Thermal Hydrolysis Boosts Biogas Production

October 12, 2021 Categories: Environmental - Opinion Piece

The conversion of sustainable biomass such as food waste, crop and processing residues, manures and sewage sludges, into biogas and advanced liquid biofuels has significant potential to contribute to the world’s renewable energy and carbon reduction goals. However, many of the most sustainable feedstocks contain high levels of woody materials known as lignin and cellulose, which are difficult to break down.

Hydrolysis is the process of breaking up of the polymers which form lignocellulosic material (such as straw and woody biomass), to release molecules such as saccharides, therefore improving the efficiency of processes such as anaerobic digestion and fermentation in the production of bioenergy and biofuels.

Thermal hydrolysis is a two-stage process combining high-pressure heat treatment of the feedstock followed by rapid decompression. This combined action breaks apart the chemical bonds in the lignocellulosic material, improving biodegradability and allowing the organisms responsible for anaerobic digestion or fermentation to act on a greater surface area of material (therefore allowing higher loading rates, or shorter retention times).

In the case of manures and sewage sludges the high temperatures involved also sterilize the feedstock, destroying any pathogens which may be present and opening up more options for the resulting digestate, such as its use as a high-quality biofertilizer.

Hydrolysis is commonly used in the water treatment industry to improve biogas production at wastewater treatment plants since the first full-scale application of thermal hydrolysis for sewage sludge in Norway in the mid-1990s. It is also employed in the production of advanced biofuels for lignocellulosic feedstocks.

HRS has developed a patented process for the continuous thermal hydrolysis of digester sludge using the Unicus Series of scraped surface heat exchangers. This involves heating the sludge to 160-170 °C of the sludge and a steam explosion step can be included for extra efficiency.

HRS offers patented processes for thermal hydrolysis both with and without steam explosion. Unlike some other systems, both are based around a continuous process using scraped surface heat exchangers to avoid fouling, allowing more efficient production. Furthermore, the systems incorporate energy recovery to provide a significant cost reduction during operation, and the technology can be retrofitted to existing installations in order to improve their output.

In the HRS process using steam explosion, the biomass is first preheated, and is then heated further at high pressure to achieve the thermal hydrolysis. The biomass is then passed to a vessel for rapid de-pressurization and the steam that is released from this is then used in the initial preheating phase, while the hydrolysed biomass is then cooled before being passed to the digester.

With experience of thermal hydrolysis, HRS is well placed to advise on the best solution for a particular purpose and to deliver a tailored solution, whatever the application.

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