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.

Activated carbon from Coconut shell

Coconut shell

Activated carbon is a non-graphite form of carbon which could be produced from any carbonaceous material such as coal, lignite, wood, paddy husk, coir pith, coconut shell, etc. Coconut shell is used for manufacturing activated carbon. Coconut Shell is carbonized by using pit, drum, destructive distillation method etc.

Uses

Shell based activated carbon is extensively used in the process of refining and bleaching of vegetable oils and chemical solutions, water purification, recovery of solvents and other vapours, recovery of gold etc. It is used in gas masks and a wide range of filters for war gases and nuclear fall outs and for the removal of colour and odour of compounds for protection against toxic gases.

Activation process

Coconut shell based activated carbon units adopt steam activation process to produce good quality activated carbon. Activated carbon manufactured from coconut shell is considered superior to those obtained from other sources mainly because of small macropores structure which renders it more effective for the adsorption of gas/vapour. Steam activation and chemical activation are the two commonly used processes for the manufacture of activated carbon. However coconut shell based activated carbon units are adopting the steam activation process to produce good quality activated carbon.

Steam Activation

The process of steam activation is carried out in two stages. First, the coconut shell is converted into shell charcoal by carbonization process which is usually carried out in mud-pits, brick kilns and metallic portable kilns. The coconut shell charcoal is activated by reaction with steam at a temperature of 900-1100 degC under controlled atmosphere in a rotary kiln. The reaction between steam and charcoal takes place at the internal surface area, creating more sites for adsorption. The temperature factor, in the process of activation is very important. Below 900degC the reaction becomes too slow and is very uneconomical. Above 1100degC the reaction becomes diffusion controlled and therefore takes place on the outer surface of the charcoal resulting in loss of charcoal.

Properties

pH Value 6.5 - 7.5, Methylene value adsorption mgm / gm 190 – 350, Adsorption capacity at % by mass (min) 45, Moisture (max.) 5%, Ash (max) 5%, Hardness 90.

Reactivation

Activated carbon is extensively used for the process of refining and bleaching, but after utilization, the "spent" carbon, as it is called, can be removed and re-activated for further use. This is done primarily with granular activated carbon because particles will be too small to be effectively re-activated. This process allows for recovery of approximately 70% of the original carbon. This number also allows for any physically lost in the shipment process. The re-activated carbon is then mixed with a portion of new carbon for higher effectiveness and is then used in the process.