Energy Program, Asian Institute of Technology, P. O. Box 4, Klong Luang

Pathumthani, Thailand

Fax No. (662) 524 5439, e-mail: bhatta@ait.ac.th

Realisation that improved cooking stoves (ICS) can relieve pressure on biomass resources led to ICS programs in most developing countries of the world. Most of the ICS programs are directed towards development of improved household cooking stoves, while relatively less work has been done on development of bigger stoves that could be used in institutional kitchens or certain traditional rural cottage industries. Three different designs of such stoves, using biomass briquettes as fuel, have been studied:

For preliminary study, a cross draft gasifier stove with natural convection was designed and tested. Based on the test results, the design was improved and a final design arrived at.

Small pieces of saw dust briquettes were used as fuel, and they showed very good results in terms of smokeless combustion after start-up. Efficiency of the stove during water boiling tests, were 15.76%, 11.80% and 13.63%.

To investigate the feasibility of multi-stage concept with top-down burning, a two-stage stove was designed and fabricated. From the experience gathered from a preliminary design, a simple and durable design was finalised.

Both small and large, vertically stacked pieces of briquettes could be used as fuel in this type of stove. Combustion was found very clean, steady and strong, and propagates from the top downwards. Flame height could be controlled by adjusting the primary air supply through the grate of the bottom stage. Efficiency of the stove, from several water boiling tests were: 21.64%, 19.91% and 20.31% using saw dust as fuel, and 27.17%, 27.27% and 26.20% using rice husk. Relatively low efficiency observed with saw dust as fuel appears to be due to higher losses caused by stronger flames. This stove could be used for relatively longer duration with a single loading of fuel.

This consisted of two concentric mild steel cylinders. The annular space was designed to be loaded with briquettes vertically. A cover provided at the bottom of the annular space can be removed and closed for loading briquettes. The inner cylinder had a grate at the bottom and had some small holes at its surface to allow pyrolysed gas to enter into the combustion zone. The pyrolysed gas would be burned inside the stove for cooking. Air for combustion could enter through the bottom of the inner cylinder. The stove was insulated externally by ceramic fibre insulation. The entire stove stands on three legs that hold it 15 cm above the ground to allow free primary air access.

Combustion in the stove was found very clean due to perfect burning of the volatiles. Water boiling tests revealed efficiency values of 15.38%, 12.96% and 12.47% for saw dust briquettes, and 11.39%, 13.20% and 15.47% for rice husk briquettes. Higher ash content could be the reason for lower efficiency observed in the case of rice husk briquettes. Charcoal, produced in one run of the stove could be used to start it the next time.

All the tested designs described above showed almost smokeless combustion with higher efficiency compared to traditional stoves. The highest stove burning efficiency was achieved with the insulated two-stage top burning stove. These stoves could be disseminated to the rural areas of developing countries.

Keywords: biomass briquettes, water boiling test, gasifier stove, pyrolysis