Oct-2023

27 years of reliable sulphur removal: Coso case study

LO-CAT® technology is a liquid reduction-oxidation (redox) process that uses catalyst in an aqueous solution to convert hydrogen sulphide into elemental sulphur.

Mark Kolar, Coso Operating Company
William Echt, Merichem Company

Article Summary

Plant History: Coso Operating Company operates a 300-megawatt electric generation facility at the China Lake Naval Weapons Station approximately 170 miles northeast of Los Angeles, California at Coso Junction. After they are tapped and gathered, the steam wells produce electricity from the renewable geothermal energy source. The produced steam is passed through a set of turbines / generators. 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 small amounts of hydrogen sulphide (H₂S) are removed. When the plant initially started 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, the first of three liquid redox units was installed. After start-up the non-condensable carbon dioxide and hydrogen sulphide are flashed, compressed and routed to the liquid redox unit for sulphur removal before being emitted into the atmosphere. The composition of the flash gas is very similar to a dilute amine acid gas.

The redox process has been removing H₂S at this site for the past 27 years. This technology greatly reduced sulphur emission exceedances and operating costs relative to technologies used prior to installing the LO-CAT unit.¹

The site has a total of four power generation facilities, two of them containing redox units: the Navy 1 power plant and Navy 2 power plant. There are a total of three LO-CAT units, per Table 1 and Table 2. Note that at the Navy 2 site, there are two units, the Navy 2 unit, and the Navy 210 unit. Only the Navy 210 unit will be discussed because the Navy 2 unit is only periodically operated.

Overall, the units have operated well throughout their history. This paper reviews the performance data from the of the Navy 1 and Navy 210 liquid redox unit operations, including the performance, stability and reliability of unit operations. The current cost per ton of sulfur produced will also be reviewed.

LO-CAT process description and process flow
The liquid redox process converts H2S contained in the raw feed gas into elemental sulfur via the following equation (see Figure 1 for the process flow scheme):

H₂S (g) + 1/2 O₂ (g) γ H₂O + Sº

Before entering the unit, raw feed gas passes through an activated carbon bed to absorb mercury and other heavy metals. The raw gas then enters the autocirculation 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 autocirculation vessel. Regeneration is achieved by contacting the solution with oxygen contained in air. The air and sweetened gas exit to the atmosphere as vent gas. The redox 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. To remove the elemental sulphur from the process, a slurry pump sends a slipstream of solution to a settler vessel which allows the sulphur to concentrate and form a more concentrated slurry. The slurry is routed to a filter which separates the sulphur from the redox solution and washes the filter cake. The sulfur is discharged into a sulphur bin while the clarified solution, i.e., filtrate, is returned to the autocirculation vessel.

Even with water washing of the sulphur filter cake, some liquid redox solution exits with the solid sulphur. Make-up catalyst is added to maintain the solution at optimum concentrations. A surfactant is added to help prevent foam and floating sulphur. Potassium hydroxide (KOH) is added for pH control.

Operations review
Two key parameters ensure consistent unit operations as follows: (1) Prevent sulphur from settling in incorrect places, and (2) Maintain proper solution chemistry. Catalyst make-up and chemical addition rates are discussed in the next section of this paper.

Operating practices keep sulphur from settling in the wrong places within the unit. The main method is to use ‘air blasts’ that are placed strategically throughout the unit in regions of low flow. Nozzles send bursts of air into stagnant areas within of the autocirculation and settler vessels, preventing sulphur build-up. When feed gas flows through the unit at the process design rate, sulphur in solution is less likely to settle in the wrong places within the unit.

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 to each sparger head (internal vapour distributors) is blocked, allowing gas pressure build-up. Water is then periodically flushed through the spargers to keep them clean. This ‘shuffling’ is done approximately every 4-8 hours to each sparger in rotation.

Because of this attention to detail, Coso runs both active liquid redox units consistently for a full year until the entire plant takes the mandated geothermal field shutdown. The annual turnaround takes 2-3 days from gas-off to gas-in. The shutdown and turnaround are always completed, even if the unit may not need it. The need for a shutdown is determined by the backpressure on the raw gas compressors. An increase in raw gas back-pressure indicates sulphur buildup on the floor or at the spargers of the autocirculation vessels. Unplanned outages due to high backpressure are very rare. Outages are typically due to low or no-flow from the upstream power plants, which causes sparger plugging.

The H₂S removal performance of the Navy 1 and Navy 210 units is summarised in Figure 2 and Figure 3.

The Navy 1 unit was designed for 1.2 vol% H₂S in the feed gas but experienced highs of 1.4-1.5 vol% during its first five years of operation. Those peaks came less often over the last 22 years and the average inlet H₂S is now about 0.7 vol% (7,000 ppmv).