Biogas for Power Generation

Today, both developed and developing countries are facing a monumental problem as it relates to effective and efficient disposal of organic wastes. Sustainable waste management and waste prevention is at the forefront of global initiatives, with the ultimate goal of reducing pollution and greenhouse gas emissions and to mitigate global climate changes. Uncontrolled waste dumping is no longer acceptable today and even controlled landfill disposal and incineration of organic wastes are not considered optimal practices.

Production of biogas through anaerobic digestion (AD) of animal manure and slurries as well as of a wide range of digestible organic wastes, converts these substrates into renewable energy and offers a natural fertiliser for agriculture. At the same time, it removes the organic fraction from the overall waste streams, increasing the efficiency of energy conversion by incineration of the remaining wastes and the biochemical stability of landfill sites.

So what exactly is Anaerobic Digestion?

Anaerobic Digestion, also referred to as AD, is a microbiological process of decomposition of organic matter, in the absence of oxygen. Biogas is therefore produced in airtight reactor tanks, commonly named digesters. A wide range of microorganisms are involved in the anaerobic process which has two main end products:

  • biogas, and
  • digestate

Biogas is a combustible gas consisting of methane (CH4), carbon dioxide (CO2) and small amounts of other gases and trace elements. Digestate is the decomposed substrate, rich in macro- and
micro nutrients and therefore suitable to be used as plant fertiliser.

If the substrate for AD is a homogenous mixture of two or more feedstock types (e.g. animal slurries and organic wastes from food industries), the process is called “co-digestion” and is common to most biogas applications today.

Flexibility in use of Feedstock

Various types of feedstock can be used for the production of biogas:

  • animal manure and slurries
  • crop residues
  • organic wastes from dairy production, food industries and agro-industries
  • wastewater sludge
  • organic fraction of municipal solid wastes
  • organic wastes from households and from catering business
  • energy crops
  • biogas can also be collected from landfill sites

One main advantage of biogas production is the ability to use “wet biomass” types as feedstock, all characterised by moisture content higher than 60–70% (e.g. sewage sludge, animal slurries, flotation sludge from food processing etc.). In recent years, a number of energy crops (grains, maize, rapeseed), have been largely used as feedstock for biogas production in countries like Austria or Germany. Besides energy crops, all kinds of agricultural residues, damaged crops, unsuitable for food or resulting from unfavourable growing and weather conditions, can be used to produce biogas and fertiliser. A number of animal by-products, not suitable for human consumption, can also be processed in biogas plants.

Pre-feasibility Study Questions

The following questions are key to determining if a Bioenergy (electricity and/or heat) Project has merits to offer a viable solution:

  1. What is the source of the Biomass Feedstock?
    1. Primary Source (example: harvesting residues (corn stalks))
    2. Secondary Source (example: waste from industries using agro-forestry products (rice husks))
    3. Tertiary Source (example: post-consumer waste (slaughter waste))
  2. How much feedstock is available on a daily basis?
    1. Primary Source
    2. Secondary Source
    3. Tertiary Source
  3. Is there availability of land to grow dedicated energy crops?
    1. How much land can be made available to grow energy crops?
  4. Is there availability of MSW (municipal solid waste)
    1. How much MSW is available daily?
  5. Is there a landfill nearby?
    1. How much MSW is delivered to the landfill on a daily basis?

The goal here is to under the daily demand on electricity or heat and to determine if there is adequate availability of feedstock to meet heat and power demand.