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Mercaptans, also known as thiols, are organic sulfur compounds that can create serious operational and maintenance challenges in oil and gas facilities. This is especially true when they appear in the fuel gas supplied to Once-Through Steam Generators (OTSGs) used in SAGD operations. While most people associate mercaptans with their strong and unpleasant odor, their impact on OTSGs extends well beyond smell. They contribute to increased sulfur oxide emissions, accelerate corrosion in critical equipment, and can lead to higher maintenance costs and reduced reliability of steam generation systems.
In SAGD facilities across Alberta, OTSGs play a vital role in producing the steam needed for bitumen recovery. These units burn fuel gas, which is often a combination of purchased natural gas and produced gas from the facility. When mercaptans are present in this fuel gas, they introduce technical problems during combustion that can affect both equipment life and environmental performance. This article provides a detailed look at mercaptans in the context of OTSGs, including their physical behavior, the specific problems they create, available treatment options, and practical recommendations for operators.
What Are Mercaptans and Why Do They Matter?
Mercaptans are a family of organosulfur compounds that contain a thiol group (–SH). Their general chemical structure is RSH, where R represents an alkyl group. The most common mercaptans found in oil and gas streams are methyl mercaptan and ethyl mercaptan, although heavier species such as propyl mercaptan and butyl mercaptan can also be present depending on the reservoir and processing conditions.
One of the most important characteristics of mercaptans is their high volatility. Lighter mercaptans have extremely high vapor pressures. Methyl mercaptan, for example, has a vapor pressure well above 1,500 mmHg at room temperature, while ethyl mercaptan is around 440 to 530 mmHg. Because of this, these compounds exist almost entirely in the gas phase under typical pipeline pressures and temperatures. This high volatility means that even relatively low concentrations in produced gas can carry through treating systems and end up in the fuel gas sent to OTSGs.
Unlike hydrogen sulfide, which is usually removed quite effectively by amine treating units, mercaptans are only partially removed by conventional amines. This is one of the main reasons they can persist into downstream fuel gas systems even after primary treating.
Why Mercaptans Create Problems in OTSGs
OTSGs are direct-fired steam generators commonly used in SAGD operations. They burn fuel gas to heat water and produce the high-pressure steam injected into the reservoir. When mercaptans are present in the fuel gas, several problems occur during the combustion process.
During burning, the sulfur contained in mercaptans is oxidized. This leads to the formation of sulfur dioxide (SO₂) and, more importantly, sulfur trioxide (SO₃). SO₃ is particularly problematic because it reacts readily with any moisture present in the flue gas to form sulfuric acid. As the flue gas travels through the convection section and economizer and begins to cool, this sulfuric acid can condense on metal surfaces once the temperature falls below the acid dew point. The result is accelerated corrosion, especially in carbon steel components.
In addition to corrosion, the presence of mercaptans increases the overall sulfur content in the flue gas. This raises SOx emissions from the OTSG stack and can create challenges with environmental compliance. While the odor of mercaptans is the most noticeable effect, the more serious long-term consequences for OTSG operation are equipment corrosion and increased maintenance requirements.
Specific Problems Caused by Mercaptans
Acid Dew Point Corrosion
This is the most significant issue associated with sulfur compounds in OTSG fuel gas. Sulfuric acid condensation in the cooler sections of the boiler leads to pitting, thinning of tubes, and potential leaks. Over time, this type of corrosion can force unplanned outages and expensive repairs to convection sections and stacks.
Increased Maintenance and Reduced Reliability
Facilities that operate with elevated sulfur levels in their fuel gas often experience higher rates of convection section inspections and component replacements. Corrosion-related problems are a recurring source of maintenance cost and downtime in many SAGD operations.
Emissions and Regulatory Considerations
Higher sulfur content in the fuel gas directly contributes to higher SOx emissions. This can affect a facility’s ability to stay within permitted emission limits and may require operational changes or additional controls.
Treatment Options for Mercaptans in OTSG Feed Gas
There are several technologies available to manage mercaptans in fuel gas. The best choice depends on the concentration of mercaptans, whether H₂S is also present, and whether the main goal is odor control or actual reduction of total sulfur.
| Technology | Removes Sulfur? | Converts RSH to RSSR? | Speed of Deployment | Best For |
|---|---|---|---|---|
| Chemical Scavenger Injection | Partial | Yes | Very Fast | Sudden appearance, polishing |
| Caustic Scrubbing | Yes | Partial | Fast | Consistent removal |
| Merox / Catalytic Caustic | Yes (extraction mode) | Yes | Medium | Larger streams |
| Iron Oxide Adsorbent | Yes | Limited | Medium | H₂S + light mercaptans |
| Copper Mixed Metal Oxide | Yes | Better | Medium | Mercaptans + H₂S |
Scavenger Injection for Rapid Response
When mercaptans suddenly appear in OTSG feed gas and there is no existing treating system, chemical scavenger injection is usually the fastest and most practical solution. Modern non-amine scavengers can be injected directly into the gas line using a relatively simple pump and quill arrangement. This approach allows operators to gain control within a few days while longer-term solutions are evaluated.
Scavenger injection is particularly useful during upset conditions or when new wells with higher mercaptan content are brought online. However, if mercaptan levels remain consistently high, the ongoing chemical cost can become significant, and facilities typically look to install a more permanent treating system.
Solid Adsorbent Options
Iron oxide-based adsorbents have been used for many years to remove H₂S from gas streams. They work through reactive adsorption and are very effective for hydrogen sulfide. However, their performance with mercaptans is more limited. They can remove some lighter mercaptans but generally have lower capacity and less consistent results when mercaptans are the main concern.
Copper-containing mixed metal oxide adsorbents offer better performance with mercaptans. The presence of copper helps promote both adsorption of the mercaptans and some degree of conversion to disulfides. These materials are a reasonable solid-bed option for facilities that prefer to avoid liquid treating chemicals, although the spent adsorbent must be properly handled and disposed of at the end of its life.
Conversion to Disulfides vs Actual Sulfur Removal
One of the most important concepts operators need to understand is the difference between converting mercaptans and actually removing sulfur from the gas stream.
Some treatment methods primarily convert mercaptans (RSH) into disulfides (RSSR). This approach can significantly reduce odor and may lower certain types of corrosivity. However, the sulfur is still present in the gas. When this gas is burned in an OTSG, the disulfides will still oxidize and contribute to SOx and SO₃ formation in the flue gas.
For OTSG applications, where protecting equipment from acid dew point corrosion is a priority, methods that remove sulfur from the gas stream are generally more effective than methods that only convert mercaptans into disulfides.
Do Disulfides Cause Problems in OTSGs?
Yes, disulfides can still create issues. Although they are much less odorous than mercaptans, they contain sulfur. During combustion, this sulfur is oxidized and contributes to the formation of SO₂ and SO₃. As a result, disulfides can still lead to acid dew point corrosion in the convection section and stack of an OTSG. Simply converting mercaptans to disulfides does not fully solve the underlying problems associated with sulfur in the fuel gas.
Practical Recommendations
Dealing with Sudden Increases in Mercaptans
When mercaptan levels in OTSG feed gas rise unexpectedly, the first priority should be to regain control quickly. Starting chemical scavenger injection is usually the fastest way to bring levels down while a more permanent solution is developed. At the same time, operators should investigate the source of the increase, whether it comes from new well tie-ins, changes in produced gas composition, or other factors.
Long-Term Approach
For ongoing operations, a layered treating strategy is often the most effective. Amine treating can handle the majority of H₂S, while a secondary step such as caustic scrubbing or a copper-based adsorbent bed can be used to manage mercaptans. Facilities should focus on technologies that reduce total sulfur in the fuel gas when equipment protection and emissions control are the main objectives.
Regular monitoring of both inlet mercaptan concentrations and convection section conditions helps operators detect problems early and take corrective action before significant corrosion occurs.
Conclusion
Mercaptans in OTSG feed gas are more than just an odor nuisance. Their high volatility allows them to pass through many treating systems and enter the combustion process, where they contribute to SO₃ formation and acid dew point corrosion. While converting mercaptans to disulfides can help manage odor, it does not eliminate the sulfur load that affects equipment reliability and emissions performance.
Operators have several effective options available, ranging from fast-deploying scavenger injection for sudden problems to more permanent caustic or adsorbent systems for long-term control. The most successful strategies usually combine the ability to respond quickly with a clear focus on actual sulfur reduction rather than simple conversion of mercaptans to disulfides.
By understanding these issues and selecting the right treatment approach, facilities can better protect their OTSGs, reduce maintenance costs, and maintain reliable steam generation for SAGD operations.
