A reviewed interest in renewable energy and related conversion technologies is emerging again. Although the eventual depletion of fossil fuels lurks in the background as a long-term incentive for the development of sustainable energy forms, more urgent incentives to re-emphasize renewable energy are related to global environmental quality. The first concern to emerge was release of toxic compounds and oxides of nitrogen and sulphur resulting from combustion of fossil fuels. These air pollutants contribute globally to health and environment problems, the most common of which is referred to as acid rain. The greatest threat is that of global warming related to an increased concentration of carbon dioxide and other upper atmospheric pollutants resulting from anthropogenic activities.
Use of renewable biomass (including energy crops and organic wastes) as an energy resource is not only greener with respect to most pollutants, but it’s use represents a closed balanced carbon cycle with respect to atmospheric carbon dioxide. It could also mitigate atmospheric carbon dioxide levels through replacement of fossil fuels. A third concern is the recognized need for effective methods of treatments and disposal of large quantities of municipal, industrial and agricultural organic wastes. These wastes may not only represent a threat to environmental quality, but also represents a significant renewable energy resource.
Biomass may be converted to a variety of energy forms including heat, steam, electricity, hydrogen, ethanol, methanol and methane. Selection of product for conversion is dependent upon a number of factors including need for direct heat or steam, conversion efficiencies, conversion and use of hardware and environmental impact of conversion process, waste stream and product use. Compared to other fossil fuels methane produces few atmospheric pollutants and generates less carbon dioxide per unit energy because methane is comparatively a clean fuel. The trend is towards its increased use for appliances, vehicles, industrial applications and power generation. Ethanol is becoming a popular biomass – derived fuel.
Methane can be produced from biomass by either thermal gasification or biological classification. Economic application of thermal processes is limited to feeds with either low water content or those having the potential to be mechanically dewatered inexpensively. Feedstocks containing 15% of total solids require all of the feed energy for water removal. Thermal processes for methane production also are only economic at large scales and generate a mixture of gaseous products that must be upgraded to methane. The product gas is composed primarily of methane and carbon dioxide with traces of hydrogen sulphide and water vapour. The major limitation of biological gasification is that conversion is usually incomplete, often leaving as much as 50% of the organic matter unconverted.
Principles of anaerobic digestion
It is the application of biological methanogenesis, which is anaerobic process responsible for degradation of much of the carbonaceous matter in natural environment, where organic accumulation results in depletion of oxygen for aerobic metabolism. This process, which is carried out by a consortium of different organisms is found in numerous environments, including sediments, flooded soils and land fills.
In generalized scheme for anaerobic digestion feedstock is harvested, shredded and placed into a reactor which has an active inoculum of microorganisms required for methane fermentation. A conventional reactor is mixed, fed once or more per day, heated to a temperature of 350C and operated at a hydraulic retention time of 20 – 30 days and loading rate of 1.7 kg VS m3d-1. Under these conditions about 60% reduction in organic matter is achieved corresponding to a methane yield of 0.24 m3 per kg VS added. The biogas composition is typically 60% methane and 40% carbon dioxide with traces of hydrogen sulphide and water vapour. The conventional design is being replaced by more innovative designs influenced primarily by feed suspended solids content.
The objective of these designs is to increase solids and microorganism retention, decrease reactor size and reduce process energy requirements. Improved designs have increased possible loading rates 20 fold, reduced residence times and improved process stability.
Renewable methane from biomass
Resource potential estimates for terrestrial biomass is estimated to be 22 EJ while for feed stocks like grass, wood, seaweed it is 7 EJ.The potential for marine biomass is huge at greater than 100 EJ per year.
As biomethanogenesis decomposes organic matter with production of useful energy products, anaerobic digestion of organic matter is receiving increased attention. Solid and agricultural wastes release undesired methane into the atmosphere due to anaerobic digestion in landfills, lagoons or stockpiles. Treatment and recovery of this gas in reactors would reduce this source of atmospheric methane. An attractive option for treatment of the organic fraction of these wastes is to separately treat organic fraction by composting and applying the stabilized residues in land as a soil amendment. The residues would reduce water needs and prevent erosion.
As population increases and technology development begin to result in significant resource depletion and environmental deterioration, we must take a universal view on the ground rules for sustaining our species in a manner that is compatible with preservation of biosphere. This will require production of feed, food and energy by technologies that are indefinitely sustainable and which have minimal environmental impacts. This will involve a major shift to renewable resources of energy, sustainable agricultural practices for production of food, feed and energy and recycle of all non- renewable resources. Derivation of methane from energy crops and organic wastes has an important role towards achieving this objective.