Biogas is a gaseous fuel containing large amounts of methane (biomethane) which is produced by the anaerobic digestion of biological material under controlled conditions.
BIOGAS AND ANAEROBIC DIGESTION
As well as biogas, anaerobic digestion (or AD) also produces a co-product called digestate, which can be a solid, liquid or combination of the two.
HRS Heat Exchangers produce 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 and evaporation
- Thermal hydrolysis for enhanced gas production
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 HRS G Series heat exchangers with corrugated tubes on the exhaust recovers energy which can be used elsewhere in the plant, including feedstock and digester heating, pasteurisation 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, HRS recommends DTI Series heat exchangers. Thanks to the corrugated tube in tube design, a high level of heat transfer is maintained, without the risk of obstruction or blockages associated with spiral heat exchanger systems, thus ensuring continuous operation in such a harsh environment.
FEEDSTOCK OR DIGESTATE PASTEURISATION
Where required by legislation or quality demands (for example when treating animal by-products or producing digestate to the PAS110 quality standard) pasteurisation may be required at some stage during the anaerobic digestion process. In addition, the operators of AD plants are increasingly seeing pasteurisation as a way of demonstrating the quality of their digestate product and increasing its value.
Whether you are pasteurising the feedstock prior to digestion, or the digestate produced by the AD process, the HRS DPS (Digestate Pasteurisation Systems) can cope with these difficult materials while maximising energy efficiency.
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 fertiliser, 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 HRS DCS (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%. At the same time, unlike conventional digestate driers, the HRS DCS actually increases the availability of crop nutrients. Recapturing energy for use in subsequent concentration phases increases efficiency and reusing water elsewhere in the AD plant prevents additional discharges to the environment.
THERMAL HYDROLYSIS FOR ENHANCED BIOGAS PRODUCTION
HRS has developed a process for the continuous thermal hydrolysis of digester sludge. This 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. Using the Unicus Series scraped surface heat exchanger prevents fouling and makes continuous heat treatment possible. More information about HRS thermal hydrolysis systems can be found here.
BIOGAS DEHUMIDIFICATION SYSTEM
The HRS BDS Series is an efficient solution to cool and dehumidify biogas for combustion with two standard options. The system condenses up to 90% of the water contained in the gas, which is continuously separated before the lean biogas is ready for use. This is a necessary process for all bio-energy plants that use biogas as fuel in CHP engines.
A heat recovery step can be included as a standard option thus reducing energy costs up to 20%, Cold gas at 5°C is used to precool the incoming gas at 40°C, whilst raising the temperature of the dehumidified gas to approximately 36°C before use. A chiller is included to provide the energy required for the process.
The BDS comes complete with controls in a packaged ‘plug and play’ skid.