Biogas Stoves

Introduction

Biogas production depends on the availability of sufficient biomass feedstock, water and space for the digester. Biogas is produced by the anaerobic digestion of organic matter under anaerobic conditions. Biogas is comprised primarily of methane and carbon dioxide. At household level, biogas systems can be used to produce fertilizer and for providing energy for cooking and lighting.

This article focusses on biogas cook stoves. For more information on the production of biogas for domestic cooking use please read this article Cooking with Biogas.

Stove Technology

Cookstoves and ovens for biogas application are similar to those of conventional appliances running on commercial gas-fuels. A biogas stove usually has a single or double burner with varying gas consumption rates, which is influenced by the pressure provided by the biogas plant and the diameter of the inlet pipe. The burner itself has several parts. The amount of gas that flows into the burner is controlled by the jet, a hole which is carefully sized and defines the power output of the burner. Compared to other gases, biogas needs less air for combustion. Therefore, conventional gas appliances need larger gas jets when they are used for biogas combustion. Most of these conventional appliances can be adapted for the use with biogas by the modification of the burners to ensure proper combustion and efficient use of energy. Stoves running on biogas contain a valve to premix the biogas with the right amount of oxygen, a burner to combust the mixture and a structure to hold a pot. Piping is needed to transport the biogas from the digester to the point of use in the cook stove.

More information on biogas burners can also be found in this energypedia article: biogas appliances. [LF1] 

Efficiency of Biogas Stoves

For achieving a high efficiency of biogas stoves, the important factors to be considered in designing a biogas stove are:

• Burner types (orientations of holes, shape and size of holes, burner size)
• Space between burner and the tripod or other vessel supporting mechanism
• air control mechanism and optimization on burning

Furthermore, gas composition, gas pressure, flame speed (velocity), and pot to burner distance are of relevance. A test report of SNV mentions also more general criteria for biogas stove efficiency. This Popular Summary of the Test Reports on Biogas Stoves and Lamps presents the test results of biogas stoves of eight countries (Bangladesh, Cambodia, Ethiopia, India, Lesotho, Nepal, Rwanda and Vietnam) carried out in 2007. Most stoves did not meet the general standards for biogas stoves as defined in China and India. On the basis of thermal efficiency alone, the stoves from Bangladesh and Cambodia met the prescribed minimum efficiency of 55% under both the Chinese and the Indian standards. The stove from Rwanda was very close to the prescribed efficiency.

A Water-Boiling-Test of eight different biogas stoves carried out by the Center for Research in Energy and Energy Conservation at the Makerere University in Kampala, Uganda resulted in a much lower efficiency. Many of the locally available gas burners that were tested were of poor quality: they had an overall efficiency of 20% on average and their designs did not follow gas burner theory adequately. Furthermore, the test results showed high levels of carbon monoxide emissions for most of the stoves.

However, lab test results can be very different from using stoves under real life conditions. Unfortunately, field operations and respective efficiency analyses (e.g. through kitchen performance tests) are rarely described and available.  

Consumption of Biogas for Cooking

Household burners consume approximately 200-450 liters of biogas per hour. The gas consumption for cooking per person lies between 300 and 900 liter per day, the gas consumption per 5-member family for 2 cooked meals between 1500 and 2400 liter per day.   With 1 kg cattle dung in warm tropical countries approximately 40 liters of biogas can be produced.

Advantages and Disadvantages of Biogas Stoves

For the user, cooking on biogas stoves has several advantages but also some disadvantages.

Advantages:

• Biogas stoves provide instant heat upon ignition, no pre-heating of fuel or waiting time is needed.
• In most biogas burners the flow-rate can be regulated to turn down the fire-power from high heat to small low heat for simmering.
• Biogas burns very cleanly
• Burning biogas releases less greenhouse gases than if the dung was left on the ground to decompose naturally.

Disadvantages:

• High investment costs for the digester, gas stove and installations for the gas to get into the kitchen can prevent the uptake of biogas as cooking fuel.
• Cultural rules might limit the acceptance of handling dung or feces and their use for cooking.
• Cooking with biogas stoves requires the change of cooking habits, which might prevent the adoption.