Straw, India

Grinding straw to increase regional energy production

Wood and fibres

Incinerating organic waste contributes to air pollution and climate change by releasing pollutants and carbon dioxide into the atmosphere. India incurs large amounts of agricultural waste, including crop residue, which are mostly incinerated in the fields. That is why the Indian government has created incentives to re-process biomass, such as rice straw, into renewable energy sources instead. Before being pulped, this biomass must first be crushed – which can be done using Tietjen’s hammer mill.

Using straw for energy

Extracting energy as sustainably as possible from straw primarily involves extracting biogas. In general, biogas is obtained by fermenting various organic materials and can be used in a variety of ways: as fuel for vehicles, to generate electricity (e.g. in cogeneration plants) or to feed into a gas network.To date, the process has mainly been used on an industrial scale to ferment maize silage and food waste. Fibrous raw materials, such as straw, are usually only used as co-substrates in small proportions during fermentation. The lignin structures (wood-like fibres) they contain resist degradation. Moreover, the straw tends to float in the biogas plant due to its hydrophobic properties.

Therefore, an important factor for a good biogas yield from this difficult substrate is a specific surface area as large as possible. It is important to break down the component fibres, especially lignin, as completely as possible, taking cost-effectiveness into account. This facilitates the mixing into the fermentation substrate and biodegradation. It is possible to use the SD straw mill to grind the straw before fermentation or use Imprasyn to reduce the size afterwards in the bypass. Both systems can also be combined to increase yield.

Grinding with the SD straw hammer mill

Tietjen’s SD straw hammer mill is suitable for grinding to a particle size that is optimal for biogas plants. This mill was specially developed to grind fibrous products such as straw, hay or alfalfa. A comb in the inlet area as well as the special geometry of the grinding chamber crush the fibres efficiently. A special frame system to which the beaters are attached makes it easier to change the beaters and ensures high availability.

As it is ground, the straw is pneumatically transported through the mill; it leaves the grinding chamber through the sieve once it has been crushed finely enough. The ground straw can then be used directly as a substrate or co-substrate.

Using Imprasyn to regrind the straw substrate also makes it possible to further increase the yield of the hard-to-access nutrients in the lignin fibres by means of an additional mechanical pulping process in combination with the addition of microorganisms. Substrate is taken from the fermenter and, after processing, is passed back into the fermenter or to the secondary digester. This further increases the effectiveness of the biogas process.

A sustainable cycle

The fermentation residue of the biogas process is excellently suited as an organic fertiliser and can be used again in agriculture. This closes the loop for the sustainable production of energy from straw.

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