Safely remove the toxic and corrosive H2S components
Biogas is produced by fermentation of biological waste products and is considered to be one of the most important renewable energies as replacement of natural gas. Syngas is produced by gasification of a carbon-containing fuel and used as intermediate in generating synthetic natural gas. Both gases can contain hydrogen sulphide (H₂S), which needs to be removed to prevent corrosion problems and SOx emissions. Gas desulphurization is therefore essential.
DMT has developed a vast amount of biogas treatment technologies. For the key biogas contaminants, DMT can offer one or a combination of technologies to economically manage them. A technology selection is made by analyzing the mixture of contaminants, the gas flow, the pollution load and the application. By not pushing one technology, DMT can provide the customer with the best solution for that specific project.
The basic principle of the technologies DMT offers to desulfurize gas, is the absorption of H2S to a liquid. An oxidation process converts the H2S to elemental sulfur or sulfate. DMT offers a pure chemical process Sulfurex®CR (Chemical Reaction), a biological process Sulfurex®BF (Biotrickling Filter), and a biochemical process Sulfurex®BR (Biological Regeneration) with integrates a bioreactor for the biological regeneration of the solvent.
Sulfurex®BR is a desulfurization process that combines chemical desulfurization at medium to high pH with biological regeneration of the solvent (caustic). The system consists of a packed column, a biological reactor and a settler.
Sulfurex®BR is a flexible desulphurization technology that achieves low hydrogen sulfide outlet concentrations with low operational expenses thanks to the regeneration step. Since the air injection takes place in the bioreactor, the technology is suitable for biogas upgrading. The technology is suitable for high loads of sulfur, since the reduction in OPEX compensates the higher initial investment cost (in opposition to purely chemical or biological processes).
The biogas enters the scrubber at the bottom of the column and flows upwards through a packed column. A caustic solution is distributed on top of the column over the packing media and falls down through the packing material in counter-current direction of the gas. The packing material inside the column ensures good contact between hydrogen sulfide and the caustic for maximum efficiency.
While the biogas flows through the packed column, H2S is absorbed in the solvent. The biogas leaves the column free of hydrogen sulfide at the top. The saturated process liquid is collected in the sump at the bottom of the scrubber and flows under gravity to the bioreactor.
In the bioreactor, the hydrogen sulfide present in the liquid is biologically oxidized into elemental sulfur by Thiobacillus bacteria. The oxygen required is supplied by an aeration system installed at the bottom of the bioreactor.
During the oxidation, the caustic solution is regenerated before being reused for another washing step in the scrubber.
In the bioreactor, water, nutrients and caustic are automatically refreshed for cellular growth and guarantee good operation conditions. Whenever the measured reactor fluid level is higher than the set point, part of the liquid will be discharged. The bleed stream is harmless and can generally be discharged without any form of post-treatment.
Under optimal conditions, 98% of the hydrogen sulfide is converted into elemental sulfur.
The elemental sulfur in separated from the process liquid in the settler, which can be integrated inside the bioreactor. The settler is fed with a small part of the effluent coming from the bioreactor. Sulfur sludge is removed from the bottom of the settler with a high dry matter content of 5-10% mass.
This sulfur sludge can then be used as a high quality fertilizer.
The Sulfurex®CR technology operates as a counter-current scrubber to ensure the intense contact between (bio)gas and caustic solution in order to reach maximum removal efficiency. During this process H2S gas is absorbed by the caustic solution and converted into NaHS. Continuously H2S-measurement controls the caustic input to ensure high efficiency and fast response to fluctuations in inlet gas. Selective desulfurization is enabled through measuring and control of the gas speed, chemical concentrations, pH values, temperature and pressure at which sulfur is removed from the gas. By controlling the physical and chemical processes in a smart way we are able to minimize caustic consumption.
In the scrubber, both H2S and CO2 are absorbed by the caustic solution. The gas enters the scrubber at the bottom of the tower and flows upwards through a packed column of approximately 4 – 5 meters in height. The caustic solution is sprayed on top of the column and falls down through the packing material in the counter-current direction of the gas. The packing material inside the column ensures good contact between hydrogen sulphide and caustic for maximum efficiency. The gas stream leaves the column almost entirely free of hydrogen sulphide at the top. The solvent is collected in the sump at the bottom of the scrubber and is partly recirculated to the top again and partly discharged.
The selectivity of the Sulfurex®CR process ensures that minimum amounts of CO2 are absorbed to ensure a low caustic consumption. Also, a second scrubber can be added to minimize the use of caustic. The discharge from the first scrubber is re-used to remove the bulk of the inlet H2S.
Cooling and drying of biogas
The Sulfurex is equipped with temperature and H2S concentration measurements in the outlet gasflow. The dosing of caustic to the system is controlled by the hydrogen sulphide concentration measurement in the outlet stream to guarantee good gas quality. Before the gas enters the scrubbing tower, it is introduced in a heat exchanger to reduce its temperature to assure low caustic consumption. Additionally, this heat exchanger acts also as a dryer for the outlet stream, so an extra drying step after the desulfurization unit can be saved in this case.
DMT has developed a new technology where desulfurization is integrated with cooling and drying in one system. The absorption of H2S is more efficient at low temperatures, resulting in a further caustic reduction. In addition, dried biogas prevents condensation and mechanical problems in CHP engines.
Sulfurex®BF technology is a simple way of desulfurization without the use of chemicals. The system operates as a biotrickling filter. In the bioreactor, bacteria convert the H2S into sulfuric acid. By creating the optimal conditions, the most effective desulfurization bacteria are selected. Monitoring and control of process conditions like pH, temperature, nutrients and oxygen level ensure maximal removal efficiency. DMT Environmental Technology always selects the best packing media for the bacteria to grow on. This guarantees a small footprint and easy operation.
Biological desulfurization explained
Bacteria convert H2S into elemental sulphur and/or sulphuric acid, depending on the applied oxygen concentration. Oxygen is added to the gas, by an automatic control system, which adjusts the air flow in relation to the biogas flow and outlet oxygen concentration.
Inside the Sulfurex®BF, water and nutrients are automatically refreshed, sprayed on top of the bacteria and circulated over the bioreactor. The water is heated through a heat-exchanger to create the optimum temperature for the biological process. The produced sulphuric acid and excess biomass are removed from the process through the drain.