Performance Evaluation Of Downdraft Gasifier For Syngas Production Using Rice Husk

Performance Evaluation Of Downdraft Gasifier For Syngas Production Using Rice Husk

Abstract

Biomass gasification is a thermochemical process that converts biomass to a combination of gases known as syngas comprising mainly of CO, H2and CH4 as a result of partial combustion with a gasifying agent. It is considered to be a promising clean energy option for reduction of greenhouse gas emissions and a way of utilizing agricultural wastes like rice husk. The syngas can be used not only to produce heat and power but in synthesis of liquid fuels and chemicals. This research comprises of rice husk characterization, mathematical model formulation to predict rice husk gasification theoretically and gasification of rice husk using both air and oxygen-enriched air as gasifying agents experimentally. Theoretical rice husk air gasification was done by inputting the composition of the characterizedrice husk into set of mathematical equations derived based on thermodynamics, mass and energy balances using equilibrium approach and resulting equationswere computed using MATLAB as a toolto predictsyngas composition and calorific value between temperature of 500 and 1100 °C. Experimental ricehusk gasification was conducted using a downdraft gasification system installed at National Research Institute for Chemical Technology, Zaria, Nigeria, comprising of a gasifier as reactor, cyclone, filter and air blower. The gasification was done with two different gasifying agents; air and oxygen-enriched air. For the air gasification, effect of 6.4, 3.0 and 0.7 L/min flow rates were studied while for the oxygen-enriched air gasification, 30 to 100% oxygen enrichment in air were examined. Temperature, syngas composition and calorific value were monitored during the experiment using online portable infrared syngas analyser (Gasboard 3100P series), digital thermometer (UT 350) and K-type (chromel-alumel) thermocouple. The results of the model indicated an optimum temperature at 800 °C with syngas composition of 18.72 CO%, 16.68% H2, 13.05% CO2, and 0.39% CH4, and 4.47 MJ/m3 calorific value. The best experimentalsyngas composition was at 6.4 L/min air flow rate with composition of 10.83 CO%, 9.51% CO2, 2.12% H2 and 1.18 CH4%, desired syngas composition of 14.13 % and equivalence ratio of 0.128, with an average temperature of 567°C and 2.53 MJ/Nm3 calorific value.Root mean square error value of 7.58 was calculated when the model developed was validated with the best results obtained from rice husk air gasification. For oxygen enriched- air rice husk gasification, the best point was considered at 50% oxygen enrichment in air having the highest CO to CO2ratio 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% H2, 2.28% CH4, and calorific value of 3.67 MJ/m3.Performance analysis shows that for air gasification the highest Carbon Conversion efficiency (CCE) and Cold Gas Efficiency (CGE)was achieved at the highest air flow rate (6.4L/min) as 21.27 and 12.55% respectively. 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%, respectively.

Chapter One

Introduction

1.1 Background of the Study

Access to cheap, reliable, and sustainable energy is a precursor for attaining and sustaining socio economic development. In fact it is fundamental requirement for poverty reduction. Currently about 90% of the world primary energy consumption is from fossil (petroleum, gas and coal), (Melgaraetal., 2009). However depleting of these fossil energy sources, the rate at which carbon dioxide (CO2) is released into the atmosphere when they are burnt and increasing demand of the world energy due to population coupled with technological advancement are the current challenges. These challenges have served as motivation globally to develop alternative and renewable energy like biomass and solar that can help the present generation to meet their energy demand without jeopardizing the ability of the future generation to meet their energy demand.

Biomass is a non-fossilized and biodegradable organic material originating from plants, animals and micro-organisms. They include products, by-products, residues and waste from agriculture, forestry and related industries as well as the non-fossilized and biodegradable organic fractions of industrial and municipal wastes. Biomass has high but variable moisture content and is made up of carbon, hydrogen, oxygen, nitrogen, sulphur and inorganic elements (Bhavanam and Sastry, 2011).The biomass is the only source of carbon-based renewable energy (Pandeyetal., 2013) and the most dominant renewable energy source used in the world today, comprising almost 80 per cent of the total supply. By 2050 energy from biomasscould contribute 15%–50% of the world’s primary energy (Beoharaetal., 2014). Presently about 25% of biomass is used by developed countries, while 75% is used by developing countries to produce heat (Sahito, 2013).

Nigeria covers land area of 923,768 square km and the total land available for agriculture and vegetation is a measure of its biomass potential (Diyokeet al., 2014). Nigeria has about 71.2 million hectares of available agricultural land, out of which about 36 million hectares of land are being currently utilized for agricultural production (Oladeji, 2011). Nigerian biomass energy resource is estimated to be 144 million tonnes/year (Diyokeetal., 2014).Sambo (2009) estimated Nigerian agricultural waste resources in million tonnes per annum as 11.2, with energy content of 147.7 GJ.

About 120 million tonnes of rice husks are generated annually in theworld (Omatola and Onojah, 2012). In Nigeria about 2.0 million tonnes of rice is produced annually and 400 thousand tonnes of rice husk is generated out of it (Abalaka, 2012). These large quantities of biomass resources in Nigeria offer much potential for renewable energy and can play a significant role in meeting the country’s energy demand if properly harnessed in modern and sustainable way.

Direct combustion has been the major way of utilization of biomass in Nigeria especially in rural areas. Fuel wood is used by over 60% of Nigerians living in the rural areas. Nigeria consumes over 50 million metric tonnes of fuel wood annually with alarming rate of deforestation. The rate of deforestation is about 350,000 hectares per year, which is equivalence to 3.6% of the present area of forests and woodlands, whereas reforestation is only at about 10% of the deforestation rate (Sambo, 2009).

The conversion of biomass to useful forms of energy can be achieved using a number of different biomass utilization technologies that can be separated into either thermochemical processes or biochemical/biological processes as in fermentation and aerobic digestion (Caputo etal., 2005).

The thermochemical conversions of biomass are combustion, pyrolysis and gasification. They constitute one of the promising routes among the renewable energy options of future energy because virtually all types of biomass can be used as feedstock even waste unlike their counterpart fermentation and aerobic digestion which are very specific in their biomass feedstock requirement. Biomass gasification has attracted the highest interest as it offers higher efficiencies compared to combustion and pyrolysis (Sheth and Babu, 2009). Gasification converts biomass into a combustible gas called producer gas or syngas consisting mainly of carbon monoxide, hydrogen and methane by partial oxidation. Gasification is also favoured among the other thermochemical conversion processes because it provides a syngas that can be used not only to produce heat and power but also in synthesis of liquid fuels and chemicals, such as biodiesel, methanol etc.

At present, the research on biomass gasification focuses on the optimization of the syngas production by means of a proper design, gasifier configuration, which includes fixed bed, fluidized bed, and entrained type gasifier. The right choice of the parameter values (type and amount of oxidant) and types of biomass also play a vital role in harnessing properly the energyin biomass using gasification technology.

1.2 Research Problem Statement

Currently the utilization of rice husk produced in Nigeria does not include producing energy from it. Inefficient way of utilizing biomass through combustion in Nigeria (wood fuel and charcoal) has contributed to desertification, deforestation and erosion in the country. Biomass gasification technology as an efficient and sustainable way of utilizing biomass is not properly harness in Nigeria.

1.3 Justification of the Research

Gasification is an efficient way among other options to produce energy from biomass and is considered a very promising clean energy option for reduction of greenhouse gas emissions.Biomass gasification contribution to global warming is considered almost zero because carbon dioxide (CO2) released when burned as a fuel in any form is naturally sequestered by photosynthesis (Basu, 2010). In addition, biomass fuels contain negligible amount of sulphur, so their contribution to acid rain is minimal (Luby, 2003).Gasification is part of clean development mechanism projects (CDM) which can earn Nigeria carbon credits and can be traded to countries who are trying to achieve their emission limits.Unlike other energy resource, using rice husk to produce energy is often a way to dispose of the biomass as waste that could otherwise create environmental risks. Energy from biomass gasification will reduce the dependency on the conventional source of energy from non-renewable fossil fuels. It will also serve as a major source of energy to rural areas where the convectional way of supplying energy could not be reached and this will help to reduce the rate of deforestation.Furthermore, it will create more value to growing of rice in Nigeria because economic value will be given to the rice husk generated as waste from rice production. This will translate to overall food security of the nation and new jobs will be created.

1.4 Aim and Objectives of the Research

The aim of the research was 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 husk.
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.

1.5 The Scope of the Research

The research was limited to:

1. Using only rich husk as biomass feedstock characterized via proximate and ultimate analyses
2. Using downdraft gasifier installed at National Research Institute for Chemical Technology, Zaria.
3. Utilizing air and oxygen enriched air as gasifying agents.
4. Monitoring of the gasification products (CO, CO2, H2, CH4, O2) and calorific value.
5. The inside temperature of the gasification unit was monitored only at the combustion zone.

Chapter Five

Conclusion and Recommendation

5.1 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% H2 and 1.18 CH4%, desired syngas composition of 14.13 % and equivalence ratio of 0.128, with an average temperature of 567°C and 2.75 MJ/Nm3calorific value.
2. A mathematical model was successfully developed using equilibrium approach to predict rice husk gasification using air as gasifying agent between 500 and 1100 °C. The results of the model suggested an optimum temperature at 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 a root square mean error value of 7.58.
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% oxygen enrichment 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% H2, 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%, respectively.

5.2 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 model.
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 the best simulated value.
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 varying both the flow rate and percentage oxygen enrichment. This will lead to having 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.

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