Cooking with Biogas

Introduction

Biogas is a mixture of mainly methane and carbon dioxide produced during the digestion of organic matter, such as animal and kitchen waste as well as crop residues, in the absence of oxygen. Anaerobic bacteria ferment biodegradable matter into methane (40-70%), carbon dioxide (30-60%), hydrogen (0-1%) and hydrogen sulfide (0-3%). The ideal process temperature for the fermentation process is at about 35°C, which might require additional heat or insulation of the digester in regions with daily or seasonal temperature fluctuations.

Viability of Biogas Production

Biogas production depends on the availability of sufficient biomass feedstock, water and space for the digester. As the biogas cannot be transported over long distances, the digester has to be placed close to the home of its user, which requires sufficient space. Yet, changing from a three-stone fire to using a biogas stove requires complex changes for a rural household, as well as for the user.

In order to check the viability of biogas in a certain area some basic points might be helpful to consider:

• The complex fermentation process in the biogas plant needs a continuous supply of suitable feedstock (preferably dung or other agricultural waste). This requires an appropriate farming system: a sufficient amount of livestock must be kept confined (no free-range grazing).
• It requires a continuous supply of water all year round, which can be a limiting factor in arid areas or areas with a pronounced dry season.
• Households must be able to afford the construction of a digester with a corresponding stove. The economic viability increases if the slurry from the digester can be applied as fertilizer on fields close-by.
• There must be enough labor available for the daily feeding and maintenance of the digester.
• Biogas might not be advisable in cultures where maintenance and preventive action is not a common habit.
• The rule of thumb says that 3-4 cows are needed to make biogas production viable at household level. This may be a problem especially for poorer families.

The Production of Biogas for Domestic Use

Biogas production for domestic cooking depends on an affordable appropriate digester at a suitable scale for domestic use. In any digester, the waste is mixed with water to create the right environment for the bacteria to decompose the biomass. As this is an anaerobic process, this has to happen without the presence of oxygen in an airtight tank. The biogas accumulates at the top of the tank where it is collected and taken by pipe to the user. The slurry has to be removed regularly from the tank. It can be used further, e.g. as agricultural fertilizer. Based on this principle a number of designs have emerged. Digesters come in many forms and sizes, which may range from 1 m³ for a small household unit to some 10 m³ for a typical farm plant and more than 1,000 m³ for a large installation.

A small-scale biogas system for household use will typically consist of the following components:

• Collection space: raw, liquid, slurry, semi-solid and solid animal, human or agricultural waste
• Anaerobic digester
• Slurry storage
• Gas handling: piping; gas pump or blower; gas meter; pressure regulator; and condensate drain(s).
• End-use device: cooker, boiler or lighting equipment

The article Types of Biogas Digesters and Plants displays various designs from different materials and at different costs.

 

The Use of Biogas for Cooking

For transporting biogas from the digester to the cooking place a tube is needed. Stoves for this system 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 cook-pot. Stoves and ovens for biogas application are similar to those of conventional appliances running on commercial gas-fuels. Most of these conventional appliances can be adapted for the use with biogas by special measures (particularly the modification of the burners) to ensure proper combustion and efficient use of energy.

Advantages:

• Biogas burns very cleanly, and produces fewer pollutants during cooking than any other fuel except electricity.
• Biogas provides instant heat upon ignition, no pre-heating or waiting time is required.
• Most biogas burners are able to regulate the flow-rate to turn down fire-power from high heat to small low heat for simmering.
• Biogas can be used for lighting as well.
• The by-product (slurry) from the digester can be used as fertilizer.
• Biogas is a renewable fuel that is ‘carbon negative’: unless there are leakages in the system, burning biogas in a cookstove releases less greenhouse gases than if the dung was left on the ground to decompose naturally.