Biomass Direct Combustion

1. Combustion of Biomass
BET 1 2017
Jackson Mutegeki,
Nyabyeya Forestry College

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When a substance burns,
it is said to combust.
A fuel is a substance that reacts with oxygen (combusts) to
release useful energy.
Combustion- Definition
Many fractions obtained from crude oil are used as fuels
because they contain hydrocarbons that burn easily and
release a large amount of useful energy.
Combustion is a rapid
reaction between a
substance and oxygen
that releases heat and
light energy.

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What are the products of the
complete combustion of a hydrocarbon?
Plenty of air is needed to provide enough oxygen for a
hydrocarbon fuel to burn completely.
oxygen
carbon
dioxide
fuel + +
 water
The blue flame of a gas hob
or a Bunsen burner is an
example of complete
combustion of a hydrocarbon
(in this case, natural gas).
Complete combustion of fuel

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Example - Complete combustion of
propane
Propane is a hydrocarbon used in camping gas. What is the
equation for its combustion?
oxygen
carbon
dioxide
propane + +
 water
5O2 3CO2
C3H8 + +
 4H20
How would the equations change if wood was used?

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If there is a shortage of air (oxygen), incomplete combustion
of fuel takes place.
Instead of producing just
carbon dioxide and water,
incomplete combustion
also produces carbon
monoxide and/or carbon
(soot). It also releases less
energy than complete
combustion.
Carbon monoxide is a poisonous gas because it reduces the
ability of blood to carry oxygen.
Incomplete combustion of fuel

7. Energy Release
• During the free radical reactions, carbon and hydrogen in
the fuel react with oxygen to form carbon dioxide and
water while liberating useful heat
– C+O2 CO2 -111 MJ/kmol
– H+ ½O2 H2O -283 MJ/kmol
• Oxygen is provided in excess to ensure complete
combustion of the fuel. This minimizes the production of
CO and maximizes heat generation

8. Combustion - Applications
• Designed on the basis of different factors:
– fuels
– fuel particle sizes
– moisture contents
– ash contents and compositions
– application purposes
– application sizes
– emission regulations
– etc.

9. SOLID FUEL COMBUSTION

10. Solid Biomass Composition

11. Fuelwood Combustion Theory
• When wood gets hot it releases volatile gases
which then burn if there is sufficient oxygen and
temperature
• Some of the radiant heat from the flame strikes
the wood, heating it and causing further
decomposition hence maintaining the cycle
• The gases are released at about 200 - 300oC
• But complete combustion takes place at about
650oC
• The unburnt gases escape as smoke – hence
smoke is wasted energy
John Kuteesakwe

15. Combustion Technologies
Applications
• For solid biomass utilisation, combustion
technologies depend on the application
– Domestic applications involve stoves
– Small furnaces up to a thermal output of 15 kWth,
– Middle-sized firing systems up to 1 MWth,
– Industrial plants above 1 MWth
• For liquid and gaseous biofuels, some form of
burner must be used