Desulphurisation
Desulfurization selection made easy
Authors: R. Garcia-Cano, R.M. van der Kloet, B. Bakker, J. Langerak, H. Dekker, E.H.M. Dirkse
Keywords: biogas, CHP upgrading, biogas desulfurization, Sulfurex CR, Sulfurex BF, Sulfurex CR, Selection Tool(SSweet), business case calculation
Biogas streams coming from fermentation processes of organic matter in food industry, anaerobic waste water treatment, anaerobic digesters or landfills normally contain hydrogen sulfide (H2S). H2S is a toxic gas which has a highly corrosive character to mechanical equipment, can combust and form sulfur oxides (SOx) which are harmful to the environment, and is lethal even at small concentration. Therefore desulfurization of the gas is recommended.
In time, several desulfurization technologies have been developed, varying from simple ones through wash operations to multi-step recycle systems. DMT Environmental Technology has more than 20 years of experience in desulfurization and our portfolio includes Sulfurex® CR, Sulfurex® BR, Sulfurex® BF and activated carbon. In a Sulfurex® CR, H2S is eliminated through its reaction with caustic in either one or two chemical scrubber(s). Sulfurex® BF is a biotrickling filter in which H2S is removed by bacteria through its conversion into sulfuric acid. Sulfurex® BR uses a combination between a caustic scrubber for H2S removal and a biological reactor for caustic regeneration.
The selection of the most suitable desulfurization technology depends on many factors, such as site conditions, uptime, removal efficiency, final application of the biogas or business case. DMT acknowledges that more than one technology can fit the project and the technology selection might become a tedious work. Our Sulfurex Selection Work Expert Tool (SSweet) incorporates a TCO model to tailor makes the business case by considering specific conditions of the project, so we can help our clients decide on the most suitable technology for their projects, based on Capital Expenditures (CAPEX) versus Operation Expenditures (OPEX) or reliability versus cost.
Introduction
Hydrogen sulfide (H2S) is emitted during regular activities of many industries, such as natural gas or petroleum refining in the Oil & Gas industry, or energy production in power plants (Aliakbar Roosta, 2011). In addition, gasification plants, anaerobic digesters, wastewater treatment plants and landfills normally deal with H2S in any of their activities.
H2S is a colourless and very toxic gas that has a strong odour of rotten eggs. H2S concentrations above 1000 ppmv cause immediate collapse with loss of breathing, even after inhalation of a single breath (ADS Gillies, 2000). Pure gaseous H2S is heavier than air (M.K. Amosa, 2010), and can therefore accumulate in lower areas. It forms flammable mixtures in air in the range of 4.5-45 % vol. and its combustion leads to sulfur dioxide emissions, which have harmful environmental effects. H2S is corrosive to most equipment (pipelines, compressors, gas, storage tanks, engines, etc.).
When biogas is used in an engine or CHP, the trouble-free operating window is only 100-500 ppm of H2S (Steinhauser, 2008). Above these concentrations, the lifetime of the equipment significantly decreases and can reach down to less than one year at H2S concentrations above 1000 ppm. In such cases, desulfurization technologies have a Return On Investment (ROI) which is 50% or higher. H2S also acts as strong poison for fuel cells and reformer catalysts (Laura Bailon Allegue, 2014). Therefore, H2S has to be removed due to potential mechanical, environmental and safety reasons.
In the gas treatment industry, the term desulfurization refers to processes used for the removal of sulfur compounds, such as H2S, SO2 (sulfur dioxide), sulfur trioxide (SO3), carbonyl sulfide (COS), carbon disulfide (CS2), sulfur vapor (Sx) or mercaptans (R-SH). This article focuses on gas treatment technologies to remove H2S from biogas.
Biogas desulfurization technologies
Several technologies have been developed to accomplish gas desulfurization. These processes vary from simple once-through wash operations to complex multiple-step recycle systems (Arthur Kohl, 1997). In general, the process complexity arises from the introduction of a recovery process in order to regenerate the consumables employed in the desulfurization process.
In the case of biogas, H2S can be removed in-situ during its anaerobic production in digesters, or from crude/raw biogas before its final application.
In digesters, H2S can be removed by air injection or addition of iron salts/oxides. Air injection is only suitable for bulk removal. It presents the inconvenient of decreasing biogas production in the digester and decreasing the calorific value of the biogas, making the biogas unsuitable for upgrading purposes. The remaining sulfur or sulfate in the system (produced during the desulfurization step) can lead to the renewed formation of H2S or the deposition of yellow clusters of sulfur on surfaces, increasing risks of corrosion. The addition of iron salts/oxides is effective in reducing high H2S levels, but less effective in attaining a low and stable level of H2S in the range of vehicle fuel and/or gas grid injection requirements (Laura Bailon Allegue, 2014). In addition, the most typical iron salt used in such a process is iron chloride, which can cause severe corrosion in case of high dosing flows.
In the case of raw biogas desulfurization, technologies are normally classified based on the nature of the separation process: physical-chemical or biotechnological (Nicolas Abatzoglou, 2009) (Laura Bailon Allegue, 2014). Physical-chemical processes normally involve the absorption of H2S into a liquid or its adsorption on a solid, usually followed by the conversion of H2S into another compound. These technologies traditionally and currently still dominate the desulfurization market. However, in the past two decades, increasing attention has been paid to biotechnological methods for the biological degradation of H2S. These technologies can achieve high removal efficiencies with low OPEX. Nevertheless, basic and applied research for optimization is still performed to enhance the performances of the system (René van der Kloet, 2011). Methods that combine physical-chemical and biotechnological methods have also been developed to benefit from the advantages of both desulfurization technologies and achieve higher H2S removal.
The selection of the best technology depends on many factors, such as flow rate, inlet concentration of H2S, final application of the biogas (maximum allowed concentrations of H2S and oxygen) and site specific conditions.
For boilers or engines, the addition of iron chloride or air/oxygen to the digester is the common process for a bulk removal of hydrogen sulfide. In case of simultaneous production of heat and electricity in CPH units, biological processes are commonly used to perform the desulfurization. If concentrations below 100 ppm are required, a post-treatment is regularly required after the biological process. For high concentrations of H2S, chemical scrubbers are the typical choice, with the possibility of integrating a regeneration step to reduce the OPEX.
In the case of biogas upgrading, low concentrations of oxygen are critical for the production of biomethane, CNG or bio-LNG (e.g maximum oxygen concentrations of 0.5 % for gas grid requirements). Activated carbon filters are typically used as a pre-treatment to remove H2S prior to the upgrading process for low H2S concentrations and loads. For medium or high loads, alkaline scrubbers are normally used, with the integration of regeneration processes at high flows and/or high concentrations. Upgrading technologies based on physical sorption, such as water scrubbers or non-water sorbents, can simultaneously remove H2S and CO2, but usually require a pretreatment to achieve low concentrations. Otherwise a post treatment is required to remove H2S from the exhaust air.
DMT desulfurization technologies
DMT acknowledges that no biogas stream has the same properties and that more than one technology can fit for one project. As a specialist in gas treatment technologies, we offer a wide portfolio for biogas and landfill desulfurization purposes.
The principle of our technologies is the absorption of H2S in a liquid and further oxidized to elemental sulfur or sulfate. Sulfurex® BF (Biotrickling Filter) is a biological process in which the H2S is absorbed in an aqueous solution at low pH and then biologically oxidized to sulfur/sulfate in-situ. Sulfurex® CR (Chemical Reaction) is a chemical scrubber in which H2S is absorbed into a liquid at high pH to enhance the absorption capacity, and then further oxidized during the aerobic treatment of the effluent in a wastewater treatment plant (WWTP). Sulfurex® BR (Biological Regeneration) is the combination of Sulfurex® CR with a bioreactor for the biological regeneration of caustic. In Sulfurex® BR, H2S is absorbed into a solvent under alkaline conditions, then further oxidized into sulfur in the bioreactor.
The principles of our Sulfurex® products are described below.
Biological desulfurization Sulfurex® BF is a biological desulfurization technology to remove H2S from biogas and landfill gas. The principle of the technology is the biological oxidation of H2S, with the addition of air, by Thiobacillus bacteria. Depending on the amount of oxygen available, H2S is biologically oxidized into elementary sulfur or sulfuric acid
The main element of the system is a packed column. Its packing material is simultaneously used for nurturing the microorganisms and enhancing the contact between the bacteria H2S. Water is continuously recirculated and sprayed over the bacteria, and H2S is absorbed into the liquid phase and then biologically converted. Oxygen is added by an automatic control system, which adjusts the airflow injected, to produce selectively sulfuric acid. The process incorporates a heat exchanger to maintain an optimal temperature. Nutrients are added for the biological growth and the produced sulfuric acid and excess of biomass are removed from the process through the drain.
Sulfurex® BF is cost-effective, requires low investment (CAPEX) and provides efficient removal of H2S (95 % of removal efficiency) without the use of chemicals. The system only requires water and nutrients, which leads to low operational expenses. However, the process is not suitable for all applications since the addition of oxygen is limiting the final application of the biogas (such as biogas upgrading).
Sulfurex® BF can be used for biogas and landfill gas desulfurization before its utilization in CHP units for low and medium loads. The main cost of this technology is make-up water and its disposal, so Sulfurex® BF is interesting in locations with cheap access to make-up water and disposal, such as WWTP.
Chemical desulfurization
Sulfurex® CR is a chemical desulfurization process specifically designed to treat biogas streams with high concentrations of H2S. The principle of the technology is the neutralization of H2S by caustic soda according to the following reaction: H_2 S +NaOH↔NaHS+H_2 O [3]
However, part of the caustic reacts with carbon dioxide (if present in the inlet gas), producing carbonates CO〗_2 +2NaOH↔〖Na〗_2 CO_3+H_2 O [4]
The main element of the system is one packed column, which is used to put in contact the biogas and the caustic solution in countercurrent. H2S is absorbed into the liquid phase and then reacts with caustic. The technology incorporates a system control to continuously measure the outgoing concentration of H2S. The dosing of caustic is adjusted to guarantee high efficiency and fast reaction to fluctuations in the inlet gas, providing a full control on H2S outlet concentration.
DMT gives the possibility of adding an extra column, in a double stage process. The spent solvent in the first column is recirculated in the extra column, where the caustic is reused and more H2S is removed according to the reaction: H_2 S +Na_2 CO_3↔NaHS+〖NaHCO〗_3 [5]
Both processes can integrate a cooling step, in which the biogas is simultaneously cooled down and dried in one step. Since the scrubbing process of H2S has been proved to be more efficient at low temperature, the caustic consumption can be thus reduced. In addition, dried biogas prevents condensation and mechanical problems during the final application of the biogas. The integration of a second column in addition to the cooling step can save up to 30 % of caustic consumption.
The chemical scrubber is a robust and flexible technology that allows an exhaustive control of the outlet concentration. It is a compact unit which requires low initial investment, but leads to high operational expenses due to caustic consumption. Since oxygen is not injected, this technology is suitable for biogas upgrading purposes.
Figure 2. One of our references in the UK (Sulfurex® CR 2-stage ) (left). Schematic process of Sulfurex® CR, 1-stage (right) Bio-chemical desulfurization. Sulfurex® BR is a desulfurization process that combines chemical desulfurization at medium to high pH with biological regeneration of the solvent. The system consists in a packed column, a biological reactor and a settler which can be integrated in the bioreactor. Figure 3 shows a basic process flow diagram of a Sulfurex® BR process.
In the packed column, an alkali solution containing caustic is sprayed over the column in countercurrent. H2S is absorbed into the liquid phase and reacts with caustic according.
The absorbed sulfide is then, with the addition of air, biologically converted into elementary sulfur by the Thiobacillus bacteria in the bioreactor. The sulfur produced is then recovered from the washing liquid in settler for further treatment. The liquid is recirculated in the packed column to remove more H2S.
The production of these components is minimized by controlling the addition of air in the bioreactor. Sulfurex® BR is a flexible desulfurization technology that achieves low hydrogen sulfide outlet concentrations with low operational expenses thanks to the regeneration step. Since sulfur can be used as fertilizer, our technology does not generate waste streams. Since the air injection takes place in the bioreactor, our technology is suitable for biogas upgrading. In comparison to Sulfurex® CR, the addition of the bioreactor leads to a higher CAPEX but a lower OPEX, since the caustic consumption decreases significantly thanks to the regeneration step. This technology is more suitable for high loads of sulfur, in which the reduction of the OPEX compensates the high initial investment.
The selection of the right desulfurization technology can be hard, with many factors playing an important role such as CAPEX, OPEX, final application of the gas, site conditions, uptime, reliability, etc. In many cases, more than one desulfurization technology can be applied as a solution. As DMT, we find it very important to guide our clients to the best solution for their projects, so we have developed our very own tool, the Sulfurex Selection Work Expert Tool (SSweet).
The SSweet includes our Desulfurization Sweet Spot Selection graph and mathematical model to estimate the Total Cost of Ownership (TCO). The graph is used as a preselection overview of the most suitable technology for our client’s project based on biogas flow rate and inlet H2S concentration. This preliminary selection is based on our long-standing experience of more than 20 years in desulfurization projects. In addition to this graph, a tailor-made business case is generated by considering specific site conditions site of the client (such as electricity price, water price, caustic price, discharge cost or labor cost), providing the final CAPEX and OPEX for the project. With our SSweet, we can thus help our customers build an otherwise complex business case and choose the ideal solution for their projects, based on CAPEX versus OPEX or reliability versus costs.
In the following paragraphs, we illustrate the features of our SSweet with a general example. The gas stream, with a flow rate of 350 Nm³/h, contains 55% of CH4, 44% of CO2 and 4000 ppm of H2S. In the preselection phase, based on inlet flow and concentration, more than one desulfurization technology is shown to be suitable without any additional information (Figure 4). However, when our tool is filled out accordance to a client’s specific needs, the best-fitted solution to can be easily pinpointed through the adjustment of the TCO, as shown in the following examples based on the general case.
Paper industry
In a Paper Factory, the client obtains biogas from the anaerobic treatment of the effluent. The client wants to use the biogas in a boiler unit, requiring H2S concentrations lower than 100 ppm in the biogas. Since the customer is using caustic during the pulping process, caustic is already available in-situ and its consumption is not the main concern. The client however, specifies that uptime and easy operation will be leading parameters in the final decision.
Wastewater treatment plant (WWTP)
In a WWTP facility, the client obtains biogas from the anaerobic biological treatment of the effluent. The client wants to use the biogas in a CHP unit for electricity production, requiring H2S concentrations lower than 200 ppm in the biogas. The customer has access to water and effluent of the technology can be disposed into the wastewater treatment facility. The client prioritizes a cheap and environmentally friendly solution. Figure 4 analyzes the TCO for the specific site conditions of the client. Considering the specific site conditions, along with the preferences of the client, Sulfurex® BF is the most attractive solution for the client.
Biogas Upgrading
The client wants to upgrade raw biogas from a mono-digester with chicken manure to biomethane. The desulfurization technology has to be reliable, with high uptime and oxygen injection is not allowed. In this case, the Sulfurex® BF is not an option.
This customer’s most attractive choice of desulfurization technology (prior to upgrading) is Sulfurex® BR. The biological regeneration reduces significantly the OPEX of the process, and compensates the higher initial investment costs .
Conclusions
Hydrogen sulfide (H2S) is produced by many industries as an undesirable gaseous by-product. Due to its characteristics (high toxicity, corrosive attributes and the own characteristics of its oxidized form – sulfur oxides, SOx), an H2S removal step is usually a sine qua non part of the overall process, whether the resulting gas stream is to be used in another part of the process or simply released into the atmosphere. However, this removal step can take many forms: the processes can first be distinguished between pre- and post-treatment processes. In pretreatment processes, the problem is (partly) solved at its source whereas post-treatment processes inherit the issue. Post-treatment processes can then be distinguished between biotechnical and physical-chemical processes.
In DMT, we have made the choice to focus on post-treatment technologies for H2S removal more than 20 years ago. Thanks to the high versatility of technologies, enhanced with the possibility of combination, we have developed a chemical solution (Sulfurex® CR), biological solution (Sulfurex® BF) and a biochemical solution (Sulfurex® BR), and we are also offering activated carbon filters as a solution for small loads. This portfolio of products enables us to respond to most of our customers’ specific needs, from small loads and flows to high loads and high flows, from Oil & Gas industrials to AD plants owners’ needs.
Nevertheless, offering a solution is not enough for us. We have to offer the best solution to our customers, a solution that will be best-fitted to their projects and will respond to their expectations. However, the choice of the ideal technology for a specific project is dependent on a myriad of factors – whether technical and financial. The technical factors are usually intrinsic to the type of technology used. The financial factors are also intrinsic to the type of technology used but cannot be distinguished from more extrinsic specifications such as on-site conditions or capacity of investment amongst others.
This problematic led us to the creation of the Sulfurex Selection Work Expert Tool (SSweet). Combining a preselection step based on our experience in the field of desulfurization with a tailor-made business case, consumables and costs calculations (CAPEX, OPEX, TCO), this tool enables us to select, together with our customers, the most attractive technology for their projects – both from a technical and financial standpoint. Each project has its unique characteristics and so it should have its unique business case, which is what we, like DMT, want to offer with the help of our tool.
With the Sulfurex Selection Work Expert Tool (SSweet) and our expertise, we believe that helping our customers with building their tailor-made business case and choosing the best solution based on facts contributes to the success of a project and to a clear and prosperous future