Design, Construction, and Simulation of Maize Cobs Fluidized Bed Combustor
Chapter One
Aim and Objectives of the Study
This project aims to design, construct, and simulate an agricultural waste bubbling fluidized-bed combustor.
The specific objectives of the work are:
- To design the various parts of the combustor
- To select appropriate materials for the fluidized bed combustor
- To simulate the performance and validate the results by test on the fluidized bed combustor constructed
- Estimate the cost of a prototype of the fluidized bed combustor.
CHAPTER TWO
LITERATURE REVIEW
Background History
Development of the Fluidised Bed Combustors started in 1922, with the Fritz Winkler patent for gasification of lignite. In 1960, Winkler’s efforts ultimately resulted in the design of coal firing test units. The first bubbling fluidised bed test facility started operating in 1965, and was used primarily to establish potential for air emission control (Koornneefet al., 2007). Since then, fluidised bed technologies have been developed and vigorously used for different operations such as transportation systems and chemical reaction processes. Increased utilisation of coal triggered by oil crisis during the 1970s, followed by the pressure to limit environmental pollution due to coal combustion led to the development of coal-based technologies. These technologies include pre-combustion coal cleaning, post-combustion environmental emission controls, advanced coal combustion, gasification and liquefaction (Tavoulareas, 1991).
Renewable Solid Fuels
Over the years, the definition of biomass has evolved to include diverse sources like construction debris and algae. Biomass can be defined as the non-fossilised and biodegradable organic matter of living or recently living plants, animals and micro-organisms. This also includes industrial products, by-products, residues, agricultural and municipal wastes (UNFCCC/CCNUCC, 2008). The line between biomass and waste is drawn differently from country to country. Some countries refer to biomass as plant-derived organic matter available on a renewable basis, thereby including dedicated energy plants, crop residues, agricultural and municipal waste, aquatic plants and animals. Other countries categorise biomass as waste fuels from waste products of human
and industrial processes. Biomass residues refer to biomass by-products, residues and waste from agriculture, forestry and related industries (UNFCCC/CCNUCC, 2008).
There are numerous conversion technologies from biomass resource to fuel, heat and power. Figure 2.1 shows a summary of the various biomass conversion processes.
CHAPTER THREE
MATERIALS AND METHOD
Description of Bubbling Fluidised Bed Combustor
The design of the fluidised bed system using maize cobs as fuel consists of three units. These are the main combustor or fluidised bed column, the distributor plate and the cyclone separator.
The fluidised bed combustor contains sand as the bed material and is capable of burning a wide range of fuel to generate heat energy. Air from the force draft fans or blower is supplied into the fluidised bed column. The air passes through holes in the distributor grid, lifting the bed material. This makes the sand particles behave like a fluid. The solid fuel (biomass feed) is then introduced into the bubbling bed and is immediately burned as the bed is agitated. Hot flue gases generated during combustion flow into the cyclone where the combustion gas and entrained solids are separated. The temperature of the gas is then measured using the thermocouple wire to determine its suitability for power generation.Fig. 3.1 shows a schematic of the fluidised bed model.
CHAPTER FOUR
RESULTS AND DISCUSSION
Ignition/Testing Procedure
The system was assembled outside the boiler room, mechanical engineering departmental workshop, ABU Zaria. The bed material (sand) was prepared by removing larger particles which can potentially affect the quality of fluidisation using a sieve. The bed material was then poured over the distributor grid. A shallow bed of 150mm was maintained in this case owing to the size of the sand particles.
The three-phase constant speed blower used during this test was converted to a single-phase blower with the help of a 50 microfarad capacitor.
CHAPTER FIVE
SUMMARY, CONCLUSION AND RECOMMENDATION
The maize cobs sample was obtained from soba local government of Kaduna State. The fluidised bed combustor along with all its relevant components was fabricated at the national board for technology incubation in Kano State, and tested for fluidisation. The insulation of the fabricated combustor was then completed at the mechanical engineering workshop, Ahmadu Bello University, Zaria. The completed system was tested for fluidisation and fired with agricultural waste (Maize Cobs). The velocities and temperatures of the flue gas and fluid bed were then measured and the results were analysed.
The system was also simulated using Ergun 6.2. The error margin between calculated and simulated values was then deduced.
Conclusion
Fluidised bed combustion is a reliable method of obtaining clean and renewable energy from maize cobs. The combustor has generated bed temperatures of 837 °C, sufficient for the production of steam, and hence, power generation. Furthermore, flue gases at over 220 °C have been generated by the system, thereby providing alternative ways for drying and other low thermal applications.
Char produced from maize cobs combustion could be converted to form charcoal briquettes for use in advanced wood stoves, hence, improving the total economic output of the combustor.
From an environmental stand point, the low concentration of CO, NOx and SOx in the order of 2p, 5p and 1p respectively is an indication of emission reduction potential of fluidised bed combustors.
Recommendations
The following recommendations are proposed to improve the fluidised bed combustor
- Measuring key bed parameters like temperature, bubble size, bubble velocity, bed voidage and volume fraction of bubbles will continue to present challenges in fluidised bed research, and hence, the need to procure necessary equipment for fluidised bed experiments.
- Incorporating a variable speed blower will enhance the ignition phase of the fluidised bed combustor, and allow for the bed temperature to be controlled more effectively.
- Since the pressure of the system is higher than the atmospheric pressure, light weight fuels like rice husk and groundnut shells will be blown out of the feed hopper. It is therefore necessary to modify the feed hopper to accommodate fuels of all sizes.
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