Geothermal sulphur removal
Vent gases from a geothermal power plant have consistently met H2S emissions specifications for over 25 years
David Jackson, Merichem Company
Mark Kolar, Coso Operating Company
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Dubbed “the sun beneath our feet”, geothermal energy has moved from being a niche player to becoming a viable contender in making 100% clean electricity available to the world. It is a reliable source of power that has a small land footprint compared to other renewable sources and can be harnessed at both large and small scales. The industry is expanding, and its infrastructure is long-lasting.
In the eastern portion of Central California on the military-owned Naval Air Weapons Station near China Lake, the Coso Geothermal Field, operated by Coso Operating Company, has been producing geothermal power continuously since 1987. It is one of the top three producers of geothermal electrical power in the US. Coso provides power to the southern California power grid and plays an important role in supporting the State’s mandated Renewable Portfolio Standard (RPS). It also supplies approximately 8% of the entire geothermal power in the US.
The Coso generating facility consists of four separate but interlinked geothermal power plants with nine 30 MW turbine-generator sets for a total of 270 MW of rated capacity, enough power to supply 250,000 homes. Due to the high pressures and temperatures encountered in the field, which allow the units to operate above their initial rated capacity, the net running capacity is higher than the rated capacity at 302 MW.
Between 80 and 90 production wells operate at any given time, producing a mass flow rate of more than 14 million pounds per hour. Depending on the volume of fluid that needs to be handled and where pressure support is required, the Coso field can employ 30-40 injection wells. Because of the high-temperature fluids, the power plants utilise double-flash technology for steam extraction. Wellhead pressures range from 85-500 psig. Produced fluids are moderately saline chloride brines with total dissolved solids from 7,000-18,000 ppm. Non-condensable gases account for 6% of the gas fraction, with 98% of that from CO₂. Hydrogen sulphide ranges from <10-85 ppm.
After the wells are tapped and gathered, the steam wells produce electricity from the renewable geothermal energy source. The produced steam passes through a set of turbines/generators, and the non-condensable vapours are separated from the condensed steam (water) at low pressure. Finally, the brine is reinjected into the geothermal field.
The non-condensable vapours cannot be vented to the atmosphere until the particles of hydrogen sulphide (H₂S) are removed. During the initial facility start-up, the H₂S-laden vapours were reinjected into the geothermal field with the water. Over time, this H₂S abatement method became more costly, mostly due to compressor maintenance. In 1993, a Merichem Lo-Cat unit was installed, the first of three. Post-start-up, the non-condensable carbon dioxide (CO2)and H₂S are flashed, compressed, and routed to the Lo-Cat unit for sulphur removal before being emitted into the atmosphere. The Lo-Cat process has been removing H₂S at this facility for over 25 years and has significantly reduced sulphur emission exceedances and operating costs compared to other technologies previously employed.
The site now has a total of four power generation facilities, two of them containing Lo-Cat units: the Navy 1 power plant and Navy 2 power plant with three Lo-Cat units (see Tables 1 and 2). There are two Lo-Cat units at the Navy 2 site: Navy 2 and Navy 210. Only Navy 210 will be discussed here because Navy 2 is operated periodically.
The Lo-Cat process converts H₂S contained in the following equation:
H₂S (g) + 1/2 O₂ (g) H₂O + S0
Before entering the Lo-Cat unit, raw feed gas passes through an activated carbon bed to absorb mercury and other heavy metals. The raw gas then enters the auto-circulation vessel, where the H₂S is absorbed into a proprietary Lo-Cat catalyst solution. The catalyst is deactivated in the absorber section, where H₂S is converted to elemental sulphur. Subsequently, the catalyst is regenerated in the oxidiser section of the same auto-circulation vessel. Regeneration is achieved by contacting the catalyst solution with oxygen contained in air. The air and sweetened gas exit into the atmosphere as vent gas. The solution is circulated between the absorber and oxidiser sections via a system of baffles and weirs with density difference as the driving force.
Elemental sulphur formed via the reaction becomes suspended in the catalyst solution. A circulation pump sends a slipstream of solution to a settler vessel to remove the elemental sulphur from the process, which allows the sulphur to concentrate and form a slurry The slurry is routed to a filter that separates the sulphur from the catalyst solution and washes the filter cake. The sulphur is discharged into a sulphur bin while the clarified solution (i.e., filtrate) is returned to the auto-circulation vessel.
Even with water washing of the sulphur filter cake, some catalyst solution exits with the solid sulphur. Makeup catalyst is added to maintain the solution at optimum concentrations. A surfactant is also added to help prevent foam and floating sulphur. Potassium hydroxide (KOH) is added for pH control.
Two key parameters ensure consistent Lo-Cat operations as follows: (1) Prevent sulphur from settling in undesired locations, and (2) Maintain proper solution chemistry. Catalyst makeup and chemical addition rates are discussed later.
The main method to prevent undesired sulphur settling is to use ‘air blasts’ placed strategically throughout the unit in regions of low flow. Nozzles send bursts of air into these areas within the auto-circulation and settler vessels, preventing sulphur build-up. When feed gas flows through the unit at the process design rate, undesired sulphur settling is less likely to occur.
Coso and Merichem have developed special flushing and ‘sparger shuffling’ methods to prevent sulphur settling when the unit is operating at low flow rates. The gas flow is routed to selected distributors to maintain the desired flow patterns. Water is then periodically flushed through these distributors to keep them clean. This ‘shuffling’ is done approximately every 4-8 hours to each sparger in rotation.
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