ENERGY  ALTERNATIVES: RENEWABLE
ENERGY AND ENERGY CONSERVATION
TECHNOLOGIES



Wood Gsification :

  Biomass gasification is basically conversion of solid biomass 
  (wood,  wood  waste ,  agricultural  residues ,  etc. )  into  
  combustible gas mixture  normally called producer  or low Btu 
  gas. The  process  is  typically  used for woody  biomass and 
  involves  partial  combustion  of   such   biomass .  Partial 
  combustion  occurs  when air supply is less than adequate for 
  complete  combustion of biomass.  It produces carbon monoxide 
  (CO) as well as  hydrogen (H2), both  are  combustible  gases. 
  Conversion to gas results in loss of up to 25% energy. Use of 
  gas  can be  highly  efficient  and  hence overall efficiency 
  could be very  high. Gas  can be fed  directly  into internal 
  combustion  engines (I.C. engines) thereby saving  commercial 
  fuels. Also, it can be  employed at any scale (above few kilo 
  watts ),  and  hence  is  ideally  suited  for  decentralised 
  applications -shaft power, electricity  or thermal energy. In 
  case of shaft power / electricity, the gas is basically burnt 
  inside an engine (diesel based - compression ignition engine) 
  with pilot diesel injection to start combustion. 

  Biomass gasification provides a valuable fuel for both mobile 
  and non mobile  uses. Producer  gas can  replace  natural gas, 
  gasoline  or  fuel  oils  used to make  steam  for generating 
  electricity and fire boilers, and produce heat for industries 
  and homes.It can also be used as fuel for internal combustion 
  engines  for  a  wide  array  of purposes. Gasification is an 
  efficient way to  extract heat from  biomass. It is estimated 
  that for  each  100 kcal of  potential  energy in solid fuels, 
  gasification can  extract  about 80 kcal in hot raw gas. This 
  is  more  efficient  than  many  devices  that  burn  biomass 
  directly in a hearth  or  fire box. Producer gas can be piped 
  short  distances  and used for industrial purposes like  fuel 
  kilns: brick kilns, ceramics, glass pottery, etc.,for boilers 
  in  rice  mills,  saw  mills,  cashew  industry  or for power 
  generation. In any case, while using producer gas for heating, 
  the burner must be designed for operation on low energy gas. 

  The function of a gasifier  is to convert  solid carbonaceous 
  fuel, such as wood  into  combustible gas by a combination of 
  oxidation,  pyrolysis  and  reduction  process (Talib,  et al., 
  1987).  The  major  chemical  reaction  taking place are both 
  exothermic  ( C+O2 -> CO2,  C+0.5O2 -> CO )  and  endothermic 
  (C+H2O-> CO+H2,  CO2+C-> 2CO).  In order to effectively carry 
  out  these  reactions,  the  solid  fuel  passes  through the 
  following zones: drying, pyrolysis,  combustion and reduction. 
  In drying zone the  temperature is around  150°C and moisture 
  in the biomass is driven  off. In  pyrolysis  zone, at  400°C 
  thermal break down of  wood / bio residues takes place in the 
  absence  of  air,  resulting  in  the  formation  of methanol, 
  acetic acid and  heavy  hydro carbon including tar. The solid 
  material left after  pyrolysis  is  primarily fixed carbon in 
  the form  of  charcoal. The pyrolysis material along with the 
  gases and the  organic  vapors  produced  passes  through the 
  combustion  zone. In  combustion  zone,  exothermic  reaction 
  takes place and the heat released is used for sustaining both 
  the pyrolysis and the reduction reactions. The temperature in 
  this  zone  ranges  from  1000-1500°C.  Combustion  zone  has 
  controlled introduction of air.

  By  forcing  the  vapors  through  a  narrow  area   directly 
  underneath  the  combustion  zone, organic  liquids  and  tar 
  formed  in  the  pyrolysis  zone are cracked. In the cracking 
  process, heat intensity  increases. The gases thus formed are 
  drawn  to  reduction  zone  where  endothermic reaction takes 
  place.  The  mixture  of  final  product  gases (producer gas) 
  consisting  of  18-25%  CO,  13-15%  H2,  3-5%  methane  (CH4), 
  0.2-0.4% heavy hydro carbon, 5-10% CO2, 45-54% N2, 10-15% H2O 
  and  particulate  matter is drawn into the clean up system at 
  about  250°C. In  clean  up  system, the  gas  is  cooled and   
  cleaned to remove  particulates and later introduced into the 
  engine. Commercially  available  biomass  gasification system 
  for power  generation covers the range of 3-500 kW. The major 
  areas  where this  system is relevant for power generation in 
  a decentralised manner  are:
  (a)   Village  electrification  in  remote  areas  which have 
        adequate biomass resources,
  (b)   Energisation  of  a  number  of  pumpsets  located in a 
        cluster,
  (c)   Captive  power  for  industrial  units located in rural 
        areas  having  extensive land area which can be devoted 
        for energy plantations, and
  (d)   Captive  power  for  industries that have biomass waste 
        processes such as paper mills, saw mills,rice mills,etc