By Antonio Guillermo Jara Ponce, Systems Sales Manager, HRS Heat Exchangers
Southern Europe has huge potential to expand its use of anaerobic digestion (AD) and biogas, and in the process reduce greenhouse gas emissions and the reliance on imported natural gas, as well as preventing environmental pollution, dealing with organic wastes and producing a valuable agricultural input in the form of digestate.
The key to maximising the adoption and profitability of AD is to maximise efficiency at every stage of the process, and this includes maximising the efficient use of the high quality digestate biofertiliser produced during the anaerobic digestion (AD) process, as well as maximising biogas production. One way of maximising process efficiency is to utilise heat regeneration (or heat recovery), where heat from a process which would otherwise be lost or wasted is recaptured and used for useful heating purposes. Heat regeneration should not be confused with ‘regenerative heat exchangers,’ which are a specific type of heat exchanger in which the product and service fluids flow alternately and the heat is stored in the structure of the heat exchanger.
The benefits of digestate
One example of heat recovery is the HRS Digestate Pasteurisation System (DPS), which is widely used to prevent the spread of potential crop or animal pathogens which may be in the feedstock, allowing digestate to be used in a range of situations, including crop production. Using digestate in agricultural systems improves soil health and, with long term use, can increase the ability of soils to sequester carbon.
Using digestate to fertilise soils also reduces the need for synthetic nitrogen, phosphorus, and potassium fertilisers. Depending on the feedstocks used, one tonne of digestate can contain: 2.5-27 kgs of nitrogen; 0.5-10 kgs of phosphorus; 1.5-13.5 kgs of potassium; as well as sulfur, magnesium and valuable trace elements such as zinc, copper and nickel. As synthetic fertiliser prices rise, the ‘free’ nutrition in digestate becomes increasingly attractive and valuable to growers.
HRS Digestate Pasteurisation System
Because the HRS Digestate Pasteurisation System (DPS) is based on heat exchangers rather than tanks with heating jackets, it is one of the most energy- and cost-efficient solutions for digestate pasteurisation. Using heat exchangers means that effective digestate pasteurisation is possible using surplus heat from elsewhere in the AD plant (such as the CHP unit), rather than needing to install an additional heat source such as a biomass boiler, which could add hundreds of thousands of Euros to a project. Â It also allows heat regeneration of up to 60%, providing further energy savings as this saved heat can then be used for other processes, such as evaporation of the digestate to remove water.
The standard 3-tank DPS provides continuous pasteurisation, with one tank being pasteurised while one is filling, and another being emptied. The HRS pasteuriser uses a double tube heat exchanger to heat the digestate to 75°C above the required pasteurisation temperature. This allows for variation in the sludge consistency and its incoming temperature, making sure that the digestate is always properly pasteurised. The tanks can also be used individually, for example to allow for routine maintenance.
