How is biogas turned into electricity?

Biogas is converted to mechanical energy through an internal combustion engine. The mechanical energy rotates an electric generator which produces the electricity.

Biogas Engine

Biogas engine – uses the biogas to generate mechanical energy. Biogas is carefully mixed with the correct proportion of air and drawn into the biogas engine by the force of the engine pistons moving downwards, creating a vacuum. 

‍The air and biogas mixture is then compressed as the piston moves up. Biogas is a slow burning fuel, and a higher compression ratio engine is essential for efficient combustion. A high energy spark plug ignites the compressed air and biogas mixture. The burning biogas air mixture heats rapidly, expanding and then forces the piston down to create torque to rotate the engine. 

‍The biogas engine exhaust valve opens,  releasing the spent air and fuel mixture into a heat exchanger to extract remaining combustion energy. Inoplex has carefully optimised this conventional four-stroke engine operation to suit a range of biogas, which burns very differently to petrol, gasoline or diesel. Inoplex biogas engines include High compression ratios, high energy ignition sparks, swirling of air-fuel mixture and low exhaust temperatures.

Generator

Generator – the generator produces the electrical output from the engine-generated mechanical energy. This mechanical energy rotates an iron core wrapped in copper wire inside a powerful magnet to create an electrical current. The Inoplex Permanent Magnet Generator allows considerable benefits. Physical benefits with reductions in weight and length, coupled with electronic benefits, ensure a superior technology devised to maximise the efficiency of the complete power system.

Commonly combustion engines are used to convert the biogas to electricity. This has an added bonus of easy to find new and replacement engines, spare parts, as well as simple local repairs making it a viable solution for regional or even remote use.

Diesel engines can be used in dual-fuel mode, where a small amount of diesel is used to help the ignition of the biogas. Gas motors can operate solely on biogas sometimes using a small amount of LPG to start the engine if the biogas is particularly weak, or low in methane. 

Energy from methane

Biogas can be utilised in many of the same ways natural gas or LPG can be used such as cooking, heating, and lighting as well as to produce electricity. Methane is the usable part of the biogas which supplies the energy. The remaining components of the biogas include carbon dioxide, water vapour, and other trace elements such as nitrogen, oxygen, ammonia and hydrogen sulphide.

Biogas is lower in methane than natural gas – biogas 50-75% to natural gas 90 – 98 %. The energy value of the biogas is dependant on a range of factors including the type of feedstock (raw material) used to generate the gas, temperature in fermentation, length of fermentation, and the specific plant design. 

Crops such as maize silage create biogas with a much higher energy value than alternatives such as cow manure, around eight times as much in fact. Biogas power generation using cow manure is still a viable endeavour, however, 8 times more gas will be needed to generate the same amount of electricity. Factors such as this will be considered during the engineering and design phase of the bioenergy plant set up.