Drying digestate from biogas plants has major benefits. In the advanced TEMA Process™ the residual waste after digestion (solid and fibrous portion of the digestate slurry) is dewatered to 70-75% MC (MC = Moisture Content) followed by fluidised bed thermal drying, using waste heat from the biogas CHP heat exchangers.
The liquid portion which is drained away first during screening in a filter-press or a digestate separator is already a valuable fertiliser. The digestate liquid is close to 95% water and is used on-farm where it is produced and is sold locally to other farms (subject to local regulations regarding such as the Animal By-products Regs., the presence of heavy metals, and crop benefit requirements).
Advances in Digestate Drying Mean that Making Natural Renewable Fertiliser has Become Very Viable
More efficient processes have been developed to extract the maximum benefit from Municipal Solid Waste (MSW) and organic waste as the world becomes more focused on recycling waste materials.
The Dutch company TEMA Process offers fluid bed dryers for the following applications:
- making a granular fertiliser from digestate
- increasing the calorific value and stability of RDF (Refuse Derived Fuel), biomass, and sludge.
They do this as experts with extensive experience in the design and supply of tailored solutions for the drying of bulk materials.
Tema Process fluid bed dryers can operate at relatively low drying air temperatures, waste heat from co-gen sets, boilers, or other sources is their preferred energy source for dryer heating.
The Benefits of Drying Digestate
Drying creates an odourless dry fertilizer that can be applied to crops. The dried material is light and compact. This significantly reduces storage and transportation costs. Secondly, once dried the digestate can be stored to be used when it is needed for crop growth at any time of the year.
Drying digestate from biogas plants can be accomplished using a variety of energy sources in addition to the waste gas engine jacket heat or waste heat from a combined heat and power unit (CHP). In many cases, solar energy, or exhaust air from a microturbine are used to dry the digestate. On rare occasions, waste biogas plant heat may also be converted into electricity and directed to the process of digestate drying. However, as this is usually done more to maximise the sale of green energy for tax break reasons it is not a technically-recommended option.
During drying, digestates emit ammonia gas. This is an air pollutant but can be put through a water-stripper to remove it from the dryer off-gas to make a futher nitrogen fertiliser product.
Digestate Dryers in Use Today
Currently, approximately 500-700 digestate dryers are in use in Germany. This technology meets strict EU regulations while producing transportable and storable fertilizer. In addition, the process reduces emissions when compared to land spreading of the mixed wet digestate. It also meets the strict hygienic standards required by the EU. The end result is a biogas fertilizer that emits significantly less ammonia than moisture.
Digestate Drying Solves Past Problems
A growing number of biogas plants have been built across the world and are an important source of revenue for many farmers. However, this technology has until now had serious drawbacks. First of all, digestates have a high amount of ammonia, and they can be difficult to separate into liquid and solid fractions. The second problem was storage space for the wet materials and the lack of availability of farmland spreading space, all year round, due to run-off causing water pollution.
Too much digestate spreading on any one area is counterproductive, and great care must be taken to avoid run-off resulting in the nearby rivers and/or groundwater pollution.
Drying digestate from biogas plants is a good way to combat these problems and makes the AD process yet more efficient and environmentally friendly.
How Drying Creates Added-Value
Another benefit of drying digestate from biogas plants is that it creates added value as a product. The dried digestate fertiliser product has a far wider market and is a highly valuable fertilizer. This material can be stored for long periods and has a high dry matter content of 85% to 90%.
The dried digestate is also easy to transport and can be spread over larger areas than when in liquid form. Furthermore, it is better accepted by local communities because it does not cause a significant risk of contaminating the environment or causing river and groundwater pollution which is damaging agricultural areas globally.
Many biogas plants, in operation now, can generate up to 10,000 tons of digestate per year. The digestate from these plants is mainly used as liquid fertilizer and, until now, only a minor part was dried in a hybrid waste-heat or solar dryer.
But, there are signs that this will rise rapidly. The change will be rapid now that low-energy consumption CHP heated digestate drying technology is available. The driving forces behind this development are:
- the high cost of energy which has meant that fossil-fuel/ mineral fertilisers have risen dramatically in price
- the war in Ukraine which has led to shortages in the supply of the natural gas needed for mineral fertiliser production, and
- shortages of mineral mined/chemical fertilisers due to factories closing.
Why Stop at Belt Dryer Dewatering?
A high-performance belt dryer can be used to produce a solid and easier-to-handle fertiliser alone.
Systems are available that are suitable for drying up to 100 tons of digestate per day and can reduce sludge storage space substantially. Additionally, a belt drying system can be cost-effective, enabling operators to save hundreds of Euros a day and in jurisdictions of subsidy for renewable heat production receive a CHP RHI bonus.
Many biogas plant operators go no further. However, the dried product is far more in demand and will only rise in popularity.
Conclusion
A high-quality digestate dryer can turn an organic waste which would cost in excess of $100/tonne to dispose of to landfill or incineration into a valuable fertiliser commodity. Drying digestate from biogas plants can be profitable if the right methods are followed.
More information is available from TEMA PROCESS
