A Primer on Agricultural Residues

The term agricultural residue is used to describe all the organic materials which are produced as by-products from harvesting and processing of agricultural crops. These residues can be further categorized into primary residues and secondary residues. Agricultural residues, which are generated in the field at the time of harvest, are defined as primary or field based residues whereas those co-produced during processing are called secondary or processing based residues.

  • Primary residues – paddy straw, sugarcane top, maize stalks, coconut empty bunches and frond, palm oil frond and bunches;
  • Secondary residues – paddy husk, bagasse, maize cob, coconut shell, coconut husk, coir dust, saw dust, palm oil shell, fiber and empty bunches, wastewater, black liquor.

Agricultural residues are highly important sources of biomass fuels for both the domestic and industrial sectors. Availability of primary residues for energy application is usually low since collection is difficult and they have other uses as fertilizer, animal feed etc. However secondary residues are usually available in relatively large quantities at the processing site and may be used as captive energy source for the same processing plant involving minimal transportation and handling cost.

Crop residues encompasses all agricultural wastes such as straw, stem, stalk, leaves, husk, shell, peel, pulp, stubble, etc. which come from cereals (rice, wheat, maize or corn, sorghum, barley, millet), cotton, groundnut, jute, legumes (tomato, bean, soy) coffee, cacao, tea, fruits (banana, mango, coco, cashew) and palm oil.

Rice produces both straw and rice husks at the processing plant which can be conveniently and easily converted into energy. Significant quantities of biomass remain in the fields in the form of cob when maize is harvested which can be converted into energy. Sugar cane harvesting leads to harvest residues in the fields while processing produces fibrous bagasse, both of which are good sources of energy. Harvesting and processing of coconuts produces quantities of shell and fibre that can be utilised while peanuts leave shells. All these materials can be converted into useful energy by a wide range of technologies..

Miscanthus: Reducing the Establishment Costs

Miscanthus-Elephant-GrassMiscanthus has been lauded as a dynamic high potential biomass crop for some time now due to its high yields, low input requirements and perennial nature. Miscanthus is commonly used as a biomass fuel to produce heat and electricity through combustion, but studies have found that miscanthus can produce similar biogas yields to maize when harvested at certain times of the year.  Miscanthus is a C4 grass closely related to maize and sugarcane, it can grow to heights of three metres in a single growing season.

High Establishment Costs

However, high establishment costs have impeded the popularity of the crop. High establishment costs of miscanthus are as a result of the sterile nature of the crop, which means that miscanthus cannot be propagated from seed and instead must be propagated from vegetative material. The vegetative material commonly used is taken from the root structure known as rhizomes; rhizome harvesting is a laborious process and when combined with low multiplication rates, results in a high cost for miscanthus rhizomes. The current figure based on Irish figures is €1,900 ha for rhizomes.

Promising Breakthrough

Research conducted in Teagasc Oak Park Carlow Ireland, suggests that there may be a cost effective of method of propagating miscanthus by using the stem as the vegetative material rather than having to dig up expensive rhizomes. The system has been proven in a field setting over two growing seasons and plants have been shown to be perennial.

A prototype planter suitable for commercial up scaling has been developed to sow stem segments of miscanthus. Initial costs are predicted at €130 ha for plant material. The image below shows the initial stem that was planted in a field setting and the shoots, roots, and rhizome developed by the stem at the end of the first growing season.

miscanthus-stem

Feedstock for AD Plants

Switching from maize to miscanthus as a feedstock for anaerobic digestion plants would increase profitability and boost the GHG abatement credentials of the systems. Miscanthus is a perennial crop which would provide a harvest every year once established for 20 years in a row without having to be replanted compared to maize which is replanted every year. This would provide an obvious economic saving as well as allowing carbon sequestration in the undisturbed soil.

There would be further GHG savings from the reduced diesel consumption required for the single planting as opposed to carrying out heavy seedbed cultivation each year for maize. Miscanthus harvested as an AD feedstock would also alleviate soil compaction problems associated with maize production through an earlier harvest in more favourable conditions.

Future Perspectives

Miscanthus is a nutrient efficient crop due to nutrient cycling. With the onset of senescence nutrients in the stem are transferred back to the rhizome and over-wintered for the following year’s growth. However the optimum date to harvest biomass to produce biogas is before senescence. This would mean that a significant proportion of the plants nutrient stores would be removed which would need to be replaced. Fertiliser in the form of digestate generated from a biogas plant could be land spread to bridge nutrient deficiencies. However additional more readily available chemical N fertiliser may have to be applied.

Some work at Oak Park on September harvested miscanthus crops has seen significant responses from a range of N application rates. With dwindling subsidies to support anaerobic digestion finding a low cost perennial high yielding feedstock could be key to ensuring economic viability.