Agricultural wastes
Agricultural wastes are organic by-products generated during the agricultural production process, encompassing residues from both crop and animal farming. They include materials such as leaves, straw, husks, and manures, which can be categorized into primary, secondary, and tertiary wastes based on their occurrence along the agricultural value chain. Despite their potential as a renewable energy resource—estimated to equate to about 50 billion tons of oil—the energy potential of agricultural residues remains largely untapped globally. These residues can be converted into various energy forms using technologies such as direct combustion, biogas generation, and fermentation, depending on their specific properties.
The use of agricultural residues as energy sources presents a viable alternative to fossil fuels, particularly as global energy prices rise. However, competition for these residues exists, as they are also valuable for maintaining soil fertility and as animal fodder, especially in developing regions. Environmental concerns arise from the improper disposal and burning of these wastes, leading to greenhouse gas emissions that contribute to climate change. Innovations in valorization techniques aim to recover valuable bioactive compounds from agricultural wastes, enhancing sustainable management practices. Overall, the increasing recognition of agricultural residues as a significant energy source underscores the need for strategic approaches to their utilization while considering environmental impacts and competing uses.
Agricultural wastes
Summary: Agricultural wastes are by-products of the agricultural system that can be converted into various forms of energy using different conversion technologies. Most of the worldwide energy potential from agricultural residues is currently unexploited.
Agricultural wastes are defined as all types of organic substrates, which arise during the process of agricultural production. These organic wastes can come from crop farming, in the form of leaves, straw, husks, shells, and hulls, as well as from animal farming in the form of manures, slurries, and animal bedding such as poultry litter. In crop farming, different crops produce different types of agricultural crop wastes. The nature and properties of agricultural crop wastes depend primarily on the plant species, growing conditions, and the processing required to obtain the agricultural product. The amount and composition of animal manure depend on the farming methods (intensive and extensive husbandry), type of animal, and amount and type of feeding.
Agricultural Waste Resources
Depending on where along the agricultural value chain the agricultural wastes occur—production, provision, usage, or disposal—the agricultural waste can be subdivided in three main categories: primary, secondary, and tertiary wastes. Primary wastes consist of residues remaining in fields after harvest and allocation. They include straw, stalks, cobs, and leaves. Secondary wastes, also known as processing wastes, are residues occurring during processing of the product, such as slaughterhouse wastes, milling residues, fruit peels, husks, bagasse, and filtration and extraction residues. Tertiary wastes are residues remaining after the usage of the product, such as organic municipal residues, postconsumer residues, and landfill gases.
These different types of wastes from agriculture production are a valuable renewable energy resource. Because they have a material and energy value, the term agricultural residue is frequently used instead of agricultural waste. The United Nations Environment Programme (UNEP) estimates that the worldwide energy potential of all agricultural residues combined is equivalent to about 50 billion tons of oil. Although this energy can be used to replace unsustainable fuel sources and reduce the dependence on fossil fuels, it is largely underutilized in most countries.
Using these residues as energy feedstock entails far fewer risks of land use and resource competition compared to energy pathways that use food and a energy crops to generate energy. Moreover, it is also one of the lowest-cost bioenergy feedstocks. Secondary residues are particularly suitable for commercial energy generation, because they are usually available in large quantities at the processing sites without additional transportation and handling costs.
When considering agricultural residues as commercial energy source, the following aspects need to be thought about in order to secure a steady supply stream: seasonality of crop production, availability of residues, harvesting method, livestock-keeping systems, transport distance and cost, competing uses, and possible effects of climate change. To use animal manure and bedding as energy substrates, it is further necessary to provide sufficient streams of preferably wet dung. In light of this, the main potential for commercial utilization of animal manure and bedding is allocated on large- or medium-scale cattle, pig, and poultry farms, where animals are generally raised indoors, so that the substrates can be easily collected. Most small-scale farmers, especially in developing countries, keep their livestock free-range, which makes the utilization of animal manure as feedstock a challenging task.
The overall importance of agricultural residues as an energy source will most likely increase in the coming years, because with rising energy prices economical and environmentally feasible alternatives need to be established. Agricultural residues represent a large and available potential source of energy that is ready to be used more extensively.
Competing Uses of Agricultural Residues
Despite their energy potential, agricultural residues—both animal manure and crop residues—have competing uses, which need to be considered. For instance, agricultural residues are often used as fertilizer to maintain soil carbon content and fertility, because they contain important nutrients. They also provide protection against erosion and can contribute to soil biodiversity. Environmentally sustainable biomass operations need to consider to what extent residues can be removed and what quantities have to be put back on the field in order to preserve the nutrient cycle. Commonly it is recommended to leave, on average, 30 percent of the agricultural crop residue on the fields. Besides their use as fertilizer, crop residues in developing countries are often used as animal fodder. Although agricultural residues are a poor fuel source for direct combustion, they are widely used for cooking and heating in areas with scarce wood fuel, causing health and environmental problems.
Elsewhere, agricultural residues have no use and are improperly disposed or burned on the field or farm, causing environmental problems such as air, water, and soil pollution. Agricultural residues account for a large percentage of biomass burned annually worldwide. Particularly critical are methane, carbon dioxide, and other greenhouse gas emissions that are released during the decomposition process and open burning of agricultural residues. These emissions are known to contribute to atmospheric warming and climate change.
Conversion Technologies
A variety of conversion technologies for energy generation with agricultural residues exist, for use in both large and small plants. Further technologies are in development and testing stages. Based on the nature of the transformation process, the different conversion technologies can be divided into three main categories: physical, thermal, and biological methods. The final products can be electricity, heat, and liquid fuel. Electricity and heat can be generated from agricultural residues through direct combustion, combined heat and power technology, or the generation of biogas. Liquid fuels from residues can be produced using fermentation (ethanol) or by pressing or extraction (biodiesel) processes. The physical and chemical characteristics of agricultural residues vary widely.
Their specific properties determine their quality as energy resources and the suitability for the different conversion technologies. Substrates with high moisture content (greater than 60 percent), such as fruit peels and animal manure, are most suitable for biogas production, whereas their moisture content makes them energetically inefficient to use for combustion or gasification. For these processes, a relatively low moisture content (<10–15 percent) is required. Moisture contents can be reduced before or as part of the combustion or gasification process, but in both cases the overall efficiency and final energy output are reduced.
During the 2020s, world governments and the scientific community realized that modern agricultural processes created dangerous levels of agricultural waste. To utilize this waste safely, some researchers began experimenting with valorization techniques. These techniques involved using environmentally friendly physical, chemical, and biological processes to recover bioactive compounds.
Bibliography
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