By Matt Hale, Global Key Account Director, HRS Heat Exchangers

The Irish Government’s planned Biomethane Strategy, which was published earlier this year has the potential to displace fossil gas in many hard-to-decarbonise sectors and play a significant role in the decarbonisation of Ireland’s agriculture sector. The corresponding increase in anaerobic digestion and digestate production can boost soil health and reduce the reliance on synthetic fertilisers.

Ireland currently has one of the lowest levels of biogas production and biomethane deployment in Europe, with a few tens of biogas plants currently in operation, and just three plants focused on the production of biomethane¹. However, the country’s potential is huge. The European Commission has identified Ireland as the country in the EU with the highest potential per capita for biomethane development, and Gas Network Ireland (GNI) has shown that 176 prospective producers have an interest in developing projects in Ireland, with an overall potential to produce 14.8TWh of biomethane each year.

The Irish Biomethane Strategy has a strong focus on the benefits for the agricultural sector, including providing farmers with options for the diversification of activities and revenue streams, and a mixture of cattle slurry and grass silage as key feedstocks. It sets a target of supplying 5.7 TWh of biogenic methane by 2030.

If the country is to meet these targets for biomethane production and utilisation, it is important that equal emphasis is given to the production and utilisation of digestate – a renewable, low carbon biological fertiliser. Such a focus is necessary to maximise both the environmental and economic benefits from the strategy.

Using the right systems

Using the right systems as part of anaerobic digestion plant can maximise the economic potential of digestate. The HRS Digestate Pasteurisation System (DPS) and HRS Digestate Concentration System (DCS) both help to increase the potential economic and agronomic value of digestate in terms of nutrient value, physical attributes and safety. Pasteurising the digestate prevents the spread of pathogens, weed seeds and crop diseases, while concentration reduces the volume, making it easier to handle.

The HRS DPS is one of the most energy-and cost-efficient methods for pasteurising digestate, which is based on the use of heat exchangers rather than tanks with heating jackets. This means effective digestate pasteurization is possible using surplus heat while allowing additional thermal regeneration levels of up to 60%.

The standard 3-tank DPS provides continuous pasteurisation, with one tank being pasteurised while one is filling, and another being emptied. The DPS 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.

The HRS DCS uses patented technology to reduce the volume of liquid digestate up to 90%, raising the concentration of total solids to 20% solids while also maximising the nutrient content, using heat from the AD plant’s CHP engine. Lower water content also means reduced transport costs and field traffic, bringing further benefits in terms of reduced compaction caused by the application of the digestate to land.

Another benefit of the DCS is odour and ammonia control, which helps increase the nutrient content of the digestate. The high temperatures and vacuum conditions needed to concentrate digestate can cause the release of ammonia, largely responsible for the odours associated with digestate. The DCS overcomes this by acid-dosing the digestate with sulphuric acid, thereby decreasing the pH levels. This turns the ammonia into ammonium sulphate, which is not only less odorous, but is also an ideal crop nutrient.

In addition to these specialist systems for digestate management, HRS also produce a Biogas Dehumidification System (BDS), which removes water from biogas to protect boilers and CHP engines from corrosion and cavitation. With heat recovery included as standard, the BDS helps to increase the overall energy efficiency of AD plants.