Pyrolysis/ Gasification
Pyrolysis is also refered to as destructive distillation or carbonization. It is the process of thermal decomposition of organic matter at high temperature (about 9000C) in an inert (oxygen deficient) atmosphere or vacuum, producing a mixture of combustible Carbon Monoxide, Methane, Hydrogen, Ethane [CO, CH4, H2, C2H6] and non-combustible Carbon Dioxide, water, Nitrogen [CO2, H2O, N2] gases, pyroligenous liquid, chemicals and charcoal. The pyroligenous liquid has high heat value and is a feasible substitute of industrial fuel oil. Amount of each end-product depends on the chemical composition of the organic matter and operating conditions. Quantity and chemical composition of each product changes with pyrolysis temperature, residence time, pressure, feed stock and other variables.
Gasification involves thermal decomposition of organic matter at high temperatures in presence of limited amounts of air/ oxygen, producing mainly a mixture of combustible and non-combustible gas (carbon Monoxide, Hydrogen and Carbon Dioxide). This process is similar to Pyrolysis, involving some secondary/ different high temperature (>1000oC) chemistry which improves the heating value of gaseous output and increases the gaseous yield (mainly combustible gases CO+H2) and lesser quantity of other residues. The gas can be cooled, cleaned and then utilized in IC engines to generate electricity.
Pyrolysis/ Gasification is already a proven method for homogenous organic matter like wood, pulp etc. and is now being recognised as an attractive option for MSW also. In these processes, besides net energy recovery, proper destruction of the waste is also ensured. The products are easy to store and handle. These processes are therefore being increasingly favoured in place of incineration.
15.3.4.1 Different Types of Pyrolysis/ Gasification Systems
The salient features of different types of Pyrolysis/ Gasification Systems so far developed are given below.
(i) Garrets Flash Pyrolysis Process:
This low temperature pyrolysis process has been developed by Garrett Research and Development Company. In a 4 tonnes per day pilot plant set up by the company at La Varne, California, the solid waste is initially coarse shredded to less than 50mm size, air classified to separate organics / inerts and dried through an air drier. The organic portion is then screened, passed through a hammer mill to reduce the particle size to less than 3mm and then pyrolysed in a reactor at atmospheric pressure. The proprietary heat exchange system enables pyrolytic conversion of the solid waste to a viscous oil at 5000C. A schematic diagram of this system is given in Fig. 15.10.
(ii) Pyrolysis Process developed by Energy Research Centre of Bureau of Mines, Pittsburg:
This is a high temperature pyrolysis process to produce both fuel oil and fuel gas and has been investigated mainly at laboratory scale. The waste charge is heated in a furnace with nickel-chromium resistors to the desired temperature. The produced gases are cooled in an air trap where tar and heavy oil condense out.
Uncondensed vapours pass through a series of water-cooled condensers where additional oil and aqueous liquors are condensed. The gases are then scrubbed in an electrostatic precipitator before further use. It is claimed that one tonne of dried solid waste produces 300-500 m3 of gas, but the process is yet to be tested at full scale. A schematic diagram of this system is given in Fig.15.11.
(iii) Destrugas Gasification System:
In this system (Fig. 15.12) the raw solid waste is first subjected to shredding / size reduction in an enclosed shed. The air from this shed is taken up as intake air in the plant so as to avoid odour problems. The shredded waste is fed to retorts (heated indirectly by burning gas in a chamber enveloping it) through which it sinks under gravity and gets subjected to thermal decomposition. The produced gas is washed and most of it (85%) used for heating the retorts. The remaining 15% is available as fuel. The slag consists of mostly char.
