Road to recovery

Many companies are discovering the effectiveness of using organic waste effluents to generate biogas in an anaerobic digestion (AD) process. But while biogas can be used for generating electricity in a CHP engine, it has many other benefits.

Combined heat and power plant (CHP) engines that are capable of generating 2 MW of electrical power will generate roughly the same amount of thermal energy. This thermal energy must be removed from the engine to assure its correct operation. So wherever cogeneration is applied, thermal energy is available.

But with so many processes available, how can companies best optimise heat integration?

One option is to use the use the heat from the engine cooling water (90 ºC). This heating water can be sent to an external heat exchanger for heating the digester mass or to the heating water to coils that are positioned in the digester reactor walls.

For some types of AD feed stocks thermal hydrolysis is needed. In this process the biomass is heated up to elevated temperatures (usually above 100 ºC). This process breaks down cell structures inside the biomass. The result is a higher biogas yield in the anaerobic digesters. For this process a higher temperature heat source is needed. Normally heat from the CHP exhaust gases (around 500 ºC) is ideal for this.

Certain AD feed stocks require a pasteurisation process. This is usually the case where animal by-products are present to make sure all pathogens are neutralised. In general, this means heating the product to 70 ºC and maintaining it at this temperature for one hour. The heat source to apply for this process is usually the 90 ºC CHP cooling water.

CHP cooling water can also be applied during digestate evaporation. In many cases, considerable costs are involved for removing the liquid digestate off the AD site. Digestate evaporation recovers water from the liquid digestate. This allows both the digestate volume and associated costs to be reduced. It is also possible to use the exhaust gases as the heat source for the evaporation process.

CHP thermal energy can be applied in different applications that run at different temperature ranges. Once it has been decided what applications, process companies must apply 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. Many times, all waste heat is available in the form hot water only. This means that the CHP suppliers has set the engine up in such a way that the exhaust gas heat is transferred to the CHP cooling water loop and from there on the heat is distributed to the rest of the site in one form only.

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

In the early stages of AD plant design, the designers of the AD process should sit together with thermal engineers and the supplier of CHP engines to come up with the right mix of thermal applications. This cooperation is vital to making sure every kW of energy is applied in the most optimal way.