Performance Evaluation of Downdraft Gasifier for Syngas Production Using Rice Husk

Performance Evaluation of Downdraft Gasifier for Syngas Production Using Rice Husk

Chapter One

 Aim and Objectives of the Research

The research aimed to study the performance of a downdraft gasifier using rice husk as feed stock.

The objectives of the research were:

1. To determine the proximate and ultimate analysis of rice
2. To use equilibrium model to predict syngas composition using air gasifying agent.
3. To study the effect of air and oxygen-enriched air as oxidants on the syngas composition in a gasification experiment.

CHAPTER TWO

LITERATURE REVIEW

 Historical Background and Current Status of Gasification Technology

The salient features of town gas from coal were demonstrated to the British Royal Society in 1733, but the scientists of the time saw no use for it (Basu,  2010).Gasification was first used in 1792 when W. Murdoch, a Scottish engineer, used syngas from coal for domestic lighting purposes. In the 19th century, coal gasification became commercialized, illuminating several cities including New York City (Rivas, 2008).

The discovery of petroleum limits the use of gasification fuel. However during the World War II, shortage in petroleum supplies led to re-introduction of gasification and by 1945 the gas was being used to power trucks, buses and agricultural and industrial machines (Rajvanshi, 2014).

After the Yom Kippur War, On October 15, 1973, Arab members of the Organization of Petroleum Exporting Countries (OPEC) declared oil embargo to the United States and other western countries, which were at that time heavily reliant on oil from the Middle East. This shocked the western economy and gave a strong impetus to the development of alternative technologies like gasification (Basu, 2010).

Global warming and political instability in some oil-producing countries gave a fresh momentum to gasification during the current twenty first century. The threat of climate change stressed the need for moving away from carbon-rich fossil fuels. Gasification came out as a natural choice for conversion of renewable carbon-neutral biomass into gas (Basu, 2010).

Types of Biomass

 Biomass may be divided into two broad groups: (a) Virgin biomassand (b) Waste. Primary or virgin biomass comes directly from plants or animals while waste or derived biomass comes from different biomass derived products. Table 2.1, shows these classification.

 Components of Biomass

Cellulose, hemicellulose, lignin and extractives are found to be the major components  of biomass. Cellulose and hemicellulose are formed by long chains of carbohydrates (such as glucose), whereas lignin is a polymeric phenolics. Lignin has a close relationship with hemicellulose, as it acts as a glue fixing the bunches of cellulose chains and plant tissues together. Thus it gives mechanical strength to the plant. Lignin is rich in carbon and hydrogen, which are the main heat producing elements. Hence lignin has a higher heating value than carbohydrates (Vos, 2005). Table 2.2 shows the composition of biomass in terms of those components.

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CHAPTER THREE

METHODS AND MATERIALS

Materials and Equipment

The experiments were conducted with a downdraft throated gasifier developed by the National Research Institute for Chemical Technology, Zaria (NARICT) consisting of the reactor, cyclone, filter and air blower. Rice husk was used as biomass feedstock,whichwas collected from rice milling centre in Zaria.

Other equipment and materials used for the experiment were:

1. Portable infrared gas analyser (Gasboard 3100P series)
2. Digital thermometer (UT 350)
3. K-type (chromel-alumel)thermocouple

1. Flowmeter(MF5706)
2. Compressed air(21% O₂, 79N₂)
3. Compressed Oxygen( 99.9%purity)
4. Silicone gasketmaker

Research Methodology

The methodology employed for this research is presented in Figure 3.1.It involves firstly, characterization of the rice husk used to determine its proximate and ultimate composition. Therice husk composition was used to simulate syngas compositionand calorific value using equilibrium model. The experimental gasification was carried out using rice husk with both air and oxygen – enriched air as gasifying agents while varying the flow rates for air and percentage oxygen enrichment for oxygen – enriched gasification. The variation effects on syngas composition, gasification temperature and calorific value were then monitored. The overall results from the simulation and the experiments were then compared.

CHAPTER FOUR

RESULTS AND DISCUSSION

Characterization of Rice Husk

Figures 4.1 and 4.2 showthe chemical composition of the rice husk in terms of proximate and ultimate analysis respectively. The chemical composition of any biomass has impact on the gasification behaviour of the biomass and the syngas composition and quality.

 

CHAPTER FIVE

CONCLUSION AND RECOMMENDATION

CONCLUSIONS

The following conclusions are drawn from the gasification study carried out using downdraft gasifier with rice husk feedstock:

1. Increase in air flow rate (0.7, 3.0 and 6.4 L/min were considered) during air gasification favours oxidation temperature, equivalence ratio, syngas composition, and calorific value. The best syngas composition recorded using air as the gasifying agent was at 6.4 L/min with composition of10.83% CO, 9.51% CO₂, 2.12% H₂and 1.18 CH₄%, desired syngas composition of 14.13 % and equivalence ratio of 0.128, withan average temperature of 567°C and 2.75 MJ/Nm³calorific
2. A mathematical model was successfully developed using equilibrium approach to predictrice husk gasification using air as gasifying agent between 500 and 1100 °C. The results of the model suggested an optimum temperatureat 800 °C and equivalence ratio of 0.42 with syngas composition of 18.72 CO%, 16.68% H₂, 13.05% CO₂, 0.39% CH₄, and 4.47 MJ/m³ calorific value.

3. Validation of the model developed was done with the best results obtained from rice husk air gasification and gave aroot square mean error value of58.
4. For oxygen enriched- air rice husk gasification, 30 to 100% oxygen enrichment in air were considered. It was found within this range that temperature, equivalence ratio and calorific value increased linearly with increase in enrichment.The CO to CO₂ratio was found to be greater than one and also increasing linearly between 30 to 50% enrichment, while decreasing linearly and less than one from 60 to 100% enrichment.The best point was considered at 50% oxygenenrichment in air having the highest CO to CO₂ of 1.63 with equivalence ratio of 0.494, desired syngas of 24.34%, syngas composition of 19.8% CO, 12.16% CO₂, 2.26% H_2, 2.28% CH₄, and calorific value of 3.67 MJ/m³.

5. Performance analysis of rice husk gasification using both air and oxygen- enriched air as gasifying agents showed that carbon conversion efficiency (CCE) and cold gas efficiency (CGE) increased with increase in air flow rate for air gasification and with oxygen enrichment for the case of oxygen- enriched air gasification. For air gasification highest CCE and CGE were achieved as 21.27 and 12.55%, respectively with the highest air flow rate of 6.4L/min, while for oxygen- enriched air gasification, 50 % oxygen enrichment in air gave the best values of both CCE and CGE as 46.72 and 26.24%,

 RECOMMENDATIONS

The following recommendations are suggested for further studies:

1. Other model approaches like non -equilibrium and artificial neural networks should be employed to see if lower error can be achieved and the modification of the equilibrium
2. The air blower only supplied maximum air flow rate of 6.4 L/min equivalence to equivalence ratio of 0.128 during gasification using air as gasifying agent creating a research limitation, because model suggested an optimum equivalence ratio of 0.42.Therefore further research is recommended at higher flow rates that will provide equivalence ratio up to and beyond thebest simulated
3. During the oxygen- enriched air gasification the flow rates were kept constant while varying the percentage oxygen of the gasifying agent. Further research is recommended to be carried out on gasification using oxygen-enriched air by varyingboth the flow rate and percentage oxygen This will lead tohaving different equivalence ratio at each enrichment level, thereby giving a better option to optimize the process.

4. Research is strongly recommended for the possibility of separating carbon dioxide in syngas and use as supplementary gasifying agent of the gasification system.

REFERENCE

• Abalaka, A.E. (2012). Effects of Method of Incineration on Rice Husk Ash Blended Concrete.ATBU Journal of Environmental Technology. 5 (1) 34-47.
• Allegue, B. and Hinge, J. (2012). Biogas and bio-syngas upgrading (Report).Danish Technological Institute Laura.
• Arena, U. (2012).Process and technological aspects of municipal solid waste gasification.A review.Waste Management journal, 32, 625-639.
• Arnavat, P. M. (2011). Performance Modelling and Validation of Biomass Gasifiers for Trigeneration Plants. (Unpublished Doctoral dissertation),UniversitatRovira,Virgili.
• Barman, S. N. Ghosh, S., De, S. (2012). Gasification of biomass in a fixed bed downdraft gasifier – A realistic model including tar,BioresourceTechnology , 107, 505–511.
• Basu, P. (2010). Biomass Gasification and Pyrolysis Practical Design and Theory.Elsevier : Oxford
• Beohara, H. Guptaa, B., Sethib, V. K., Pandeyb M., (2012).Parametric Study of Fixed Bed Biomass Gasifier: A review.International Journal of Thermal Technologies.1 (2).
• Bhavanam, A and Sastry, R. C. (2011).Biomass Gasification Processes in Downdraft Fixed Bed Reactors: A Review. International Journal of Chemical Engineering and Applications, 6 (2), 425-433.

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