Renewable Energy Sources for the mCCHP-SE-RES Systems - Design for Micro-Combined Cooling, Heating and Power Systems - page 110

first two functions must be exercised in an absolute syn-
chronization, so that adjustment of the heat developed by combustion is conducted
with an optimal ratio between the air and fuel

It is obvious that the

flow rates, a ratio which usually
depends on the load of the combustion chamber. The third function is designed to
be independent of the first two, considering the combustion chamber large enough,

which makes it independent of the uid flow that passes through it. This means that,

by maintaining constant the flow rates of fuel and air entering the combustion

chamber, pressure inside the furnace can be changed by changing the

flow of gases

extracted, by changing the speed of the flue gas fan driving engine.

The main indices of the furnaces allowing comparison of both combustion
chambers and their sizing are:

volumetric thermal load:

￿

B LHV
V f

m 3

q v ¼ ½ kW = ð 8 Þ

cross-section thermal load:

￿

B LHV

S

m 2

q S ¼ ½ kW = ð 9 Þ

ow rate, kg/s or Nm 3 /s; LHV

where: B fuel low heating value, kJ/kg or kJ/

Nm 3 ; V f — furnace volume, m 3 ; S — furnace cross section, m 2 .
The combustion process is an exothermic chemical reaction, a reaction that
releases energy as it occurs. The amount of heat generated by complete combustion
of a unit weight of a fuel is a constant for any given fuel (combination of com-
bustible elements and compounds) and is not affected by the manner in which the
combustion occurs. This constant is called

— fuel —

“ heat of combustion ” , “ calori c value ” ,

or . The heat of combustion is determined by direct measurement in

a calorimeter of heat evolved during combustion of a certain amount of fuel.
Combustion products inside the calorimeter are cooled to the initial temperature and
the heat transferred to the cooling medium is measured to determine the higher
heating value (HHV) as the water vapors in

“ heating value ”

flue gas are transformed into liquid
water (by cooling) loosing their heat of vaporization. When the flue gas contains

water vapors, the heating value is called LHV. In combustion practice the lower
heating value is used.
A very important issue in combustion technology is to ensure the proper amount
of air that minimizes pollutant emissions and fuel consumption (energy ef

ciency).
In practice are used two terms: the air to fuel ratio, AF (kg air/kg fuel), and the
excess air factor de ned as:

m air
m fuel
m air

AF
AF stoich ¼

m air
m air ¼

V air
V air

k ¼

m fuel ¼

ð 10 Þ