Heating for the biogas reactor is a major issue since the average ambient air and ground temperatures (as discussed here and shown here) are below freezing most of the year. The slurry temperature inside the reactor needs to be maintained around 30°C for optimum gas production.
Traditional heating methods
As the temperature lowers, the gas production rate drops significantly. Near freezing temperature, no gas will be produced. Although some heat from the slurry is lost to the air, the majority of the heat loss is actually through the walls of the reactor, to the surrounding earth. This is a significant loss, and must be overcome to sustain the production of methane.
Possible methods to mitigate heat loss include a greenhouse, compost heaping, solar heating, inlet heating, insulation, and a double-walled reactor.
- A greenhouse can provide a warm environment to elevate the ambient air and, for approximately a meter, ground temperatures, and to warm or thaw stored water and waste products. The greenhouse can also provide an environment for crop growth. To further minimize heat losses, the greenhouse roof can be equipped with a night curtain.
- Compost heaping on top of the reactor , as pictured above, generates heat to warm the ground temperature of the earth on top of the dome, minimizing heat loss through the top of the dome.
- Solar heating options are either photovoltaic panels connected to electric-resistance heating elements , or solar water heating panels with heat exchangers. These would be installed in or around the reactor, accessible through the effluent exit. Some case studies have found that heating coils corrode inside bioreactors, so the heating coils will be removable for ease of management.
- Inlet heating elevates the temperature of the water or slurry mix prior to feeding it into the reactor. Solar dishes—which are capable of heating water to over 70°C could be used to heat the water or slurry mix.
- Insulation can be applied to exterior reactor tank walls, slab and outlet chamber, and to the water storage tank.
- A double-walled reactor can be constructed that has a cavity between the inner and outer walls and slabs to minimize heat losses to the earth. This design is similar to a double pane window design. A double wall reactor would require additional structural considerations due to the pressure of the contents of the reactor on the walls of the reactor.
Because of the unique heating challenges posed by Mt. Everest, we will be using a combination of solar heating, insulation, inlet heating and a greenhouse.
The greenhouse aids our heating design in a number of ways. It raises the air temperature surrounding the reactor and shelters it from winds, which negates convective heat-losses. The greenhouse creates a warm place to store the waste and water and keep it from freezing, a passive way to melt any ice that may have formed, and a roof-surface for collecting rainwater. It could also be used to grow plants using the fertilizer from the biogas reactor.
The biggest heat loss is through the ground, so adding insulation like hard Styrofoam will be required to reduce the amount of heat needed to be added to the reactor.
There are two passive options for heating the slurry before being put into the inlet. The slurry can defrost in the greenhouse or be heated by using a solar cooker. If solar cookers were used, it would pasteurize the human waste before it enters the system, thus reducing the required retention time from 90 days to 30 days (which gives a significant increase to the gas output).
Solar heating system.
A non-passive heating method provides some control over the internal temperature of the reactor . There is a heater that is removable so that it can be maintained (by sliding it into the effluent exit, as shown below) and shut off if, during the warmer summer months, when the reactor begins to get too hot.