This particular study explored the feasibility of an appropriate instrumentation and control system for a pilot scale atmospheric fluidized bed biomass gasification unit to facilitate its operation. The most important parameters were properly identified and evaluated to make the control of the gasification system much easier and facilitated.
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These parameters include the gasification temperature, pressure, the air flow rate and the fuel feed rate. The necessary devices were installed in the gasification system and calibrated to conveniently monitor and control these identified parameters. A control system program was developed to process all the electrical signals into readable values and monitor the sensor measurements in the gasification unit. The feasibility of the developed process control program was evaluated based on its ability to facilitate measurement, operation and control of the fluidized bed gasification system using segregated municipal solid waste (MSW) pellets as fuel feedstock.
In about 55 min reaction time, the automated control system had full control of the operation of the gasifier. The desired gasification reaction temperature of 700°C was maintained for about 1 h during the test. The control system was also able to check the fluidization conditions and other safety indicators. The appropriate equivalence ratio was controlled during gasification by the adjustment in the fuel feed rate. During the test, the feed rate was increased as the error reached more than 10. Continuous
adjustment in the fuel feed was experienced to compensate for the decrease in fuel in the bin that caused the change of the flow in the screw conveyor. It was also able to perform corrective actions with the disturbances associated with the system.
The gasification control system kept the fluidizing air velocity at an average of 48 cm s-1. This was 30% higher than the minimum fluidization velocity indicating good fluidization of the bed inside the reactor. The desired equivalence ratio at different temperature ranges was also achieved throughout the whole test.
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The temperature and pressure profiles observed during gasification further support the feasibility of the instrumentation and the process control program that was developed. The gasification temperature profile showed that the reaction temperature was well maintained at the desired value of 700°C during the operation of the gasifier. The expected start-up time was achieved without any trouble. Near isothermal condition within the reactor was also observed during the experiments. As indicated by the
pressure profile, good fluidization also occurred during gasification. The stable pressure profile within the reactor likewise indicated that the bed material loss was minimal because of the optimized gasification operation.
Another indication of the feasibility of the instrumentation and the use of the control program was the good production of the synthesis gas from the gasification of MSW. Analysis of the gas produced indicated a heating value (HV) of 7.94 MJ Nm-3. The gasification system operated at 94% carbon conversion efficiency and 59% cold gasification efficiency. Gas production went at a rate of 4.00 kg min-1 and a yield of 2.78 m3 kg-1 of fuel.
46 CHAPTER III
OPTIMIZATION OF SEGREGATED MUNICIPAL SOLID WASTE GASIFICATION
INTRODUCTION
Biomass, which can be classified as plant, animal manure or municipal solid waste, is widely considered a major potential fuel and renewable energy resource for the future (Bridgwater, 1995). Biomass resources are abundant in most parts of the world and various commercially available conversion technologies could transform the current traditional technology into modern applications for energy source (Johansson et al., 2006). Like fossil fuels, biomass contain high percentages of carbon and hydrogen and can be a good alternative source of energy (LePori and Soltes, 1985). Biomass used as energy source can reduce CO2 gas emission and SO2 and NOx pollution due to its neutral carbon contribution to the atmosphere (Cao et al., 2005).
The generation of excessive amounts of waste is a common occurrence in most developed countries. As societies develop, the amount of waste material generated increases to a level that its disposal becomes a problem. The development of processes in the disposal and utilization of industrial solid wastes has caught attention of
policymakers and researchers to address the issue of cleaner energy generation for enhanced environmental quality. Techniques in waste management should be able to generate greater recovery value from the wastes and maintain the sustainability of the process (Mastellone and Arena, 2008).
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Gasification is one of the thermochemical processes that can convert waste into a useful product known as synthesis gas and these conversion routes have an excellent future (Sipila, 1995). Without complete combustion of the fuel, conversion occurs in an oxygen deficient (partial oxidation) condition at high temperatures. The partial
oxidation process of the biomass takes place at temperature of about 800°C and produces primarily combustible gases consisting of carbon monoxide (CO), hydrogen (H2) and traces of methane and some other products like tar and char (Rajvanshi, 1986). Four stages occur during gasification of carbonaceous material: drying, volatilization,
combustion, and reduction (Knoef, 2005). The moisture within the material is heated and removed during the drying process. Continued heating volatilizes the material where volatile matter separates from the particle and comes into contact with the oxygen. The very exothermic combustion process then occurs providing the heat for the last stage, the reduction reactions, to occur. The reduction reactions include water gas reaction,
Boudouard reaction, water-gas-shift reaction, and methanation reaction (Swanson et al., 2010).
In order to effect the most efficient transformation of biomass to fuels and other forms at the desired scale of operation, an understanding of the physical and chemical characteristics of different biomass resources is needed (Goswami and Kreith, 2008). Large scale projects for gasification have been envisioned for alternative energy sources and yet many of these have remained in the proposal stage. Agricultural industries, such as the cotton gin, poultry and dairy industries, generate tons of wastes while consuming enormous amounts of heat and power in their operations. Thus, the on-site conversion of
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the generated wastes into useful products is the most practical option as it will also minimize the transport cost of the biomass. Ultimately, this system will make these industries produce their heat and power requirement thereby indirectly contributing to reduced dependence on foreign oil and generating new businesses in the farm.
This part of the research looked at the optimization of the production of synthesis gas from the gasification of segregated municipal solid waste (MSW) as feedstock. The specific objectives of the study were to: (a) characterize the segregated municipal solid waste (b) evaluate the most appropriate preparation of the feedstock for gasification and (c) apply the response surface methodology to optimize synthesis gas production.
METHODOLOGY