About 5 billion hectares of the earth’s total land surface is utilized for agricultural purposes, with 3% accounting for permanent crops. The rest include livestock farming and agricultural wetlands. Methane (CH4) and nitrous oxide (N2O) are Greenhouse Gases (GHG), emitted from agricultural lands with about 30 and 300 times the potency of global warming (GWP) respectively. CH4 and N2O can be harnessed from agricultural waste to create energy and organic fertilizers.
What is biogas and nitrous oxides?
Biogas typically consists of methane (CH4) and carbon dioxide (CO2). About 15% of global methane generation is attributed to agriculture. Methane is mainly known as natural gas due to its attachment to the fossil fuel industry. It is mainly used as fuel for energy generation, such as steam generation in boilers and cooking fuel in the far east. This is mainly due to its high energy content upon combustion at approximately 50 MJ/kg.
Figure 1 – The molecular structure of methane (CH4) and nitrous oxide (N2O) together with its GWP (source: youtube, e3solutionsincdotcom)
Nitrous oxides (N2O) is a GHG which is sometimes referred to as laughing gas. Apart from the natural occurrence in soils by nitrifying and denitrifying bacteria as well as oceans, they are also developed during the utilization of fertilizers. Nitrous oxides emissions are lower than methane and carbon dioxide. Although the emissions are to a lower extent, their potential of global warming could create a significant global warming effect. About 40% of the total N2O emissions are from anthropogenic sources. N2O concentration has increased by 20% to 330 parts per billion from 270 parts per billion over the last 250 years.
How is anthropogenic biogas and nitrous oxides generated?
Livestock generate biogas in the process of digestion as a by-product. Permanent crop areas generate biogas from decomposed organic matter buried in wetlands upon tillage. Processing of crops for production of vegetable oil release effluent containing biogas, normally containing 40% – 60% methane. The release if biogas is inevitable, as the world human population depend on agriculture as a source of food. It is only practical to convert methane to a less potent global warming GHG, such as carbon dioxide. A simulation model by MIT illustrates the potential of global warming reduction by 0.5°C if all GHG emissions, mainly methane, is eliminated.
Figure 2 – Methane generation sources, which is emitted as biogas (source: youtube, Engie)
Fertilizers contain nitrogen, comprised in a chemical structure, depending on the nature of the fertilizer. These can be either synthetic or organic fertilizers. The fertilizers are converted into nitrates by bacteria in the soil. When excess nitrates within the soil remain, they are released as nitrous oxides. Excess use of chemical fertilizers causes leachates and dissolved nitrates to be released, polluting groundwater and nearby water sources. Nitrates, being a precursor of nitrous oxide, causes water pollution.
Figure 3 – N2O emissions from agricultural sources. (source: youtube, Primary Industry and Resources)
How do we harness the biogas and nitrous oxides?
The biogas from liquid manure and underutilized solid biomass from crops, such as fronds, branches and leaves are fed into a large container. The container filled with biomass is digested by bacteria in the absence of oxygen, which is called anaerobic digestion. This is similar to a ruminant livestock digestive system. The anaerobic bacteria may digest the organic matter in days or weeks. This depends on the temperature, type and states of organic matter and its nitrogen content.
The production of biogas leaves a sludge mass behind as a by-product, containing high amount of nutrients. The remaining sludge, sometimes called as digestate, are separated into solid and liquid constituents and are used as fertilizer. Nitrogen is attached to the matrix in the fertilizer structure to be used as a means of nitrogen source for plants. In this case, the release of N2O is mitigated to some extent, preventing the valuable biomass from decomposing without proper management and releasing N2O to the atmosphere
Figure 4 – Harnessing of biogas from biomass and livestock manure (source: youtube learnfatafat).
What do we do with the biogas?
The biogas produced can be further upgraded to be directly fed into the federal gas network or used for communal heating and electricity generation. Heating occurs when gas is fed into an engine, where methane is ignited in the presence of sufficient air. The heat generated from the combustion is used to heat water and sent via pipelines for hot water utilization in communal areas. The engine piston induces a rotation to propel a generator, producing electricity. Small engines can generate a few kilowatts of power. This can be done in small acreages, owned by individual farmers. If the electricity generated exceeds individual usage, the possibility of feeding electricity to the grid generates further income for the farmer.
Figure 5 – Biogas utilization for heat and electricity generation (source: youtube, Colloide).
For larger produces of biogas, such as cattle and poultry farms, the biogas generated can be fed into a rotating gas turbine to generate electricity. The flue gas exiting the turbine, would still have enough residual heat to heat water for usage around the building at the farm. Similar processes can take place at large plantation areas of soy bean and rapeseed. The biogas needs to be purified into pure methane for use in vehicles or to be utilized as cooking gas. When methane is combusted, it produces carbon dioxide and water. Although the release of carbon dioxide is inevitable, it helps in the reduction of GHG emissions 30-fold.
How do we manage the digestate?
The digestate, which is the remnants of biomass from biogas production, is rich in nitrogen source and a source of organic fertilizer. This reduces crop growth dependency on synthetic fertilizers. The complex nature of organic fertilizer potentially allows for slower release of nitrogen and N2O than the readily convertible synthetic fertilizer. The slow release of organic fertilizer helps to retain the nitrates within the soil matrix with minimal leaching rates as well. Although good for the environment, the use of organic fertilizers is coupled with slower growth of plants. The balance of higher crop yield and environmental protection is the essence of sustainable fertilizer management.
Figure 6 – The utilization of the digestate as fertilizer. (source: youtube, Colloide)
From sustainable agriculture to sustainable fuels and products.
With sustainable agriculture, the lifecycle of organic matter is closed and less GHG emissions is released by means of sustainable agriculture. Some experts mention the GHG emissions are fully reduced as the emissions are assimilated by newly growing plants. However, the rate of plant growth and anthropogenic release is crucial to maintain the balance of emissions and sequestration. A slower release of GHG emissions to the atmosphere to meet sequestration rates is only possible if some of the carbon stocks are kept intact instead of producing biogas. The use of carbon stocks into useful product such as biodiesel, bioethanol, and bioplastics create a longer lifecycle before they are burned or degraded as carbon dioxide into the atmosphere. This may balance the carbon cycle to some extent. The use of organic fertilizers decelerates nitrous oxide emissions and water pollution, stabilizing the nitrogen cycle as well.
Conclusion
An enterprise running an agricultural entity may follow such methods to a lesser extent than individual farmers. The ownership of vast lands by a single organization for crop growth or livestock farming negates sustainable agriculture to some magnitude as profits are essential to businesses. A policy by authorities is probably needed to limit the amount of land owned by single entity. The allocation of land to smallholders may catalyze the effect of sustainable agriculture. However, the installation of the biogas plant needs to be subsidized by the relevant authorities. Taxes paid by the farmers shall act as a repayment scheme for the installation cost over a period of time. This can be projected to large corporations as well, although subsidies might be lesser due to their financial strength. Nevertheless, sustainable methods of agriculture do achieve a stable ecological cycle apart from reducing GHG emissions in the world which we live in.
Figure 7 – Biogas production creates an ecological cycle (source: youtube: Ahmed Mehdat)
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