Biogas is a gaseous fuel containing large amounts of methane (biomethane), which is produced by the anaerobic digestion of biological material under anaerobic conditions. Anaerobic digestion (or AD) produces biogas and co-product called digestate, which can be a solid, liquid or combination of the two. AD produces green energy in the form of biogas through the bacterial breakdown of organic materials under anaerobic conditions. In doing so, not only does it create a highly versatile energy carrier (biogas can be used to produce electricity, used as a replacement for natural gas or used as a transport fuel), but the residue left behind after the process is a nutrient-rich organic fertiliser and soil conditioner. Furthermore, by treating waste, including food, crop residues and livestock manures in an AD plant, any methane emissions which they may have emitted to the atmosphere during their decomposition or treatment via other means, are captured and utilised, preventing the emission of a particularly potent GHG.
In order to maximise the greenhouse gas savings which AD can provide, it is important that the feedstocks used are both sustainable and produced with as low a carbon footprint as possible. It is also important to make sure that the process is as efficient as possible and that every possible unit of energy is used. Using heat exchangers to recapture heat for reuse in the AD plant or elsewhere is one of the most cost-effective ways to do this. This article describes various heat exchangers and other systems for use throughout the anaerobic digestion process, including:
- Exhaust gas cooling and energy recapture
• Feedstock and/or sludge heating
• Feedstock or digestate pasteurisation
• Digestate concentration & evaporation
• Thermal hydrolysis for enhanced gas production
Figure 1: Exhaust Gas Cooling
Exhaust Gas Cooling
Cooling and recapturing the heat from exhaust gases can increase the efficiency of combined heat and power (CHP) plants used to generate electricity using biogas. Using heat exchangers with corrugated tubes on the exhaust recovers energy which can be used elsewhere in the plant, including feedstock and digester heating, pasteurization and digestate concentration.
Feedstock and Sludge Heating
The contents of the digester must be heated to maintain the ideal temperature for the bacteria to work. In addition, pre-heating the feedstock prior to putting it in the digester can reduce the amount of heat needed in the digester itself and improve the overall efficiency of the digestion process.
For such applications double tube heat exchangers can be used. It is an industrial double tube heat exchanger, comprising a tube within a tube. The inner tube is corrugated for increased heat transfer and reduced fouling without the risk of obstruction or blockages associated with spiral heat exchanger systems, thus, ensuring continuous operation in such a harsh environment. The product flows through the inner tube and the service fluid through the annulus between the inner and outer tube. Because of its geometry, the double tube heat exchanger is a true counter-current heat exchanger. An expansion joint (bellow) is fitted in the shell to allow for differential expansion of the inner and outer tube during operation. Multiple units can be interconnected and have the options of frame mounting, insulation and cladding in stainless steel.
Feedstock or Digestate Pasteurization
During the anaerobic digestion process, quality of digestate can be improved by pasteurization.
Digestate Pasteurization System (DPS) has been specifically designed to meet the requirements of pasteurisation in the anaerobic digestion and renewable energy sectors. The DPS is capable of pasteurizing digestate, feedstocks, sludge and similar materials and is suitable for both pre-and post-digestion pasteurization, allowing operators to maximize the efficiency of their overall process.
Figure 2: HRS make double tube heat exchanger
Digestate Concentration and Evaporation
After digestion and biogas production, the digestate is normally separated mechanically in solid and liquid phases which can then be used as biological fertilizer, although the two streams will have different storage and handling requirements. In some cases biogas operators will have to pay for waste handlers to take care of the removal of the digestate which means extra cost.
Whether or not you have a use or market for digestate, reducing its volume also reduces storage, transport, application and disposal costs. The Digestate Concentration System applies an evaporation process to concentrate the digestate. In a multi-stage evaporation process, the digestate volume can be reduced to less than 20%.
The numerous benefits of the Digestate Concentration System (DCS) system include:
- Removal of upto 80% of the water volume to produce a material containing 20% dry solids.
• Increase the nutrient concentration of the digestate accordingly with minimal loss of nutritional value.
• Reduces the handling, storage, transportation and application requirements along with associated operational, overhead and capital cost savings. For example, the amount of storage (and transport) required can be halved.
• Water removed from the digestate can be added to the feedstock before digestion to improve process efficiency and create a closed loop system.
• The heat involved in the DCS process is recovered and re-used up to a maximum of four cycles, resulting in a highly efficient process.
• Reduces odors and increases nutrient content by turning ammonia into ammonium sulphate.
• Unlike conventional digestate dryers – which use a lot of power to create a small amount of product – the DCS is extremely energy efficient.
Figure 3: Thermal hydrolysis for enhanced biogas production
Thermal Hydrolysis for Enhanced Biogas Production
A process of continuous thermal hydrolysis of digester sludge involves heating the sludge to 160-170 °C of the sludge and a steam explosion step can be included for extra efficiency. This heat treatment changes the cell structure of the compounds, breaking down lignin and hemi-cellulose chains to create free sugars which are easier for the bacteria to digest. Consequently, residence times in the digester can be reduced.
The design is based on a traditional shell and tube heat exchanger with scraping elements inside each interior tube. The reciprocating movement of the hydraulically operated scrapers mixes the fluid and cleans the heat exchange surface. This keeps heat transfer high and reduces downtime as cleaning in place (CIP) can be avoided. In addition, the scraping movement introduces turbulence in the fluid increasing levels of heat transfer.
Conclusion
There are many different ways to maximise both the efficiency of biogas plants, and the usefulness and effectiveness of digestate. With the right advice and by choosing the correct technology, it is possible to make sound investment decisions which will not only increase plant efficient, but also improve the overall environmental profile of AD – and maximise its benefits in terms of mitigating climate change.
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