Gasification of Bagasse

For countries growing sugar cane there is a large potential of sugar cane bagasse for use in energy production. Bagasse is burned in the sugar mills to produce heat for the drying process in the sugar production. But the utilisation efficiency of the bagasse is very low in the conventional processes of both combustion and heat exchange which needs improvement so that surplus of bagasse could be used for electricity production.

Gasification

Gasification is a widely studied and applied technology to produce a mixture of combustible gases consisting of several sequential processes which include drying, pyrolysis to give gases, tars and char, cracking and oxidation of tars and gasification. The solid fuels can be effectively harnessed by converting them into a gaseous combustible producer gas which has a gross calorific value of 3.5-5 MJ N/m3 comprises mainly of carbon monoxide (25% v/v) and hydrogen (15- 20% v/v). It can be combusted in suitable burners with flame temperatures exceeding 1000degC and can be used for industrial thermal applications.

Reactors

Two main classes of chemical reactors have been employed for bagasse gasification: fixed bed and fluidised bed reactors. Fixed bed, updraft, downdraft and open core reactors are, in general, of very simple construction and operation, and avoid the excessive costs of feedstock pulverisation. These reactors can operate at high carbon conversion; long solid residence times and low ash carry over. The quality of the produced gas is better for the downdraft configuration. But scaling up of this system cannot be done to large capacities. On the other hand the complex and expensive technology of fluidised bed reactors allows very huge capacities, very good solid-gas contact and easy scalability, but with pulverized feed stock.

Design

The fixed bed reactor can gasify sugar cane bagasse and wood chips for production of gas that can be used in an internal combustion engine to produce electricity for a rural community. The design of gasification units is often based on feedstock reactivity and gasification characteristics. The general system comprises of a reactor, a gas conditioning system, a bagasse feeding system and the instrumentation and controls. For example, a downdraft, throat less and open-top reactor with an internal diameter of 75 cm and an active bed height of 1.25 m. can be used for a thermal output of 1080 MJ/ h. High temperature resisting firebricks can be used for the hot face followed by cold face insulation. A gas conditioning system consisting of a dry dust collection system can be used to eliminate the problem of wastewater. A high temperature char/ash coarse settler and a high efficiency cyclone separator can be used along with a high temperature resisting induced-draft fan.

Gasification characteristics

Gasification characteristics can be grouped into: thermo chemical (ash content, volatile products, reactivity of volatile products, etc.), intraparticle rate (thermal properties, moisture content, size, kinetics and energetic of chemical processes, etc.) and extra-particle rate (heat transfer from reactor to particle, residence time and mass transfer conditions depend, in their turn, on the type of gasification unit). This point is very important because the knowledge of gasification characteristics of bagasse is a crucial factor in making the technology attractive for wide use in industrial and commercial use.In general, for the sake of economy, a gasification plant should be able to operate with different, locally available feed stocks, whose nature and condition, presumably, change in the course of the year.