THEORETICAL BACKROUND FOR WOODGAS


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REACTIONS IN GASIFYER

Gasifyer is set automatically to equilibrium point, where oxidation and reduction reactions are in balance. If the reduction reactions consume too much energy, oxidation reactions will be favoured to enhance heat generation and new equilibrium point is found. There is no need to control the amount of air fed into the gasifyer. In coal gasifyers the energy balance is excessive. Due to extra energy gas contains only little carbon dioxide and equilibrium temperature is high. When humid wood is used as fuel there is lack of energy and the product gas contains always a significant amount of carbon dioxide.

 

H2

 =

Hydrogen, appears  in air  as bimolecular gas (2 g/mol) *

Ns

=

Nitrogen, appears  in air  as bimolecular gas (28 g/mol) *

O2

=

Oxygen, appears  in air  as bimolecular gas (32 g/mol)

C

=

Carbon (12 g/mol)

CO2

=

Carbon dioxide (44 g/mol)

CO

=

Carbon monoxide (28 g/mol) *
CH4 = Methane (16 g/mol)
* = lighter than air when pure gas

When carbon (coal) is burned, most of the energy (72%) is released only after the second oxygen atom reacts to form carbon dioxide. This is the scientific bases of the whole principal of operation. If this were not true, it would be impossible to feed internal combustion engine with producer gas. When coal is gasifyed, the reactions to generate only carbon monoxide produce enough energy to sustain process to go on.

 

C

+

 ½ O2 à  CO   energy released

110,5 kJ/mol

CO

+ 

 ½ O2     à  CO2     energy released 283,0 kJ/mol

C          

+ 

 O2  à    CO2  energy released 393,5 kJ/mol

When carbon is in contact with hot flue gases in high temperature, it is capable to catch oxygen from carbon dioxide to form carbon monoxide. This reaction consumes energy and thermal energy is converted into chemical form.

 

CO2 

+

C

à  2 CO   energy consumed 172,5 kJ/mol

Carbon can trap oxygen from water molecules as well. In this case reactions produce hydrogen and carbon monoxide. Released hydrogen is excellent fuel. Hydrogen makes the gas easy to ignite and increases speed of combustion. The precondition for reaction to propagate is high enough temperature. In practice the temperature must exceed 800 °C.

 

H20

+

  C à  H2+ CO   energy consumed

131,5 kJ/mol

Water in fuel can be considered to be in liquid form. To vapourize water to steam a significant amount of energy is consumed. Addition to this, energy is consumed to heat the vapour to gasification temperature. About half of the weight of wood is built up from oxygen and hydrogen. During combustion a lot of water is formed. Even though the wood would be completely dry, enough water for gasification reactions is always released from chemically bound oxygen and hydrogen. The water coming with humidity is excessive and only disturbs gasification process.

 

H20

 (liquid) à  H20 (gas)  energy consumed

44 kJ/mol

All the reactions are equilibrium reactions and take place in both directions. The consumed or liberated energy is shown on reactions proceeding at direction of the arrow. The mass in reaction is expressed in moles. One mole contains always equal amount of molecules independent of the size of the molecule. Weight is specific for each substance. Molar volume of all gases is constant, about 22,4 litres / mole at pressure of 1 atm and at 0 °C. Volume depends only on temperature and pressure, not on chemical composition of gas. Function of internal combustion engine is based on thermal expansion of gas and pressure increase caused by temperature increase caused by oxidation (= combustion) reactions.

 

Burning of wood gas and power out put of internal combustion engines

Combustion of hydrogen and carbon monoxide liberates energy. As the reaction equations below show, combustion of main gas components lead to decrease in number of gas molecules. Inert components (nitrogen N2 and carbon dioxide CO2) do not change or take part in reactions.

 

C

+

 ½ O2 à  CO2                       energy released

283,0 kJ/mol

H2

+ 

 ½ O2     à  H20 energy released 242,0 kJ/mol

CH4         

+ 

 2 O2  à    CO2 + 2 H20 energy released 802,0 kJ/mol

Proportion of hydrogen and carbon monoxide together make about 40 % of the volume of the gas. After air addition these components make about 20 % of volume. Combustion of methane has no influence on amount of gas molecules. Total number of molecules in combustion mixture decreases by 7 % as a result of combustion, which is not favourable for power out put of internal combustion engine. Beneficial is that because of relatively large proportion of hydrogen, the combustion limit is large compared to for example to petrol. Gas which contains 21 % CO, 19 % H2 and 1,5 % CH4 the lower combustion limit is 16 % and higher combustion limit is 67 % (proportion of gas in air). This means roughly air proportion 2 to lean and about air proportion 0,5 to rich direction.

In practice the power out put is propotional to the amount of air an engine can take in. Wood gas makes about 45% of the optimal explosion mixture. This means it displaces almost one half of the volume engine could such air in. Respectively methane displaces 10 %, propane 3% and vapourized petrol only about 1% of the volume of the explosion mixture.

 

C8H16          

+

  12 O2   à     8 CO2 + 8 H20  

Combustion reaction for petrol

 

Heat value of optimal wood gas air mixture is about 2 500 kJ/m3, which is about 30 % lower compared to optimal mixture of air and petrol (3800 kJ/m3). Power loss is, however, more because there is some suction resistance in gas cleaning chain and this results as a loss of cylinder filling. Wood gas burns also more slowly. This requires earlier ignition timing results also as a power loss. Work pressure of the engine is lower because of the amount of gases is reduced during combustion.  When hydrocarbons are used as a fuel, the amount of gases slightly increases, which has a positive effect on power out put.