Heat Recovery in Biogas Plants
Sulphur
The problems involving sulphur particularly apply to biogas. If heat exchangers and steam generators are to enjoy a long service life, it is important to optimise the system design. Our project department will work with you to find the best possible solution.
The following fundamental rules apply here:
- The design of the exhaust gas heat exchanger is based on the nominal operating data (exhaust gas mass flow rate, exhaust gas temperatures, cooling water temperatures etc.)
- It should be borne in mind that diverging operating conditions may result in critical operating states.
- This particularly applies to: partial load mode, frequent stops and starts (e.g. due to insufficient biogas production), cooling water temperatures lower than specified.
In the exhaust gas SO2 and SO3 (sulphur trioxide) are in balance:
e.g. SO2 can be oxidised to SO3 via catalytic converters. While SO2 passes through the exhaust gas heat exchanger as gas, SO3 may form H2SO4 in conjunction with water.
Even low SO3 concentrations can cause the dew point temperature to rise quickly. SO3 is a decisive factor for the acid dew point temperature.
Unlike inert SO2, the SO3 present will immediately react with the water vapour always contained in the raw gas, to sulphurous acid (H2SO3) then being converted to sulphuric acid (H2SO4).
Given the extreme differences in the temperature of ebullition for H2O (100°C) and H2SO4 (338°C), condensation under dew point conditions can also result in high concentrations in the condensate with low concentrations of H2SO4 in the flue gas.
The SO3 content depends on factors such as the sulphur content of the fuel, the type of combustion, flue gas routing, the oxygen available via the air ratio, the adsorption capacity of flue dust and deposits.
The consequence of excessively high sulphur values in the combustion gas is apparent
Pressure drop
Given the increased exhaust gas outlet temperatures, it is generally possible to use "large" smoke tubes subject to compliance with the installed length specified. Experience has shown that smoke tubes with an inside diameter d < 20 mm should not be used for biogas operation. As shown in the diagram, there is a significant reduction in the pressure increase with contamination as well as the time required for cleaning due to the decrease in the number of tubes when compared with small tube diameters.
Catalytic Converters
The use of catalytic converters also results in SO2 being oxidised to SO3. This makes desulphurisation using activated carbon filters essential. The activated carbon filter functions in a defined humidity range of the gas. FriCon, the gas dehumidifier recently developed by APROVIS, allows you to adjust the humidity to ensure optimum efficiency levels. With FriCon you can:
- avoid the formation of condensate in downstream gas pipework.
- partially separate H2S, NH3 and organic silicium compounds.
- separate suspended matter.
- improve the calorific value.
