Spray Engineering Devices - Biomass Gasification
    
BIOMASS GASIFICATION
INTRODUCTION

Current energy scenario depends on the applicability of renewing technology in an efficient manner. The fast depletion of fossil fuel sources now draw plans to use non-renewable sources on a suitable basis and to replace them with renewable sources. Biomass sources are most promising, sustainable and less polluting

Why do we gasify?

All the biomass fuel materials are converted into gaseous form and then mixed with combustion air for burning.

1 kg of Petroleum fuel = 4 kg of biomass

Biomass occupies three to four times more volume. Because biomass materials are bad conductors of heat, it is also very difficult to have control over the rate of energy production. All these difficulties are overcome through gasification.

General requirements of gasification

  • The size of the feedstock should not exceed the recommended values.
  • Moisture content should not exceed 12%.
  • Bulk density should be around 500 kg/m³.
  • Interchangeability of gasifiers designed for powdery and solids is generally not possible.
  • Ash content is also an important parameter.
  • Need electricity for running the blower and fuel preparation machines.

A formula for biomass

BIOMASS - CH1.4O0.6

The process of release of thermal energy from fuel is known as combustion. Any combustion process can be depicted by the fire triangle shown in the figure.

Pyrolysis of biomass

Pyrolysis means the breaking down of materials by heat (~ 500°C) in the absence of air. First to heat, which releases gases and volatile materials according to:

CH1.4O0.6 --> Gas + vapor + Charcoal

Vapor is typically 75-90% of the fuel mass

Biomass Constituents Decomposition Temperature
Hemi cellulose 200 to 280°C
Cellulose 280 to 320°C
Lignin Beyond 320°C
Thermal decomposition of Biomass

Gasification

  • Gasification is an incomplete combustion of biomass at temperature between 700ºC and 1000ºC
  • It is basically a thermo chemical process that convert biomass materials into gaseous component i.e., Producer gas containing:

    • Carbon monoxide - 18 – 22%
    Hydrogen   13 – 19%
    Methane - 1 – 5%
    Heavier hydrocarbons - 0.2 – 0.4%
    Carbon dioxide - 9 – 12%
    Nitrogen - 45 – 55%
    Water vapor - 4%

  • According to the fuel gas end use, the gasifier system can be divided into
    1. Heat Gasifiers.
    2. Power Gasifiers.
  • Feedstock preparation is an important aspect of gasifier system planning – involves cutting of wood into required size
  • Manual, mechanical and electrical energy required for cutting fuel wood
  • Saw cutting generates 10% saw dust on weight basis
  • Perfect functioning of gasifier is highly dependent on properties of feed stock
  • Wood fuel needs processing such as size reduction and drying before combustion
  • Main constraints occurring in the existing gasifier systems are Construction Materials, Tar-free gas, Feed Stock Preparation and Clinker Formation

GASIFICATION PROCESS

Drying

The temperature above 100°C, the water is removed and converted into steam.

Pyrolysis

Thermal degradation starts at 200 – 320°C

Biomass + Heat (No O2) --> Charcoal + Oils + Tars + Gases

Combustion

Heterogeneous reaction takes place between oxygen in the air and solid carbonized fuel, producing carbon monoxide

C + O2 --> CO2

Hydrogen in fuel reacts with oxygen in the air blast, producing steam

H2 + ½ O2 --> H2O

Biomass + Unlimited O2 > Heat + CO2 + H2O + Ash

Reduction

High temperature chemical reactions take place in the absence of oxygen The principal reaction that takes place in reduction is mentioned below.

Boudouard reaction
CO2 + C --> 2CO

Water-gas reaction
C + H2 O --> CO + H2

Water shift reaction
CO2 + H2 --> CO + H2O

Methane production reaction
C + 2H2 --> CH4

APPLICATIONS Of PRODUCER GAS

Direct heat applications - where the gas is burnt directly in a boiler, furnace or kiln to provide heat. MNES report (2001) shows that the thermal gasifier systems are used for Silk Yarn drying, dyeing, Magnesium Chloride production, brick drying and large scale cooking applications.

Shaft power applications - where the gas is used to run engines and large scale electricity generation applications using gas turbines. Biomass Gasifier (5 x 100 kW) installed at Sunderban Island of West Bengal is being successfully run to provide electricity through a local grid (MNES report, 2001). VERMONT (USA) gasifier has supplied fuel for generation of 100,000 kilowatt-hours (kWh) of electricity.

CONTROL AND SAFETY MEASURES

  • Water seals
  • Automatic cut ON and cut OFF at biomass loading
  • Automatic flaring of gas
  • Automatic emergency shutdown of gasifier with hooter provision
  • Continuous extraction of ash from the grate bottom position
  • Safety valve provisions provided to avoid over pressure

GASIFICATION IS A GOOD ALTERNATIVE TECHNIQUE

  • High flexibility in terms of various biomass materials as feedstock.
  • Thermo chemical conversion efficiencies in the range of 70% to 90%, which is highest among various alternatives.
  • Gasification output capacity, especially in the high output ranges, is controlled only by availability of adequate feed materials rather than other technical considerations.
  • The area requirement for gasification equipment is lowest per unit output of energy in the form of heat and electricity.
  • The gasification equipment has high turn down ratios comparable to biogas and higher than steam turbine systems.
  • Gasification outputs are suitable as a fuel to all types of internal combustion engines with capacity in the range of 15 to 30%.
  • Positive impact on globe climate i.e. reduced threat of global warming.

GASIFICATION RULES OF THUMB

  • 1kg biomass (10%MC) generates 18MJ
  • 1kg yields 5kWh (thermal)
  • 1kg yields 1kWh (electric) when converted with 20%efficiency
  • 1kg biomass combines with 1.5kg air to give 2.5kg producer gas, which is equal to 2.5m³
  • 1m3 of producer gas weighs 1kg
  • 1ppm tar in 1m³ gas weighs 1kg