WASTE TO ENERGY
he recent report, Te Outlook for Biogas and Biomethane, from the International Energy Agency provides a welcome assessment of the huge global potential of biogas from anaerobic digestion to contribute to a clean and low carbon global energy system. However, it makes only passing reference to the importance of digestate in meeting these targets, something which depends on the conversion of a potential ‘waste’ product into a valuable biofertiliser resource. As well as producing biogas, anaerobic digestion (AD) produces a residue known as digestate, which has the potential to be a valuable organic biofertiliser for use in agriculture. While the report’s estimate that full utilisation of the world’s potential production of biogas and biomethane – some 730 Mtoe – could fulfil around 20% of global gas demand, such a ramping-up of biogas production does not come without challenges. In fact, this level of biogas and biomethane production represents a 20- fold increase on 2018 production levels, resulting not only in a huge increase in biogas availability, but also in the production of digestate. However, while the 93-page report considers many factors, digestate is mentioned in just three short paragraphs. Te report is right in its assertion that,
T
The HRS Digestate Pasteurisation System (DPS) can produce energy savings of 70% compared with other technologies
SUSTAINABLE MAKING BIOGAS
Biofertiliser use key to fulfilling biogas potential, explains Matt Hale
“using waste and residues as feedstocks avoids the land-use issues associated with energy crops,” and is also correct in saying that “digestate can be sold as a natural fertiliser, helping to offset part of the production cost [of biogas].” However, it fails to recognise that digestate produced from waste and residue feedstocks often requires further treatment before it can be used as a fertiliser due to a range of crop health, hygiene and biological reasons. To prevent the spread of weed seeds,
crop diseases and other potential pathogens,
either the feedstock or the digestate should be treated appropriately, with pasteurisation being a tried and tested technique across the world. In fact, in many areas including much of Europe, certification schemes exist to back-up such pasteurisation so that digestate that has been so treated can be used, sold, traded and applied as an agricultural and horticulture fertiliser, rather than a waste product. If the biogas potential expressed in the IEA report is to be reached, then the additional digestate produced (again a 20-fold increase on 2018 volumes) must be treated in such a way as to maximise both its value and its agricultural acceptability.
If anaerobic digestion is to meet the scale advocated by the IEA, the resulting digestate will need to be pasteurised
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HEAT EXCHANGER SOLUTION One of the most energy- and therefore cost- efficient methods to pasteurise digestate is the HRS Digestate Pasteurisation System (DPS), which is based on heat exchangers rather than tanks with heating jackets. Using heat exchangers means that effective digestate pasteurisation is possible using surplus heat while allowing additional thermal regeneration levels of up to 60%. Tis saved heat can then be used for other
processes, such as evaporation of the digestate to remove water. Te standard three-tank DPS provides continuous pasteurisation, with one tank being pasteurised while one is filling, and another being emptied. Te HRS pasteuriser uses a double tube heat exchanger to heat the digestate to 75 degrees Celsius above the required pasteurisation temperature. Tis allows for variation in the sludge consistency and its incoming temperature, making sure that the digestate is always properly pasteurised. Te tanks can also be used individually, for example to allow for routine maintenance. Te DPS is just one of a number of systems that HRS produces to improve the efficiency and sustainability of biogas plants and the anaerobic digestion process, with other examples including systems to concentrate digestate, dry biogas and recover waste heat from exhaust systems.
Matt Hale is with HRS Heat Exchangers.
www.hrs-heatexchangers.com
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