Energy Generation from Paddy Straw: An Analysis of Bioenergy Models
In India, paddy is cultivated in about 43.95 million hectares producing about 106.54 million tonnes of rice and approximately 160 million tonnes of straw with a ratio of 1:1.5 for rice grain produced to straw produced. Punjab produced 11.27 million tonnes of rice, which is 10.6 per cent of all India’s total production for the year 2013–14 and produced a total of 16.90 million tonnes of paddy straw. Of the paddy straw produced, some part is used as a fuel for modern biomass power plants, brick kilns, cardboard making, mushroom cultivation, and some portion is used to fuel domestic biomass cookstoves in rural areas. Portions of the paddy straw that remain uncollected in the fields due to a combined harvesting technology are not burned and are eventually ploughed back into the fields, which serve as beneficial manure for the upcoming crops. Flooded rice fields also add up additional methane, a potential greenhouse gas produced by the bacterial community under anoxic conditions. But, due to surplus paddy straw and problem associated with its storage, farmers sell paddy straw at an uneconomical price of `500 per metric tonne leading to nearly two– thirds of it being burned openly in the fields to quickly prepare it for sowing the next wheat crop.
Paddy Straw Burning Leads to Production of GHGs
Researchers suggest that open field burning of paddy straw contributes heavily towards production of harmful greenhouse gas (GHG) emissions including polycyclic aromatic hydrocarbons (PAHs), as well as polychlorinated dibenzo-pdioxins (PCDDs), and polychlorinated dibenzofurans (PCDFs), referred to as dioxins. Experimentally, it has been evaluated that one tonne of paddy straw releases 3 kg particulate matter, 60 kg CO, 1,460 kg CO2, 199 kg ash, and 2 kg SO2. Local paddy straw burning affects the environment as these air pollutants have significant toxicological properties and are notably potential carcinogens. This article presents experimental evaluation of paddy straw utilization via two bioenergy routes, viz., biomethanation for power generation and paddy straw pellet for household cooking needs. Biomethanation of paddy straw has been explained by actual experimental data from a demonstration scale biogas power of generation scale 3,800–4,000 m3 of biogas per day. As an alternative to cattle dung cake for cooking application, paddy straw pellets were made and its potential was experimentally validated at lab scale in micro gasifier based forced draft improved biomass cookstove. Further over citing huge potential of biogas from paddy straw, energy and cost–analysis of demonstration scale biomethanation process is presented.
Since renewable energy resources vary according to geographical conditions, bio-energy generation from paddy straw has a wide scope in Punjab and other northern states of India.
Anaerobic Digestion of Paddy Straw
The anaerobic digestion technology is a most efficient way in terms of energy output/input ratio for handling biomass resources to produce energy and bio-fertilizer. Biomethanation of paddy straw presented here consists of actual field experimental data taken from demonstration scale biomethanation plant at Fazilka, Punjab. Paddy straw is received in bales from the entire region of Fazilka, Punjab, and is stored in the storage unit. Further, the paddy straw is manually spread over the width of the conveyer belt, to be fed into the pulverization unit, for its size reduction to a level of 3–5 mm. The proximate analysis revealed that untreated paddy straw contains up to 10.0 per cent moisture and 90.0 per cent total solids on wet weight basis, while 84.0 per cent and 16.0 per cent volatile solid matter and ash matter, respectively on dry weight basis. The ultimate analysis resulted into 40.00 per cent carbon, 5.50 per cent hydrogen, and 0.75 per cent nitrogen content on a dry-weight basis. Upon elemental analysis, it has been found that the amount of nitrogen content present in rice straw biomass is very low, C/N ratio = 54.0. Compositional analysis of paddy straw revealed 39.90 per cent cellulose, 24.0 per cent hemicellulose, and 5.6 per cent lignin.
The average capacity of paddy straw pulverization unit is 1.0 tonne/h. This unit is powered by an electric motor of 75.0 kW, which consumes nearly 94 kWh energy per hour of operation. This unit also consists of a pulverized paddy straw collection system followed by aspirator system for the collection of dust generated during the pulverization process. The aspirator unit is powered through electrical power of 30 kW, which consumes 37.5 kWh energy per hour of operation.
Biomethanation is carried out in two anaerobic digesters (designed in house) of 3,400 m3 water volume capacity. The prepared paddy straw substrate is fed to the two digesters through the feeding unit using pumps. No external heating source is provided in the digester as the annual mean temperature in the area lies within mesophilic range. Loading rate was kept constant at 6.75 tonne VS/ day to maintain 8–10 per cent TS in the digester, while the digester was maintained at a hydraulic retention time (HRT) of 30 days based on previous work done by the authors. The digested slurry was passed through two horizontal solid–liquid separating machines with a slurryhandling capacity of nearly 8.0 m3 /h. The system is able to separate solid material at the rate of nearly 600 kg/h having a moisture content of about 65 per cent. The separated liquid is recycled to prepare paddy straw substrate in a blending tank.
Table 1 shows the initial parameters of conducting continuous feed anaerobic digestion of paddy straw. C/N ratio of the pretreated paddy straw was maintained by adding urea at a rate of 18–20 g/kg of paddy straw on a dry basis. The digester was fully charged with fresh cow dung for startup and feeding of paddy straw was gradually started.
The hydrogen sulphide level in biogas has to be reduced below 50 ppm for engine operation. The hydrogen sulphide scrubber unit consists of a 5.5 kW electric motor to power a booster pump, which pumps raw biogas through the scrubber unit. An electric motor having 5.5 kW power is used to circulate the digested slurry in the scrubbing unit. The total power consumption in hydrogen sulphide scrubbing unit has been 11 kW, which utilizes 13.75 kWh energy per hour of operation. Power generation unit consists of 1.0 MW 100 per cent biogas. The generator produces 1.2 MW per hour electrical energy through a three-phase 415 V alternator. Ten tonne/d of paddy straw is pulverized and fed to anaerobic digesters, which produce nearly 3,800–4,000 m3 of biogas per day with methane and carbon dioxide content in the range of 50–55 per cent and 40–45 per cent, respectively. The hydrogen sulphide content in produced biogas varies from 500 to 600 ppm. Average specific biogas production has been found in the range of 390–440 m3 /tonne of total solids fed to the plant. Specific methane production yield has been observed in the range of 200–220 m3 / tonne of total solids for standard operation of 12 months. Figure 1 shows the process flow of paddy straw to biogas generation.
Maximum and minimum biogas generated per tonne of total solids in present case is 390 and 440 m3, respectively after a short, mechanical pretreatment.
Energy and Cost– Benefit Analysis of Biogas Production
For the energy balance, calculations are made from the point of paddy straw pretreatment for biogas production. From Tables 2 and 3, it is evident that the conversion of paddy straw to biogas via pulverization achieves a net positive energy of 655 kWh/tonne and cost benefit of `6,916/ tonne of paddy straw. It was revealed that the use of rice straw for biogas production can generate a positive net energy balance between 70 per cent and 80 per cent.
This shows that pretreatment of paddy straw is necessary to reap a higher methane yield. The pretreatment followed by biomethanation will enable the economically competitive use of paddy straw for energy generation. This will lower the negative environmental impact during burning of paddy straw in open fields.
Total Energy Yield of Biogas and Bioethanol Productions
It is evident from Table 4 that the total obtainable energy yield from biomethanation route is 30 per cent more than bioethanol route. If all the surplus paddy straw biomass, which accounts for 11.70 million tonnes in Punjab, is brought to biomethane production, it will produce energy equivalent to 2.238 Mtoe and upon converting it to bioethanol, it will produce energy equivalent to 1.564 Mtoe.
Paddy Straw Pellet for Improved Biomass Cookstove
Farm-collected paddy straw biomass was dried and pulverized for pelletization. Paddy straw was air dried for five days followed by drying in hot air oven at 105±1° C for 12 hours and pulverized. Pulverized paddy straw was mixed with standard binder and pellets were made by a pelletizer of capacity 100 kg/h.
Water boiling tests were conducted in laboratory for the estimation of thermal performance of improved biomass cookstove fuelled by paddy straw pellets. Thermal efficiency of the cookstove was measured as a ratio of useful heat generated by the combustion of pellets in improved cookstove to the heat (theoretically) produced by complete combustion of a given quantity of pellet in the stove (based on the net calorific value of the fuel).
The cookstove was tested for its emissions (CO, CO2, and total particulate matter) simultaneously along with the testing of thermal efficiency. Measurement of CO and CO2 values in ppm were performed using standard procedure. Total particulate matter was monitored through the stack monitoring system. Thermal efficiency of the improved biomass cookstove was found to be 36.11±0.38 per cent when fuelled by paddy straw pellets which is equivalent when the same stove is fuelled with other fuels. Emissions were calculated on total CO2 equivalent per tonne of paddy straw pellet fuelled in the aforementioned biomass cookstove. Figure 2 presents the trends in thermal efficiency during laboratory testing and CO2 equivalent emissions when 1 tonne paddy straw pellets will be combusted in improved cookstove.
The average value for CO2 equivalent was found to be 648.76 kg/tonne of paddy straw pellet. The value shows a significant decrease in emissions when compared to CO2 equivalent emissions from burning 1 tonne of paddy straw in the open field which comes to be 2,150 kg CO2 e/ tonne as mentioned in Table 5.
Global Warming Potentials
As a part of life cycle assessment of the technologies for the utilization of paddy straw for bioenergy production, the global warming potential for biogas (power), improved biomass cookstove, and bioethanol was calculated. Global warming potential (GWP) is an index defined as the cumulative radiative forcing between the present and a chosen later time ‘horizon’ caused by a unit mass of gas emitted now. It is being used to compare the effectiveness of GHGs to trap head in the atmosphere relative to standard gas, generally CO2. The GWP for CH4 (based on a 100-year time horizon) is 21, N2O is 310, CO is 2, and particulates (PM) is 190. The GHG emissions in terms of kg CO2 e/tonne of paddy straw is presented in Table 5. Emissions from open field burning is considered to be a base case and accordingly calculations were made for each activity. It was found that 1 tonne of paddy straw if diverted from burning in open field can produce 8 GJ for biogas, 5.6 GJ for ethanol, and 5.0 GJ when used as paddy straw pellets with 36 per cent biomass cookstove efficiency. The system boundary taken into consideration while making calculations is depicted in Figure 3. Global warming potential of all three technologies as mentioned in Table 5, suggests that significant emissions can be controlled by diverting paddy straw from open field burning. Since all three routes mentioned here have nearly the same net GWP, these the global warming potential for biogas (power), improved biomass cookstove, and bioethanol was calculated. Global warming potential (GWP) is an index defined as the cumulative radiative forcing between the present and a chosen later time ‘horizon’ caused by a unit mass of gas emitted now. It is being used to compare the effectiveness of GHGs to trap head in the atmosphere relative to standard gas, generally CO2. The GWP for CH4 (based on a 100-year time horizon) is 21, N2O is 310, CO is 2, and particulates (PM) is 190. The GHG emissions in terms of kg CO2 e/tonne of paddy straw is presented in Table 5. Emissions from open field burning is considered to be a base case and accordingly calculations were made for each activity. It was found that 1 tonne of paddy straw if diverted from burning in open field can produce 8 GJ for biogas, 5.6 GJ for ethanol, and 5.0 GJ when used as paddy straw pellets with 36 per cent biomass cookstove efficiency. The system boundary taken into consideration while making calculations is depicted in Figure 3. Global warming potential of all three technologies as mentioned in Table 5, suggests that significant emissions can be controlled by diverting paddy straw from open field burning. Since all three routes mentioned here have nearly the same net GWP, these technologies have to be used based on requirement since one technology cannot be the solution provider for mitigating open field burning.
Paddy straw burning is a serious concern in India and has been driving the attention of policymakers and researchers. The authors did in-depth study for best utilization of paddy straw for sustainable energy production and to reduce resulting emissions in terms of GHGs equivalent. The analysis of biomethane production from paddy straw revealed that this route of energy conversion is most efficient in terms of obtainable useful energy and global warming potential. The power generation data showed that the biomethane results into electricity generation of 777.0 kWh/ tonne of paddy straw with output/ input energy ratio of 5.5. However, bioethanol production potential analysis showed an electricity equivalent of 544.25 kWh/tonne of paddy straw. Nevertheless, bioethanol is a ray of hope in competing with existing petrol-based motor vehicles but biomethane provides an added advantage of reaping extra energy from the same amount of paddy straw and on the other hand providing valuable manure for sustainable agriculture. The pelletized paddy straw can be used in improved biomass cookstoves to meet thermal cooking energy requirement as the results showed in reduction of indoor air pollution as compared to open field burning. The analysis further revealed that biomethanation of paddy straw reduces net global warming potential by 2,750 CO2 e kg emissions/tonne. However, bioethanol production showed net global warming potential reduction of 2,549 CO2 e kg emissions/tonne. The pelletization of paddy straw for improved cookstove showed net global warming potential reduction of 2,459 CO2 e kg emissions/tonne. The overall analysis of conducted study reveals that the utilization of paddy straw for biomethane production through anaerobic digestion route is the best way in terms of energy and environmental economics. Decentralized and centralized system of biogas production commercial plants can be suitably set up at a cluster level of villages to minimize logistic cost. The available energy can be suitably used to supply clean and green cooking fuel, power generation as well as vehicular fuel applications depending upon the need in the vicinity.
Dr Ram Chandra, Mr Abhinav Trivedi, Mr Bhaskar Jha, Mr Amit Ranjan Verma, and Dr Virendra Kumar Vijay , Centre for Rural Development and Technology, Indian Institute of Technology Delhi, New Delhi, India; Email: email@example.com.