Renewable Energy Sources for the mCCHP-SE-RES Systems - Design for Micro-Combined Cooling, Heating and Power Systems - page 107

In fixed-bed combustion plants there is the risk of ash melting at high tem-

peratures. Bottom ash represents about 60 - 90 % of the total ash produced in this

system;

cyclone-ash is the ash transported by exhaust gases and collected in cyclone. It
accounts for about 10

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- 35 % of the total ash produced in the fixed bed of the

combustion plant;
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lter fly-ash is very fine ash which can be collected by electrostatic precipitators,

wet filter, or bag filter. In the absence of advanced filtration systems, that ash is

discharged directly into the atmosphere. In fixed-bed combustion boilers, this

ash accounts for 2 - 10 % of the total ash produced.

The moisture content varies from one plant to another. It is lower for stem and
higher for roots and crown. Higher moisture content leads to decreasing in calori c

value and adiabatic flame temperature. Higher moisture content means a larger

volume of flue gas and consequently larger combustion chamber. The moisture

content in uences thermal behavior during pyrolysis affecting the physical prop-

erties and quality of the pyrolysis oil. Also high moisture content reduces the
ef ciency of biomass combustion plant.

3.2 Biofuel Combustion

The combustion process is affected by composition of biomass, especially by
moisture content and calori c value and by operational parameters: temperature,

residence time, stoichiometry, and turbulence.
The ef ciency of biomass combustion unit depends on the fuel type, excess of

combustion air, and the temperature of the exhaust ue gas.

Combustion systems consist of a combustion chamber (furnace), where the

flame and combustion gases are developed; burners or grates, with the associated
fuel supply subsystem; combustion air supply subsystem; gas exhaust system and
exhaust subsystem of combustion solid products (ash/slag), in the case of solid fuel
combustion.
The main functions of the combustion subsystems are:

ensuring the physical-chemical conditions of combustion (intimate mixing of
fuel with air, proper disposal of

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fuel gas, ignition temperature, and maintaining

the continuous burning);

￿ good combustion stability;

￿ collection and disposal of ash (slag);

￿ low pollutant emission;

combustion with appropriate fuel/air ratio.
Choosing the combustion subsystem is determined by the nature of the fuel to be
used and the thermal power of the combustion chamber. The manifold properties
and features of the fuel determine the existence of various combustion systems. The

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