Decentralized off- grid electricity generation from biooil produced using the intermediate pyrolysis of agricultural residual waste

Abstract

While the sun is the principal energy source for our earth, energy in the form of sunlight has little value in our modern global economy. To obtain food and fuel, society relies upon biomass to convert light energy into chemical energy (via photosynthesis) on the earth, which makes the biosphere act as a vast biomass generation facility. At present, nearly 100 billion tons of biomass is already present on land (Naik et al., 2010), but only a small portion of it is used at its full potential. A biomass generation facility can be seen as a large carbon sink. The issue of climate change can be resolved, to an extent, if sustainable management of these carbon sinks is put to practice. Use of biomass contributes to about 12% of today's world primary energy supply, while in many developing countries, its contribution ranges to even 40-50%. India is an agrarian country and biomass is abundantly available resource of the nation. A substantial amount of agricultural residue is produced in the country during harvesting season of crops. However, a large portion of these agricultural residues are currently not utilized efficiently, which is simply allowed to burn in the open fields, just to clear the fields from straw and stubble after the harvest of the preceding crop. The issue of this open field burning is a widespread phenomenon, specifically in Northern parts of India (Punjab and Haryana). Given the fact that 23%, 48% and 95% of rice straw residue produced in India, Thailand, and Philippines burns in open fields respectively. The open fields burning has become an environmental threat as well as health related issues (Gadde, Menke, and Wassmann 2009). Punjab is considered as the food bowl of India and, it is surprising to note that a large number of rice straw is burnt each year in Punjab, India alone. There is a strong need for an effective utilization of these agricultural residues so that the environmental impact of open fields burning can be reduced. At the same time rural area of the country is suffering with frequent power cuts and poor availability of electricity. Considering both these factors, an effective solution for rural area could be to make the use of the biomass for electricity generation. If agricultural residue is used for electricity generation, instead of burning it in open fields, it can address not only the issue of emissions, but also provide electricity to the rural area. Since fossil fuels (coal, oil and gas) have historically been cheaper and more reliable, their share is higher in India. Due to large coal availability, India has the largest percentage of coal based installed capacity for power generation. Given the specific CO2 emission number of coal as 1.04 kg of CO2/kWh, the emission from power generation for the year 2011 was high and stood at 579 Mt of CO2 (GoI 2011). It appears that emission intensive electricity generation trend would be difficult to reverse in the near future. Under such circumstances, there is a strong need to evolve a technology option that is not only environmentally neutral, but that actually absorbs emission from the environment and cut down the environment emission. Biomass is the only energy source which has the capacity to absorb carbon from the environment into its structure. Emission issues can potentially be mitigated by employing the large biomass resources of the country for the dual benefit of energy generation and carbon sequestration. However, to achieve this goal, a step change in technology and agricultural practices will be needed in rural India. In the present work the literature for the open fields burning is reviewed first and the options of utilizing agricultural residue are studied in particular. It is inferred that the solution to the issue of open fields burning and the shortage of electricity can be addressed by opting for an intermediate pyrolysis process. That is, the inputs and outputs of a smallscale intermediate pyrolysis process were investigated to obtain electricity generation and emissions data, along with the agricultural benefits. Further, chemical analysis of different samples of feedstock, biooil and its blend and biochar is undertaken for different tests to characterize the products and also to validate the process. The intermediate pyrolysis process utilizes the straw from the field and produces biooil and biochar. The biooil can be used for energy generation while biochar can be used in the field as a fertiliser. To investigate and obtain pertinent data for use of biooil blend in the engine, with respect to power generated and emissions released, an engine trial is carried out in the laboratory. Similarly, to investigate the effect of the use of biochar in the field, a field trial for growing of crops, onion and capsicum, was carried out. The data and the process parameters obtained by conducting these tests and trials are used for the further analysis in the present study. The intermediate pyrolysis process produces biooil and this biooil can be used for power generation in the blend. Since carbon emission from the biooil is environmentally neutral, power generation from biooil qualifies under renewable energy. Additionally, the use of biochar is considered as a low cost carbon sequestration as biochar could remain in the soil for the years to come. These two emissions-reduction advantages of the intermediate pyrolysis process (products of biooil & biochar) are on different time scales (few hours and several years respectively). Incorporating time value of the carbon emissions can resolve such disparity which is not put to practice till date. The present study proposed a novel method which incorporates this effect. The model is implemented to demonstrate the time value of carbon emission for the intermediate pyrolysis process. The advantage of the model is that it predicts the effect of emissions for a given process in terms of a single number (called as emissions discounting number). The model can be applied to any process in the universe. Green energy generation is environmentally neutral, while gray energy generation adds emissions in the environment. Literature suggests that biooil can be used for the power generation in the blend (up to 30%) with biodiesel or with the diesel. If biooil is blended with biodiesel the emissions resulting from fuel will be 100% environment neutral and the energy produced can be termed as green energy. If biooil is blend with diesel in that case only emissions released from burning of biooil will be environment neutral but emissions released from diesel will not. The energy in that case will be a mixture of green and gray energy. Referring to the literature, the treatment to such mixed mode of energy is not in practice and it lies outside the scope of current biofuel policy of India. Besides this, none of the present renewable energy source has carbon sequestration benefit. Therefore, the present method of accounting lacks provision to account this. To account correctly, such mixed mode of energy generation as well as carbon sequestration benefit, appropriate correction and modifications in the existing method of accounting and its implementation is must, for which a different approach is required. The present study, therefore, proposes an ISCB (Incentive scheme, Sustainability aspect, Carbon credit and Banking facility) model which accounts for a mixed mode of energy generation option. This model also considers the benefits like embedded energy, carbon credit based pricing system, banking facility and the other incentives offered by governments for the promotion of renewable energy. One of the products of the intermediate pyrolysis process is biochar and literature suggests biochar can be used as a slow-release fertiliser in the field. The use of biochar as a fertiliser can be a tool to bring down the cost of farming and hence and it can improve profitability of farmers. Besides this use of biochar in the field gives long-term carbon sequestration benefit to the farmer and absorbs the emission from the environment. On the social front co-operative activity of a central intermediate pyrolysis plant helps the farmer social benefit. Combining all these, the intermediate pyrolysis process has positive implications on all three fronts of sustainability, i.e. social, economic as well as environmental. In order to correctly estimate its implication a triple bottom line (TBL) analysis is carried out in the present study. Finally, based on the overall results, it is concluded that the intermediate pyrolysis process can prevent open-field burning, and produces biooil and biochar. The analysis of biooil and biochar helps explain the kinetics of reaction taking place in the pyrolyser. The process produces biooil and biochar, which can generate green energy, in blended with biodiesel, with very little de-rating of the engine performance, of the order of around 8% of the operating conditions, and emissions of NOx, HC and CO are higher compared to diesel. The process can produce char residue which can be used in the fields as a fertiliser and is also a low cost carbon sequestration option. The increase in the yield of a non-root crop, capsicum, was more than double. While the increase was only marginal in case of a root crop, onion, mainly due to the type of sand which was not appropriate for crop. The emissions discounting number of intermediate pyrolysis process can be as high as 10%, if entire char is used as a fertiliser in the field. Further, ISCB model can help smooth the implementation of intermediate pyrolysis process, the process accounts both green as well as gray component of energy and the effect of carbon sequestration in the model and predict the cost of emission from the process. The triple bottom line analysis shows improvement in the performance across all the three pillars of sustainability.