Pasteurisation essential for Lithuanian AD plant27 July 2022

HRS Heat Exchangers has helped a developer and operator of renewable energy projects across Europe upgrade its 10.8 MW anaerobic digestion plant near the town of Vievis in southern Lithuania.

Opened in 2019, the plant is a biodegradable materials processing plant treating some 55,000 tonnes of agricultural, industrial and local authority waste a year (equivalent to a city of 310,000 people) to produce 86,000 MWh of electricity, resulting in CO2 savings of 120,000 tonnes. Feedstocks include agricultural materials such as corn, grass and other products, as well as food industry waste including animal remains from slaughterhouses.

In order to process animal by-products and allow the digestate that results from the process to be used as an organic, renewable green fertiliser, it is necessary to heat-treat either the feedstock or the digestate. For this process, the operator selected the HRS digestate pasteurisation system (DPS). Not only does this heat-treat the digestate ensuring that it is free from pathogens and weed seeds, but it also helps to remove odours while maintaining the quality of the resulting biofertiliser.

Any plant handling animal by-products requires suitable controls and licensing, and the HRS DPS system meets all of these requirements. As European regulations on the use of conventional fertilisers become tighter, there is increasing demand for the high quality biofertiliser produced by the plant from local farmers.

Efficiency has been the watchword for the design and operation of the Vievis AD plant, and the energy efficient design of the HRS DPS was a key factor in its selection. The tank-based DPS works on a three-tank principle. While one tank is being filled, the second tank holds the digestate above 70°C, at the same time as the third tank is being emptied, with each process lasting an hour.

Traditionally, systems that heat digestate in a tank use a heating jacket – a bit like waiting for a giant kettle to boil – and then dump the heat after it has served its original purpose. The HRS system, however, employs energy recovery and is typically two to three times more efficient. By transferring energy from the hotter (pasteurised) sludge to the colder (unpasteurised) sludge, energy consumption is reduced, and the need for additional heat sources is removed.

The tube which carries the digestate through the system’s heat exchanger features a unique corrugated design, unlike the smooth tubes used in alternative exchangers. This boosts heat transfer by creating extra turbulence and helps reduce fouling, resulting in less downtime and maintenance, and increasing the system’s lifespan.

A spokesman for the operator commented: “We always invest to improve technology operating in the plant.” As an example, a change of feedstock in 2020 resulted in changes to the properties of the digestate being processed, reducing the heat transfer efficiency of the original heat exchanger in the HRS DPS system. HRS worked closely with the plant operator to upgrade the heat exchanger technology used in the system.

Arnold Kleijn, HRS sales & product development manager, explains: “Due to a change in the client’s feedstock, the system was no longer giving the required performance. We redesigned the system, incorporating a new heat exchanger designed for more viscous products, and used as many of the existing system’s components [as possible], together with existing tanks on site, in order to implement the new and improved process at the lowest cost.”

The Vievis AD plant is now capable of handling a wide range of challenging viscous feedstocks while continuing to produce green renewable energy and biofertiliser.

Operations Engineer

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