirements. This research gives promising solutions for availability of the expensive electricity in off-grid areas of Ethiopia possible form refuse derived fuels (RDF) in a micro – scale gasification plant. So far, one scientific manuscript titled ‘Equilibrium Modeling of Refuse Derived Fuels (RDF) Gasification in Imbert Type Downdraft Gasifier’ is on final stages of publication, being accepted after review from this study to an open access journal. More scientific outcomes are expected from further experimental investigations and performance analyses.

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Keywords

Gasification, RDF, Modeling, Producer Gas, Gas Cleaning

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