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Biomass Gasification by Plasma Pyrolysis: An Alternative to Stubble Burning?

Burning of crop residue is prevalent in North India although banned by the National Green Tribunal. Wheat stubble burning is a recent practice consequential to growth in machine harvesting. The bulk of the crop residue is contributed by cereal crops like wheat and paddy as well as sugarcane leaves adding up to more than 500 million tonnes annually. The burning of crop residue is estimated to have released more than 150 million tonnes of carbon dioxide (CO2), over 9 million tonnes of carbon monoxide (CO), 0.25 million tonnes of oxides of sulphur (SOX), 1.3 million tonnes of particulate matter and large amounts of black carbon. These are significant contributors to environmental degradation and result in the north Indian and the melting of Himalayan glaciers. The burning elevates the sub-surface temperature to destroy bacterial and fungal populations critical for soil fertility. Stubble burning also causes loss of soil nutrients like nitrogen, phosphorus, potassium and sulfur (1).

An alternative to burning is gasification where carbon-containing materials are combusted in the presence of Oxygen. The reactions release gaseous products containing mostly CO, H2, CO and a small content of higher hydrocarbons. A variant of the process, pyrolysis is realized when the entire heat is supplied externally and the process progresses without Oxygen. Pyrolysis is ideal for converting lignocellulosic products such as wood or straw into hydrogen or syngas.

The entire carbon/hydrogen fraction of the biomass can be used for syngas production if the reaction takes place at elevated temperatures. Maximum biomass-to-syngas conversion is realized if all the carbon is converted into Carbon monoxide. Since the Carbon atoms exceed the number of Oxygen atoms in biomass, additional Oxygen has to be supplied for the gasification by adding air or steam.

In the gasification process, a common problem is the production of tar formed from complex hydrocarbon molecules. At low temperatures, only a small fraction of CO and H is formed while the Methane and higher aromatic hydrocarbons dominate. Excess CO produced by the partial oxidation process dilutes the syngas. Partial gasification has limited flexibility to control syngas composition.

The search for optimising syngas production with controllable composition has been the driver for Plasma Pyrolysis based on external energy supply for biomass gasification. The high energy content and chemical purity of the plasma state contribute to minimising contamination and dilution of the product syngas while enabling control of the syngas composition and yielding higher calorific value. As the pyrolisis process is driven by the thermal energy of the plasma and not combustion reactions, the process can be applied to a wide range of organic materials and biomass. However, one must note the need to supply energy to produce high-temperature plasmas while evaluating the techno-economic benefits and feasibility.

Plasma pyrolysis is a thermo-chemical process that offers better control of operating temperature, higher process rates, and compact reaction volume and is capable of optimizing the composition of produced syngas. The extremely hot plasma environment is used for breaking down the biomass. The plasma environment is rendered highly reactive due to the presence of charged and excited species and can catalyse all forms of chemical reactions. The high enthalpy and temperature of plasmas compared to combustion aids in low flow rates, low product gas dilution and compact reactors. The specific gas yield is approximately 1.4–2.5 m3 per 1 kg of feedstock (2)

Plasma gasification does not need a high mass flow rate of the gasifying medium compared to that used for conventional gasification. This results in reducing the dilution of syngas product by gas supplied to the reaction and enhancing its heating value. With the endogenous gas-fed plasma torch developed and patented by FCIPT, this becomes evident. Our innovation utilises the gas generated when organic matter is pyrolysed in the torch feed. An inline suction pump sucks the product gas and sends it through a filter to remove soot particles. It is then fed to the plasma torch. We had a 35 % gain in energy efficiency with this torch. It also increases electrode life by reducing the electrode erosion rate.

Many Laboratory-scale investigations of Plasma gasification of biomass have been reported in the literature. In studies of syngas generation from wood in plasma pyrolysis using AC air plasma torches, (3) it was found that the high flow rate of plasma ensures good mixing of plasma with treated material and a uniform temperature distribution in the reactor. However, there is syngas dilution by plasma gas components if air or nitrogen are used to drive the torch (3,4). The use of inert gas mixed with Hydrogen (5,6) eliminates this disadvantage, though at a higher cost. In steam plasma gasification, the product gas is used as plasma gas in a modified plasma torch (7).

The process is a realisation of the power-to-gas (P2) concept of solar energy storage if the plasma excitation energy is derived from solar power. Thus the pyrolysis conversion of biomass into energy gases is ideal for the storage of renewable electricity with its large diurnal and seasonal variations.

The advantages of plasma gasification can be summarised as follows (8):

1. Energy for gasification is externally provided through the plasma medium making the process independent of the material under treatment, providing flexibility, fast process control, and freedom in choosing the chemistry. 2. A wide range of biomass feedstock, including agriculture waste and biodegradable fractions of municipal waste can be gasified.3. The temperature in the reactor is determined by the externally controllable power supplied to the plasma and material feed rate.4. The reactor volume can be maintained at high temperatures with uniform temperature distribution. This minimises the production of higher hydrocarbons, tars and other complex molecules.5. High energy density coupled with enhanced heat transfer efficiency achievable in the reactor allows for shorter residence times and large throughputs in compact plants.6. Highly reactive environment and easy control of the composition of reaction products.7. With the variable, external energy input, low thermal inertia and instantaneous feedback control are possible.

For India, which has major environmental problems caused by stubble burning, there are three enablers for a focused thrust on promoting biomass plasma gasification. These are:

1. Waste to energy concept is gaining ground.

2. Indigenous, small-scale gasifiers which generate electricity from biomass are being built to treat agricultural biomass by several manufacturers.

3. Microwave and Graphite electrode torches with endogenous gas feed have been developed at the FCIPT division of the Institute for Plasma Research, Gandhinagar.

(2) V. M. Bateninet al., “Thermal methods of reprocessingwood and peat for power engineering purposes”, ThermalEngineering,57, 2010, pp. 946–952.

(3) Rutberg P.G.; Bratsev A.N., Ufimtsev A.A. 2004. Plasmochemical technologies for processing of hydrocarbon raw material with syngas production. J. of High Temp.Mat. Process., 8: 433–446.

(4) Zasypkin I.M.; Nozdrenko G.V. 2001. Production of acetylene and synthesis gas from coal by plasma chemical methods. Thermal Plasma Torches and Technologies, Vol II., ed.O.P. Solonenko, Cambridge Interscience Publish.: 234–243.

(5) Zhao Z.L.; Huang H.T., Wu C.Z., Li H.B., Chen Y. 2001. Biomass pyrolysis in an argon/hydrogen plasma reactor. Chem. Engineering & Technology, 24: 197–199.

(6) Zhao Z.L. 2003. Plasma gasification of biomass in a downflow reactor. Abstract of Papers of the American Chemical Society, 226: U536-U536 048-FUEL Part 1

(7) Brothier, M., et al. 2007. Syngas production from biomass gasification by plasma torch. Proc. of 18th ISPC (ed. K Tachibana et al), Kyoto, Book of Abstracts: 193,

(8) Milan Hrabovsky, Thermal Plasma Gasification of Biomass

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