REGENERATIVE THERMAL OXIDIZER
The destruction efficiency of organics, the pressure drop and the thermal recovery efficiency of these last generation two tower RTOs is in each way comparable to the performance of previous generation three tower RTOs.
In the biogas upgrading chain, three purification steps produce vent gases that contain enough methane and organic compounds to necessitate that these gases be incinerated prior to their discharge.
Carbon dioxide rejection systems using hollow fibre membrane or vacuum pressure swing adsorption (VPSA) technology reject CO2 with 4% to 5% methane. In the case of landfill gas upgrading the nitrogen content of the feed gas is generally above the allowed limit for pipeline injection and a nitrogen rejection unit (NRU) generally based on VPSA technology is necessary.
The vent gas from these NRUs varies from manufacturer to manufacturer but is generally above 8% methane, finally the removal of non-methane hydrocarbons (NMHC) and siloxanes from the biogas stream using thermal swing adsorption (TSA), produces during the regeneration cycle of the adsorption media, a vent gas with an organic load that varies as per a bell curve during each regeneration cycle.
In other words, the organic load from two of these vent streams is relatively constant while the third one goes through a peak, regularly, every 8 to 10 hours depending on the regeneration cycle time.
The principal challenge to the thermal oxidation of these vent gas is the fact that they contain virtually no oxygen and mostly carbon dioxide. This combination has proven to be a challenge to classic thermal oxidizers as they are ill designed to manage major changes in organic load and lack of oxygen in the gas.
The experience has shown many times that such gas incinerators have problems sustaining a flame even when the methane concentration in the burner shall be sufficient for good combustion.
The regenerative thermal oxidizer, due to its heat recovery chambers, pre-heat the gas up to the incineration temperature and ensures that the oxidation reaction is complete in the combustion chamber.
The remaining challenge is the handling of the cyclic variation in the thermal load. This challenge is addressed by dilution with atmospheric air and the control of that dilution air flow to maintain a constant temperature in the combustion chamber.
The fact that the process is cyclic and repetitive allows for programming of the dilution air flow modulation to avoid hunting and overshooting of the temperature set-points.
Due to its thermal energy recovery and the relatively high thermal content of the biogas vent flow, the RTO will operate without any supplemental fuel. That is a major OPEX advantage over other types of thermal oxidizers.
The only fuel use, generally propane, is to preheat the unit to the operation temperature at the first use and after any prolonged stoppage. For any short duration stoppage the RTO can be put into a stand-by mode which will minimize the cooling of the heat recovery canisters and the combustion chamber to allow a quick restart with very little supplementary fuel use.
The two chamber RTO design for biogas vent application is the state of the art adopted by the biogas upgrading industry.