Wind comes and goes. Rivers flood. Biofuels take up farmland that could otherwise feed people. The sun sets or hides behind clouds. So among renewable energy sources, geothermal power plants are environmentally attractive since their energy resource is continuous.

Unfortunately, geothermal steam often contains varying amounts of non-condensable gases (NCGs) such as CO₂, H₂S, mercury vapor and, in some cases, benzene, which if not disposed of properly may cause environmental, health and safety problems. Effective control of these emissions can make geothermal energy a viable construct. This is evidenced by the operating history of three geothermal power facilities that have been in service for 15 years while meeting strict California emission regulations.

Facility history

Terra-Gen Power LLC operates a geothermal power facility on the China Lake Naval Weapons Center in southeastern California. As illustrated in Figure 1, hot brine is extracted from high-pressure, underground wells. The brine is flashed and the produced steam is passed through condensing turbine/generator sets, which produce approximately 250 MW of electrical power. The electricity, enough power to service approximately 250,000 homes, is then sent into the local power grid.

The unflashed brine and condensate from the turbine/generator sets are reinjected into the geothermal reservoir and the NCGs, consisting mostly of CO₂, are sent to treatment facilities. Initially, the NCGs were injected back into the reservoir. However, over time this practice began to affect the performance of the reservoir and the practice was stopped. It was then decided to remove the H₂S and mercury from the NCG mixture, and exhaust the remaining gas, CO₂ and water vapor, to the atmosphere.

In 1993, a former owner of the facility began an investigation into various H₂S and mercury removal systems. He eventually decided on the Lo-Cat, an isothermal system from Houston-based Merichem Chemicals & Refinery Services LLC. The process involved a sulfided, activated carbon media upstream of the process for mercury removal. These systems were initially installed at the Navy I and Navy II facilities. A third system was later added at Navy II due to larger gas loading than previously anticipated.

Thermal process

The treatment systems provided by Lo-Cat effectively removed the undesirable emissions from the geothermal operations at the China Lake Naval Weapons Center.

The process accomplishes direct oxidation of H₂S to elemental sulfur as depicted in the following equation:

Equation 1: H₂S + 1/2 O₂ <--> H₂O + S⁰

This reaction is accomplished in an aqueous scrubbing system by using a water-soluble metal ion. The metal is capable of oxidation in ambient air or in the process gas stream, and has a suitable electropotential for oxidizing the sulfide ion to elemental sulfur. Although there are many metals that can perform these functions, iron was chosen for the process because it is inexpensive, non-toxic and is a good oxidation catalyst.

The basic reactions of the process can be divided into the absorption and regeneration portions as follows (aq = aqueous):

Absorption:

• Equation 2: H₂S absorption: H₂S (gas) + H₂O  <--> H₂S (aq) + H₂O

• Equation 3: First Ionization: H₂S (aq)  <--> H⁺ + HS^-

• Equation 4: Oxidation by ferric ions: HS- + 2Fe³⁺  <--> S0 (solid) + 2Fe²⁺

• Equation 5: Overall absorption reaction: H₂S (gas) + 2Fe³⁺  <--> 2H+ + So + 2Fe²⁺

Regeneration:

• Equation 6: 1/2 O₂ (gas) + H₂O <--> 1/2 O₂ (aq) + H₂O

• Equation 7: Regeneration of ferrous ions: 1/2 O₂ (aq) + H₂O + 2Fe²⁺ <--> 2OH- + 2Fe³⁺

• Equation 8: Overall regeneration reaction: 1/2 O₂ (gas) + H₂O + 2Fe²⁺ <--> 2OH^- + 2Fe³⁺

Adding Equations 5 and 8 yields Equation 1.

System design

As previously mentioned, three units were installed at the facility to handle gas streams from six geothermal power plants. The design parameters for each of these units can be seen in Figure 2.

Since the treated NCGs from each unit could be exhausted to the atmosphere in accordance with air permit requirements, an auto-circulation unit was selected for these applications. In this process configuration (Figure 1), the auto-circulation vessel contains a series of baffle and weir combinations that separate the absorption sections from the regeneration sections. The system also incorporates a series of natural hydraulic pumps produced by the aerated liquid on one side of the baffle and the non-aerated liquid on the other side of the baffle. This provides the motive force for circulating the solution from one chamber to the next. The process flow diagram for each unit is shown in Figure 3.

The discharge gas from the NCG compressors is initially processed through a bed of pre-sulfided, activated carbon, in which the mercury vapor is removed from the gas stream by adsorption onto the activated carbon. The NCG is then directed into the auto-circulation vessel, where the H₂S is absorbed into the solution and rapidly converted into solid, elemental sulfur, which remains suspended in the circulating solution. The solution is then contacted with air in a separate chamber (or chambers) and the iron is re-oxidized. The spent air and sweet NCGs are combined and exhausted to the atmosphere.

A small slipstream of solution is pumped from the auto-circulation vessel and directed to a settler, where the sulfur is allowed to settle into a concentrated slurry. This slurry is then directed to a vacuum belt filter. The sulfur cake is washed with water on the filter, the filtrate is returned to the unit, and the sulfur cake is sold as fertilizer/fungicide at approximately 70-wt.-percent sulfur. The sulfur has excellent soil absorption characteristics. In addition, residual surfactant, which is employed in the process, enhances the hydrophilicity of the sulfur. The chelated iron and thiosulfate contained in the process solution also add micronutrients to the product sulfur.

Operating history

As required in the operating permits, the effluent gas streams from each unit are continuously monitored via lead acetate type monitors. Over the 15 years of operation, the three units have consistently exceeded the H₂S removal efficiency required by the operating permits, which equates to greater than 99.9 percent removal efficiency.

In June 1996, Tom Mason, the president and chief operating officer of CalEnergy Company Inc. – the owners of the facility at that time – wrote in the Geothermic Resource Council's GRC Bulletin that "The statistics clearly demonstrate the success of the company's efforts. CalEnergy believes that the [H₂S abatement systems] currently in place at Coso constitute the best available control technology."

Normally the units have fairly large solution inventories, and for units processing CO₂, as in these cases, the solution is well buffered with KHCO₃ and K₂CO₄. Consequently, changes to the solution chemistry and the unit's performance are never immediate but occur over a fairly long time period. This operating characteristic results in minimum monitoring of the operations. Generally, the only duties for the operator is to measure the pH and redox potential of the solution on a daily basis, and the iron concentration on a weekly basis, and then make the corresponding adjustments to the chemical addition pumps. This takes no longer than two to three hours per day of an operator's time.

Maintenance

The original auto-circulation and settler vessels for all three units have not experienced corrosion or had significant repair over the 15 years of operation. The original progressive cavity pumps, which were employed to transfer the sulfur slurry from the auto-circulation vessel to the settler, did experience stator erosion and were replaced with a gravity feed system. The filter cloths on each vacuum belt filter have been regularly replaced every three to six months. The rubber belt drive is replaced every two to three years. The units were designed such that the filter cloth and belt can be replaced while the unit continues to run.

The entire power trains are shut down for maintenance once a year. During that shutdown period, the units are also serviced. This service includes removal of the NCG and oxidizer air spargers for inspection, cleaning and repair as necessary, as well as cleanout of the vessels, and pump and equipment maintenance as required. Through detailed planning by plant personnel, a complete turnaround of a unit can be conducted in two days. PE