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Experiment testing of Top Lit Updraft Coal Stove for

Domestic use in Malawi

by John L Taulo, Deputy Director (Research and Development)

Introduction

Combustion of biomass remains the prevalent energy source for cooking and heating of rural

households in Malawi, where more than 84.7% of the countrys population resides (Kambewa

and Chiwaula, 2010). Approximately 11 million rural people in Malawi still use biomass for

cooking, where fuelwood represents approximately 88% of energy used by rural households and

95.4% of total energy use in rural communities (NSO, 2009). Cooking is then often done over

open fires, which are highly inefficient transferring only 5-10% of the fuel energy to the cooking

pot. The adverse health and socio-economic implications of this form of energy supply are

enormous, with women and children at particular risk. The burden of biomass fuel collection andprocessing for cooking also falls mainly upon women and children (mainly girls), who spend

significant time gathering fuel resources every day.

Steadily rising firewood consumption for cooking purposes results in deforestation of large areas

creating severe ecological problems. In order to protect the environment it is urgently required to

utilize alternative methods for cooking purposes. Therefore, providing a clean cooking energy

option for these households will yield enormous gains in terms of health and socio-economic

welfare of the weakest and the most vulnerable sections of society. At the same time, the cleaner

combustion in these devices will greatly reduce the products of incomplete combustion which

are greenhouse pollutants, thus helping combat climate change.

This study aims at using locally available materials to develop a more efficient, affordable and

safe coal-burning stove in which the use of the stove will result in lesser consumption rates of

fuel and reduce the indoor air pollution. The stove in the present study follows the principle of

producing combustible gases, primarily carbon monoxide, from coal by burning it with limited

amount of air. The coal is burnt just enough to convert the fuel into char and allow the oxygen in

the char at a higher temperature to produce combustible carbon monoxide (CO), hydrogen (H2),

and methane. Other gases, like carbon dioxide (CO2) and water vapour (H2O) which are not

combustible, are also produced during gasification. By controlling the air supply with a small

fan, the amount of air necessary to gasify coal is achieved.

TLUD Coal Stove

The TLUD stove (see Fig.1) consists of a cylindrical reactor, an outer cylinder, a gas burner, and

a fan. The cylindrical reactor having a diameter of 15 cm and a height of 49 cm is where the fuel

is gasified. It is made of 1.6 mm mild steel sheet and is provided with grate at the bottom for the

passage of primary air. The grate is made of 12 mm diameter deformed bars with 10 mm

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spacing. The outer cylinder serves as stove body and as burner support. The gas burner is where

the gas generated from the reactor, mixed with preheated air, and is ignited. The fan is attached

to the stove body and is used to supply the air needed for gasification. The primary air enters

from the bottom end of the reactor with the use of a 12 cm, 15- watt computer fan. The

secondary air, on the other hand, enters the reactor through 16 holes on top of the stove casing

having a diameter of 20 mm and is mixed with the gas generated at the small holes located at the

upper portion of the burner. Combustible gases are burned in the plate burner consisting of 40

and 45 holes at the inner and outer circles, respectively, with 10-mm diameter.

Results

Modified University of California water boiling test (WBT) version 4.12 was used for testing the

stove. Burn rate and stove efficiency were determined together with emission factors for carbon

dioxide (CO2), carbon monoxide (CO), nitric oxide (NO) and hydrocarbons(HC). Compared tothe three stone fire, the coal stove exhibited a higher burn rate (25.57 g/min (1.534 kg h -1)) but

lower efficiency. The average computed thermal efficiency of the stove was 18.3%. The CO and

CO2 emission was in the range 9 ppm to 5480ppm (10.32-6279 mg/m3) and 1700 ppm to 23, 500

ppm (3060-42,300 mg/m3), respectively. The coal stove recorded mean CO, CO2, HC and NO

emission factors of 1.658, 125.2, 0.197 and 0.236 g kg -1, respectively. The emissions and

concentrations of carbon monoxide met an emission standard of a CO: CO2 ratio of

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this study a more efficient coal-burning stove was designed, fabricated and the thermal

performance of the stove was compared with that of a traditional stove. The results obtained

showed that the combustion of coal in the stove was very slow, resulting in a slow temperature

increase in the pot. The coal stove can be operated at power outputs ranging from 1.578 to 18.93

kW at efficiency of approximately 18.3%. The study also showed that the power output of the

fire influences the efficiency and emission characteristics of the stove; decreased combustion

efficiency could lead to more or less complete combustion and emit more products of incomplete

combustion (PICs).

Further studies are also needed to explore the relationship between the efficiencies estimated

above and the actual rate of household fuel consumption. An integrated assessment of

greenhouse gas emissions, thermal efficiency, and health impacts can provide more balanced,

fair, and complete evaluation of the stove than only considering one of these parameters. It

seems, however, that the design of the coal stove must be improved before it can be promoted as

a reliable fuel-saving intervention. The domestic use of coal on a massive scale should not be

considered until a more fuel efficient and cleaner burning stove is designed. The work atMIRTDC is underway to perfect this technology by performing extensive experiments on

various models.