The blessing of crop residue is in its capacity to provide a source of affordable and easy accessible energy for every-day living essentials [1]. The curse of the crop residue is in its interference with a smooth transitioning from one crop to the next during the harvest and planting season [2]. India experiencing an un-precedent green revolution since mid-sixties. The increase in crop productivity has been largely due to an increase in yield and double cropping practices. Increased yield in grain has increased the available non-grain biomass. For rice crop, the ratio of the above ground biomass to grain is 1.5 to 2. Mechanical or manual harvest practices leave a tall stubble of at least 150 mm tall in the field [2]. The higher yield and double cropping have forced the farmer to engage in an intense race to remove rice stalks immediately from the field in order to prepare the land to sow the following wheat seeds. The farmers in Haryana and Punjab have 4 - 5 weeks to complete harvesting and begin sowing.

Mechanical chopping and incorporation of chopped rice into soil and seeding in stubble with happy seeder have helped in dealing with excess rice straw [3]. However, the economics of owning the equipment and a slow speed of operations have not solved the problem of excess straw. Farmers often resort to burning the residue to cope with the shortage of time in cleaning the field. The images in Figure 1 show that crop residue burning in Punjab and Haryana within a span of three days in the last part of October 2020. Many of the pollutants found in large quantities in biomass smoke are known or suspected carcinogens and could lead to various air borne/lung diseases. The pictures are testimony to the fact that the current policies and regulations for lessening the burning of biomass are not working to their full potential. Crop burning is not limited to Punjab and Haryana. Table 1 lists the major states that produce crop residues. Uttar Pradesh generates nearly 60 million tonnes (t) out of which 13.34 million t, or almost all surplus crop residue are burned. Punjab at more than 50 million t ranks second. The State of Haryana with 27.83 million t ranks 10^th in the list. Punjab is the first and Haryana the fourth in crop residue surplus. It is estimated that in Haryana, 10% of the straw is harvested by hand and 90% is combined harvested.

Roughly 55% of the machine harvested rice is completely burned and the rest

are partially burned. Although data in Table 1 has a few anomaly (e.g. West Bengal) but it highlights the huge amount of biomass, 91.75 million t that is burned each year in India. That is a lot of bioenergy that goes into smoke. The burning of paddy straw leads to loss of precious nutrients as nearly 25% nitrogen and phosphorus, 50% sulfur and 75% of potassium uptake from soil are retained in the crop residues. It has been estimated that burning of 1 tonne of paddy straw accounts for loss of 5.5 kg nitrogen, 2.3 kg phosphorus, 25 kg potassium and 1.2 kg sulfur [4].

In all India, 26% of the rural households use crop residues while 64% rely on wood fuel. Much of the crop residues about 45% are used as animal feed, fuel, and in various industrial processes [2]. Only 12.2% of the crop residue is used for energy production. The calorific value of the crop residue ranges from 15.1 MJ/kg for oilseed stalks to 17.47 MJ/kg for cotton stalks with an average of 16.5 MJ/kg based on the dry biomass. The amount of heat energy and power production from excess biomass can be estimated from the annual available biomass

times the heat energy content of the biomass times the efficiency of converting that energy into electricity. Using this calculation, the amount of crop burned in India at 92.81 million t (Table 1) will have the potential to produce 1531.37 million GJ or 36.46 million t oil equivalent (t_oe^, oil has roughly 42 GJ/toe). According to EIA, the primary annual energy consumption in India has nearly tripled between 1990 and 2018, reaching an estimated 916 million toe [5]. The energy use pattern by Indian national shows that more than 27% of energy in the country comes from biomass and waste materials. Diverting burned crop residue to useful energy will contribute another 4% to the energy mix [5].

Dr. Ram Chandra [6] proposed two solutions to deal with excess rice straw: 1) on-site utilization and management that would consist of incorporating crop residue in the soil, animal feeding, and minor applications for various purposes; 2) off-field utilization that would consist of direct combustion for heat, power, ethanol production, and biogas production. Identification of techno-economically viable biomass conversion technologies for crop residue management will lead to few emerging areas of development in the value chain of biofuel and biomass energy technologies. A few of these technologies are associated with the first generation of biofuels like biodiesel, bioethanol, biogas, biomass gasification, and municipal waste pyrolysis.

Figure 2 is a proposed strategy for post-harvest handling of straw to minimize burning and to increase utilization of crop residue. Land application and collecting/removing of biomass are encouraged. The hand harvested and baled biomass are fed to animal feed, produce biogas, or converted to pellets. Pelletization is the central operation in extending the use of biomass to downstream. Once pelletized, the biomass is bagged and stored for shipping to heat and/or power application and more advanced liquid and gaseous fuels. A portion of pellets can be roasted to produce charcoal/biochar for energy and land applications.

Reference [2] suggests a combination of distributed and centralized processing operation for the growth of bioenergy at scale.

Figure 3 lists the current state of technologies that use biomass feedstock. TR (technology readiness) is the developmental state of each technology. Combustion technology at TR 9 are the oldest technology that is relatively well-developed. Pellets and briquettes have an improved combustion properties compared to lose biomass. Devices and pelletized biomass can burn biomass efficiently with the least emissions. Roasting biomass at temperatures 200˚C - 300˚C in the absence of or a reduced oxygen increases the carbon content and enhances the calorific value of the biomass [9].

Large scale (production rate > 45,000 t) of producing torrefied biomass or biochar is not commercially available [10]. The cost of making torrefied biomass is a barrier against full scale commercialization of the product. Higher temperature gasification and pyrolysis of biomass are practiced but still at a pilot demonstration scale. Research and demonstrations are underway to convert the syngas from gasification to biomethane. The production of lignocellulosic ethanol and sustainable aviation fuel (SAF), and marine fuels are in the research and development stage. Advanced fuels will be discussed further in the following section.

After examination of several existing and promising biomass conversion technologies outlined in [20] [21] we concluded that a speedy cut and baling the left-over residue after combine will reduce the need of farmers to burn their residue. This will not happen unless a value proposition is created for the farmers. A pelleting plant will turn the biomass bales or chopped up stalks into pellets for storage and transport. The pellets will be available for applications like combustion for heat and power and biogas production. In future, the engineered pellets will meet stringent reaction specifications to produce advanced cellulosic liquid biofuels like ethanol, diesel and jet fuel with high conversion rates. The authors believe that these technologies will not come on stream without solving the variability in feedstock. Considerable research to date shows the pre-treatment of straw for initial hydrolysis can improve the efficiency of converting biomass to biogas and methane.

UNEP [20] published a monogram in 2013 titled “Technologies for converting waste agricultural biomass to energy”. The publication outlined the state of biomass conversion technologies as presented in Figure 3. According to a more recent study conducted by IRENA [13], it appears there has not been much progress in biomass conversion technologies since 2013. The identified barriers against the commercialization of fuels from biomass and investment risks are persisting

The technologies that are fully commercialized to-date are related to biomass densification and combustion. These two technologies are fully at the readiness TR 8 and 9. Although, bio-char has been commercialized at a relatively small scale, problems with odor, emissions, and a potent dust are yet to be worked out. Anaerobic digestion is also a proven technology at a commercial scale in some parts of the world. The low calorific value biogas is mostly used in internal combustion engines to produce power.

The specific aspects of biogas that require further research relate to operating the gasifiers in cold climates and on selecting the blends of fibrous cellulosic material for its optimal conversion to biogas. The production of pyrolysis oil is commercialized but its full application except burning as a burner fuel has not been discovered. Although cellulosic bioethanol is claimed to be at the verge of commercialization [21], the evidence from pioneer plants in the U.S. shows that challenges in variability in feedstock properties [22] [23] that would lead to a low conversion rate yet to be overcome.

The authors acknowledge the financial support received from Ministry of Human Resource Development (MHRD), Government of India for the Project ID-904 under the Scheme for Promotion of Academic and Research Collaboration (SPARC) and support of the CCS Haryana Agricultural University, India and University of British Columbia, Canada.

The authors declare no conflicts of interest regarding the publication of this paper.

Sokhansanj, S., Yadav, Y.K., Lau, A., Yadvika, Ebadian, M. and Verma, K. (2021) Identification of Techno-Economically Viable Crop Residue Utilization in India. Journal of Sustainable Bioenergy Systems, 11, 94-104. https://doi.org/10.4236/jsbs.2021.112007