A Calcium Carbonate Scale Inhibitor for Direct-Contact Binary Geothermal Service

Abstract

Auerbach, Michael H.; Pfizer Central Research Pfizer Central Research Reimer, Robert A.; Pfizer Central Research Pfizer Central Research Olander, Robert G.; Barber-Nichols Engineering Co. Rapier, Pascal M.; Lawrence Livermore Natl. Laboratory Summary A 500-kW direct-contact binary-cycle pilot geothermal power plant was constructed and successfully operated power plant was constructed and successfully operated free of scale at the U.S. DOE East Mesa geothermal test facility. The pilot plant used an Elgin tower direct-contact heat exchanger (DCHX) with isobutane as the working fluid. Initial tests with the untreated 340 deg. F (170 deg. C) geothermal brine gave heavy calcium carbonate fouling of the brine boost pump, flow monitors, and flow-control valves within 20 to 50 hours of startup. Use of 2 ppm high-temperature polymeric scale inhibitor in the feed brine enabled plant operation for more than 700 hours without formation of detrimental scale. The antiscalant was not extracted into the working fluid and did not emulsify the hydrocarbon/brine interface. On the basis of dosage and flow, the additive cost was calculated to be about 0.06 cents per net kW-hr of power generated. Introduction Electrical power is generated from liquid-dominated geothermal resources either bydirect flash of the hot brine to produce steam that drives a turbine ortransfer of sensible heat from the brine to a more volatile working fluid in an organic Rankine cycle loop without brine flash. The vaporized working fluid, usually a low-boiling-point hydrocarbon, drives the power turbine in this binary-cycle process. Heat is transferred from the geothermal brine to the binary-cycle working fluid in either a surface-type tube-and-shell heat exchanger or a DCHX, where the brine and working fluid mix together. The higher effective surface areas and heat-transfer coefficients obtainable with direct contact greatly reduce the capital cost for a given heat flux. However, extraction of steam and noncondensible gases into the working fluid increases turbine backpressure and reduces power output and working fluid recovery, making the DCHX process more complex to operate economically. The direct-flash process is generally more efficient for resources above 392 deg. F (200 deg. C); the binary-cycle process is favored below this temperature. Both processes process is favored below this temperature. Both processes can he completely interrupted or severely restricted by heavy mineral scaling, which fouls or plugs surface piping, pump impellers, and reinjection wellbores. The piping, pump impellers, and reinjection wellbores. The scaling minerals most often found are silica, heavy metal sulfides, and calcium carbonate, depending on the temperature and total dissolved salt content of the particular brine. Scale-control strategy thus depends on the particular brine. Scale-control strategy thus depends on the type of scale, its seventy, and the details of the plant process. process. In 1978 the U.S. DOE commissioned Lawrence Berkeley Laboratory to build and demonstrate a 500-kW direct-contact binary-cycle geothermal pilot plant. Design, construction, and operation of the plant were subcontracted to Barber-Nichols Engineering Co. The plant was built and tested at the U.S. DOE East Mesa plant was built and tested at the U.S. DOE East Mesa geothermal test facility in the Imperial Valley, Holtville, CA, during 1979–81. Extended operation of this plant was severly hampered by CaCO3 scaling. JPT P. 1546