HS in v ent gas ppm Linear (HS in v ent gas ppm )
Inlet gas HS v ol% Linear ( i nlet gas HS v ol%)
Sulphur production LTPD
Poly. (Sulphur production LTPD)
Figure 2 Sulphur data for Navy 1 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 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. These specifics allow Coso to consistently run both active Lo-Cat units for an entire year between shutdowns Over the year, the raw gas back pressure tends to increase. This is an indicator of sulphur build-up in the auto- circulation vessels. The shutdown and turnaround are always completed, even if the unit may not need it. This prevents unexpected shutdowns during the year. The annual turnaround takes 2-3 days from gas-off to gas-in. The H₂S removal performance of the Navy 1 and Navy 210 units is summarised in Figures 2 and 3 . Navy 1 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 17 years. The inlet H₂S has been consistent between 0.8 and
1.3 vol% (8,000 to 13,000 ppmv) with average concentrations close to 1.0 vol%. Navy 1 initially produced sulphur at design rates of 2.4 LTPD with spikes up to 2.7 LTPD. Over time, average sulphur production increased before declining to the current sulphur production of 1.25-1.75 LTPD. The reason for sulphur production decreasing over time is that water condensed from the steam wells (now free of sulphur) is injected into the geothermal reservoir. This dilutes the sulphur content of the produced steam. The Lo-Cat unit was adapted to turndown conditions via the sparger shuffling procedure mentioned earlier. During the early years of operation, Navy 1 experienced periodic high H₂S in the vent gas. These few instances of exceedance were lower than for other technologies employed to meet environmental standards before the Lo-Cat unit was installed. One incident occurred in December 2000 when the vent gas H₂S was reported at 90 ppmv. As shown in Figure 4 , this happened because the solution chemistry was out of balance. All readings above 30 ppmv correlate to rapid changes in the feed gas conditions that required operator response. Since early in 2001, the vent gas H₂S has rarely exceeded 30 ppmv.
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