### What do we mean by Gasification?

Wood biomass can either be used directly as a fuel to heat a conventional type of boiler and produce steam for power production or a completely separate process can be supported where the wood is used to produce a gas. Gasifier technology has undergone many improvements in the last 50 years because of increased fuel prices and environmental concerns. Gasification has b ecome more modern and is now a quite sophisticated technology. The advantage of this technology is the operation of a decentralized energy conversion system which can prove to be economic on a small scale.

The main component of this conversion proce ss is the “Gasifier”, in essence a simple device consisting of a cylindrical container, which is made out of stainless steel. The simplest process – downdraft gasification, can be used to produce electricity on a small scale. In such a process, the gasifier is cal led a “Downdraft Gasifier” and the process that occurs inside the gasifier is explained be low. The process of gasification can be considered as a thermo chemical process, which converts biomass materials into gaseous components, which is referred to as “P roducer Gas”, containing carbon monoxide, hydrogen, methane and some other inert gases. A simple system mixes the producer gas with air and the resulting mix can be used to run a diesel engine with only relatively minor modifications. The diesel engine can be coupled with a generator in the normal way to produce electricit y as the final out put.

Downdraft Gasifier Downdraft gasification generally produces a low particulate and low tar gas so it is suited for power generation in small scale applications. The gasifier must have a chemical reactor where the above process is taken place. The fuel wood gets dried, heated, pyrolysed, partia lly oxidized and reduced inside this reactor. The four basic processes of gasification are noted below. 1. Drying of the fuel 2. Pyrolysis 3. Oxidation (Combustion) 4. Reduction

3⁄4 Drying of Fuel

The fuel wood pellets are heated and dried at the top of the gasi fier unit. Moisture contained in the wood pellets is remove d in this region to a level below 20%.

3⁄4 Pyrolysis

The dried wood pellets enter the second zone called the “Pyrolysis” zone. The gaseous products from devolatilization ar e partially burnt with the ex isting air. This process is termed “ Pyrolysis”.Both Pyrolysis and gasification turn waste in to an energy rich fuel by heating the waste under controlled conditio ns. In contrast to incinerati on, which fully converts the input waste into energy and ash, these processes deliberately lim it the conversion so that

com bustion does not tak e place d i rectly. Instea d, the waste is converted in to valuab le inte rm ediate s that can be f u rther pro c essed f o r m a terials recycling or energy recovery i.e. its convert to Pyrolysis gas and charcoal.

Charcoal Pyrolysis G a s Wood Pellets

Pyr o lysis Pr oc ess

3⁄4 Combustion

In the com bustion zone the outputs from the a bove zone, react with th e rem a ining char in the absence of oxygen at a te m p erature of around 800-900 °C.

Basic Reactions

C + O 2 C O 2

4 H + O2 2 H 2 O

C n H m + (n/2+m /4) O 2 n C O 2 + m / 4 H 2 O

3⁄4 Reduction In this region the hot gases form ed in the above process is converted in to “ Producer Gas” by the following two endotherm ic reaction s

.

Boudouard reaction: C + CO 2
. 2CO HR = + 160.9 kJ/m ol

Water gas reaction : C + H 2 O H 2 + CO HR = + 118.4 kJ/m ol As these re a c tions p r oce e d the tem p eratur e sinks progress i vely until it be c o m e s so low (700 °C) that the reaction rates are insignificant. This m eans th at the extent of the char reduction zone is dependent on the amount of energy entering the reduction zone and consequently also on the heat losses from th e reactor. Although ther e is a considerable overlap, each process can be considered to be occupying a separate zone, in which funda m e ntally different chem ical and therm a l reaction s take place. The f u el m u st pass through all of these zones to be com p letely converted.Composition of Producer Gas Constituent of wood gas at 650 o C Composition H 2 CO CH 4 CO 2 N 2 H 2 0 (% by weight ) 18.0 18.0 1.8 10.8 41.4 10.0

Composition of producer gas of fuelwood with 10% moisture level Source: VIT Research Notes 1648-Feasibility of Electricity Produc tion from Biomass by Gasification Systems

Overall efficiency of plant

Unit Efficiency (%)

Gasifier unit 75 Engine 33

Generator 92

Overall 22.5%

Overall efficiency (Source: VIT Research Notes 1648-Feasibility of Electricity

Production from Biomass by Gasification Systems)

Basically this efficiency will vary around 16% to 24% in present systems. This will be the overall efficiency in producti on of electricity, but using fl ue gas to dry the fuel wood we can increase the usage of energy in fuel wood.