## Lesson 5. Performance evaluation of improved wood stoves

Combustion

Combustion is the thermochemical process, in which excess than the calculated stochiometric quantity of oxygen will be supplied to the system, to ensure complete combustion.  Where as, the efficiency of the combustion systems are comparatively lesser due to heat losses.  As the temperature at the combustion chamber is very high than the surrounding atmosphere, the heat transfer to the surrounding leads to heat losses.  The efficiency of the conventional wood stoves are in the range of 8 – 10 per cent.  Hence, efforts are taken for better air supply to the combustion zone and also to reduce the heat losses from the stoves, in the improved wood stoves.  The performance of the wood stoves can be assessed using simple water boiling test.  The procedure of the test is given below.

Observations

The theoretical background in assessing the performance of the wood stoves is that the quantity of thermal energy transferred to the known quantity of water kept in the wood stoves will be found out.  The proportion of the thermal energy used to heat the water to the total energy available in the known quantity of fuel wood expresses the performance of the wood stove in terms of thermal efficiency.

In this experiment, take a known quantity of water in two vessels having lids.  The diameter of the vessel should be higher than the diameter of the combustion chambers.  Place the vessels on the ports of stove. Fit thermometers on the lid of vessel to read the water temperature. Use 1 kg of firewood for every one hour for combustion.  Note down water temperature every 15 minutes. When the temperature of water reaches 98°C open the lid and continue to observe the temperature while water is boiling. When little water is remaining in the main vessel stop the test. Collect charcoal and ash from stove and weigh after cooling.

Observations

(i) Initial weight of fuel wood, W1                              =

(ii) Final weight of fuel wood, W2                              =

(iii) Total weight of fuel wood used (W1 – W2) , W    =

(iv) Volume of kerosene used ,W3                             =

(v)  Initial weight of water in the first vessel , m1        =

(vi) Final weight of water in the first vessel , m2         =

(vii) Initial weight of water in the second vessel , m3   =

(viii) Final weight of water in the second vessel , m4   =

(ix)  Weight of charcoal collected ,W4                        =

(x)  Initial temperature of water ,  t1 °C                      =

(xi) Final temperature of water , t2 °C                        =

Calculation

Weight of water taken (m1 + m3)        =          M         =

Loss in weight of water (m1-m2) + (m3 – m4) =ΔM     =

Heat utilization

Thermal energy used for water heating          =          Ho

Ho       =          M cp Δ t + L ΔM

cp        =          specific heat of water (1 kcal kg-1 oC-1)

Δ t       =          (t1 – t2 ) °­­­­C

L         =          latent heat of water (540 kcal kg-1)

Heat input

Energy input                 =          Hi

Hi        =          CVF x W + CVK x W3 – CVC x W4

CVF     =          calorific value of fire wood

CVK    =          calorific value of kerosene

CVC    =          calorific value of charcoal

Efficiency of wood stove

Thermal efficiency, η  (%)    =  Thermal energy used / Energy input

=          Ho / Hi

Power output rating

Power output rating, W =             W x CVF x 860 / 1000