Geothermal chemical engineering

Abstract

AbstractA major, national program to exploit geothermal energy requires creative chemical engineering solutions to a wide array of difficult problems. With some overlap, most of the problems fall into these categories: resource development and management, energy extraction and utilization, power cycle design and optimization, environmental engineering, and corrosion and scaling of equipment. Reinjection of waste fluids, insofar as it proves feasible, will affect the shape of the solutions in all categories.Convective hydrothermal reservoirs, the most accessible and well characterized of geothermal resources, are immense, permeable media in which heat and mass transport are coupled with chemical reaction. Despite decades of effort, no satisfactory physical or mathematical model exists for any reservoir. Even less is known about the hot rock and geopressured resources, which also may be utilized ultimately.Electric power production receives primary emphasis in the United States, both in terms of current utilization and of research and development. Worldwide, however, the total power devoted to agricultural activities, space heating and cooling, and process heat applications is greater than that to electric power conversion.Because hydrothermal reservoirs are low temperature (<350°C) sources of corrosive fluids, much current development effort aims at power cycles that promise enhanced efficiencies and that avoid some of the problems of corrosion and scaling. Among the candidates are several binary cycles which employ secondary working fluids, and the total flow concept, which utilizes two‐phase flow from the production wells.Once thought to be all but negligible, the environmental effects of geothermal installations have recently been subjected to informed scrutiny and found nonnegligible. Among the problem areas are gaseous effluents including hydrogen sulfide, mercury, radon, ammonia, and H3BO3 and aqueous emissions that contain heavy metals. Reinjection of waste geothermal fluids would ameliorate many environmental impacts but could be complicated by silica deposition in equipment and in the underground reservoir.