Articles & Case Studies

Optimizing heat integration for anaerobic digestion plants

Posted: Friday 8th November 2013

This article explores the various possibilities that exist for heat recovery in anaerobic digestion (AD) plants that work with cogeneration engines or combined heat and power plants (CHP). Anaerobic digestion has become very important in the current industrial market place with many companies discovering the effectiveness of using organic waste and effluents to generate biogas. The biogas can be used for generating electricity in a CHP engine but there are also more possibilities for its use.

Alongside electricity, CHP engines generate heat. For example, a CHP engine which generates 2MW of electrical power will approximately produce a further 2MW of thermal energy. The heat is generated in the internal combustion process and must be removed from the engine to assure its correct operation. Wherever cogeneration is used, therefore, thermal (or waste) energy is available for transfer and integration into other applications. Optimizing the integration of this heat offers some important benefits to the plant.

Why make best use of heat recovery?

AD processes require thermal energy for operation. Anaerobic digesters need to be maintained at a constant temperature above ambient to ensure that the biogas is formed at a sufficient rate. It seems, therefore, an obvious choice to obtain the needed heat from the CHP engine. Experience has shown, however, that only part of the thermal energy available is needed for heating the digester and further advantages can be gained in using the surplus in other applications within the plant. For example, Renewable Heat Incentives (RHI) can offer companies tax refunds which are available where waste heat sources, such as CHP heat, are used in an efficient way. Making use of waste heat from CHP engines will also reduce the overall energy cost of a company, which in turn reduces the carbon footprint. This allows the company to sell Renewable Obligation Certificates (ROCs) and creates benefit from an additional income stream. The extra thermal energy available can further be used to optimize the overall AD plant performance through its application within thermal applications around the plant.

Where can heat recovery be used?

Apart from digester heating, other thermal processing applications that work on using waste heat from the CHP engine can be added to the AD flow sheet. Figure 1 below highlights the various possibilities:

Digester heating:

As previously discussed, anaerobic digesters must be maintained at a constant temperature for the viable production of biogas. The heat input into the digesters, therefore, needs to overcome heat losses. This is usually done by sending heated water used for the cooling of the engine through an external heat exchanger to heat the digester mass. Alternatively, the heated water can be directed to coils that are positioned in the digester reactor walls.

Thermal hydrolysis:

Thermal hydrolysis can be an interesting option for some types of AD feed stocks. In this process the biomass is heated up to elevated temperatures (usually above 100C). The heat breaks down cell structures inside the biomass which results in a higher biogas yield in the anaerobic digesters. This requires a higher temperature heat source and the CHP exhaust gases (which are at around 500C) are ideal for this.

Sludge pasteurization:

Some AD feed stocks require a pasteurization process, usually where animal by-products are present to make sure all pathogens are neutralized. In general, this means heating the product to 70C and maintaining it at this temperature for one complete hour. The heat source usually applied to this process comes from the 90C CHP cooling water.

Digestate evaporation:

In many cases, considerable costs are involved in removing the liquid digestate from the AD site to another facility such as a waste water treatment plant. Digestate evaporation recovers water from the liquid digestate and reduces its volume which, in turn, reduces costs for disposal. This evaporation process can be done using the CHP cooling water at 90C using a vacuum evaporator. It is also possible to use the exhaust gases as the heat source for this process.

Heat recovery, do it the correct way

As shown in the examples above, CHP thermal energy can be applied in different applications that run at different temperature ranges. What processes are applicable varies from site to site, from feedstock to feedstock and from the amount of thermal energy available after the digester heating.

Once applications which will bring benefit from the use of recovered heat have been identified, careful planning of distribution of the available heat is required. For example, if a thermal hydrolysis process is selected, the CHP supplier should make sure that exhaust gases and CHP cooling water are available as independent heat sources. Often, however, waste heat is only available in the form hot water. This is because the CHP supplier has set the engine up in such a way that the exhaust gas heat is transferred to the CHP cooling water loop, meaning heat can only be transferred to the rest of the site through the hot water.

Another example can be for a site where sludge pasteurization and digestate evaporation are chosen as thermal applications. One option could be to split the CHP cooling water flow between the two applications. However, detailed analysis has shown that it is better to send all CHP water first to the evaporator and from there on to the pasteurizer. This means that the thermal energy for the evaporator is available at a higher flow rate and higher temperatures, reducing the size and cost of the evaporator installation.

Conclusion

To make best use of recovered heat on an AD plant it is recommended that the designers of the AD process (digestion and biogas production) sit together with thermal engineers and the supplier of CHP engines to come up with the right mix of thermal applications in the early stages of plant design. The result of their work can provide an optimal way of distributing the CHP thermal energy throughout the site. The right design will reduce the investment cost for the equipment and ensure every kW of energy is applied in the most optimal way, maximizing the plants operational efficiency.




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