Basic Gasification Technology

Today, I would like to explain about basic study of gasification technology. This study is to develop biomass business from waste. Beside landfill gas, many countries use gasification technology to process their waste. This technology has opportunity in term of residual product than landfill gas. It produces 5% residual product only and often utilized as building material.

Gasification is a process in which a solid material containing carbon (e.g., biomass) is converted into a gas by reacting it in high temperature with oxygen which is present at level insufficient to support complete combustion. The aim is to produce a synthesis (NOT SYNTHETIC)  gas or syngas consisting mainly of hydrogen and carbon monoxide. Syngas can then be used for chemical or fuel synthesis (hence the name), or as a fuel for direct combustion.

The main steps are :

Biomass is heated in pyrolisis stage to drive off the volatile components that typically make up 70-86% of the dry biomass, leaving a solid char (or biochar). Depending on the details of the gasifier, the heat can come from external sources or from combustion of some of the pyrolisis products. The volatile components are mainly hydrogen, carbon monoxide, carbon dioxide, methane, hydrocarbon gases, tar, and water vapor. Gas stream composition depends on pyrolisis temperature, pressure, and residence time, as well as the nature of the biomass feedstock. Where the heat for the gasification stage comes from combustion of a proportion of the pyrolisis char inside the gasifier, the exothermic reactions are represented by the equations :

C + O2 -> CO2  ∆H = -393.8 kJ/mol
C + 1/2O2 -> CO2  ∆H = -123.1 kJ/mol

Next comes the gasification stage proper, where higher temperature crack tars and hydrocarbons in the pyrolisis gas stream and char are partially oxidized. Carbon is converted into CO and hydrogen in a reaction called the water gas reaction in which carbon reacts with water vapor derived from the original biomass :

C + H2O-> CO + H2  ∆H = 118.5 kJ/mol

Another key gasification reaction is the Boudouard reaction :

C + CO2 -> 2CO    ∆H = 159.9 kJ/mol

In these reversible, endothermic reaction (3 and 4), higher temperature favor the production of hydrogen and carbon monoxide. Lower pressures also favor the production of carbon monoxide, while higher pressures favor the production of carbon dioxide.

The other main reaction is the exothermic water gas shift reaction in which CO reacts with steam to form CO2 and additional hydrogen :

CO + H2O-> H2  + CO2  ∆H = -41 kJ/mol

There is also an important methanation reaction :

C + 2H2-> H2  + CH4  ∆H = -87.5 kJ/mol

The above reactions and other involving feedstock impurities take place simultaneously during the gasification process. The relative proportions of gases at the gasifier exit depend on process conditions and the composition of the biomass feedstock. The position of the steady-state equilibrium depends in the normal way on the temperature and pressure, but at low temperatures the rate of reaction may be so low that equilibrium compositions are never reached in practice. For example, below 700 C, the water gas shift reaction proceeds so slowly that the product composition is said to be ‘frozen'[2]. Gas-solid reactions are slow compared with the gas-phase reactions. Another key parameter affecting the outlet gas composition is the amount of oxygen relative to what is required to support complete combustion

Tentang andihendra

Andi hendra paluseri adalah perantau asal medan yang menyelesaikan pendidikannya S1 di ITB jurusan teknik elektro (Tenaga Listrik).
Pos ini dipublikasikan di pengalaman di ITB. Tandai permalink.

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