Farmers are always looking to save on costs and make more profits. They have started to look at their waste to see if they can make money from it - literally. The trend is that larger dairy farms across the country are converting cow manure into Biogas - methane to create renewable natural gas (RNG) to reduce the country’s dependence on fossil fuel use and reduce greenhouse gas emissions.
In the past, dairy farms would store their manure and waste in tanks or covered compost piles until they could spread it across crop fields. Now, anaerobic digesters are being built on farms where the manure is collected in these large tanks where they now produce Biogas – methane. The methane is then captured and refined into RNG to use as energy for operations on the farm or sold to local utilities. The entire digestion process leaves little additional waste to process as the liquid separated will be used as fertilizer for the fields and solids that the farms can use for bedding.
The Science of an Anaerobic Digester
Anaerobic digestion is a sequence of processes by which microorganisms break down biodegradable material without oxygen. The liquid waste is agitated and circulated to maintain a uniform temperature, and the process is temperature sensitive. Optimum biological action occurs between 85°F and 140°F (25-60°C) depending on the process and can take between 15 and 40 days to break down the organic material completely.
Heat exchangers are commonly used as heating systems for this process but at what expense? Shell-in-tube, plate, and frame or spiral heat exchangers transfer energy through a metal barrier that does not allow complete heat transfer. On average, 20% of that energy (sensible heat) is lost to condensate and returned to its source (boiler). This heat is also lost to flash evaporation, resulting in system inefficiencies. In addition, heat exchangers require substantial floor space and are subject to scaling and fouling resulting in significant maintenance activities.
The Hydro-Thermal Advantage
Hydro-Thermal developed the Non-Obstructing Heater (NOH) for many benefits for its end-users in the agriculture market looking to produce RNG. The NOH's ability to efficiently give you the precise temperature you require for your anaerobic digester is critical, whether you are heating for the Mesophilic process at 85-100°F (29-38°C) or Thermophilic process at 120-140°F (49-60°C). In either condition, it should optimize biogas generation and reduce the detention time in the digester.
Biological activity can increase when maintaining a tight temperature band and eliminating hot or cold spots in the digester, as consistent digester temperature improves methane production and sludge stabilization. Less digestion time is possible due to the sludge maintaining its temperature more consistently throughout the digester. Realizing significant cost savings is possible through optimizing the anaerobic digestion process by keeping microorganisms at their required temperatures, optimizing effectiveness, and increasing methane gas production.
No special tools are required to maintain the rugged, flow-through NOH design, helping minimize downtime caused by plugging in heat exchangers. If material gathers and plugs, these units must be opened and manually cleaned to remove the solids plugging the system. Solids in the sludge can also burn onto a heat exchanger's hotter surface (150°F or 10°C). As a result, no maintenance is required to keep the NOH clean. The NOH can be produced using wear and corrosion-resistant metallurgies to withstand grit and highly abrasive materials.
The NOH is available in 2", 4", 8", and 12" sizes and can handle a wide range of flows with its industry-leading Process Flow Turndown of 15:1. With a straight flow-through design and nothing projecting in the path of that flow, it eliminates any chance of clogging and minimizes the pressure drop across the heater. A minimal pressure drop allows for low steam pressure (15 psig) to be used, achieving trim heating.
Conclusion
Dairy farmers can convert their cow’s manure into Biogas-methane to create RNG or sell it back to the grid to generate new income. To ensure the process runs smoothly in the newly built anaerobic digesters, Hydro-Thermal's Direct Steam Injection (DSI) heaters should be installed as the heating source over other systems. The NOH is ideal for heating and maintaining the digester's sludge temperature for breakdown, as it utilizes a smaller footprint and precise temperature control (+/- 1°). In addition, NOH's straight-through design reduces maintenance by operating continuously without plugging or fouling.
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