1. Field of the Invention
The present invention relates generally to the production of electric power by use of geothermal fluids and more particularly to processes for reducing the silica content of flashed, silica-rich geothermal brine by inducing crystallization of the silica from the brine onto seed crystals.
2. Discussion of the Prior Art
Large subterranean aquifers of naturally produced (geothermal) steam or hot aqueous liquids, specifically water or brine, are found throughout the world. These aquifers, which often have vast amounts of energy potential, are most commonly found where the earth's near-surface thermal gradient is abnormally high, as evidenced by unusually great volcanic, fumarole or geyser activity. Thus, as an example, geothermal aquifers are fairly common along the rim of the Pacific Ocean, long known for its volcanic activity.
Geothermal steam or water has, in some regions of the world, been used for centuries for therapeutic treatment of physical infirmities and diseases. In other regions, such geothermal fluids have long been used to heat dwellings and in industrial processes. Although efforts to further develop geothermal resources for these site-restrictive uses continue, considerable recent research and development has, instead, been directed to exploitation of geothermal resources for production of electrical power which can be conducted, often over existing power grids, for long distances from the geothermal sources. In particular, recent steep increases in the cost of petroleum products used for conventional production of electric power, as well as actual or threatened petroleum fuel shortages or embargos have intensified the interest in use of geothermal fluids as an alternative and generally self-renewing source of power plant "fuel".
General processes by which geothermal fluids can be used to generate electric power are known and have been known for some time. As an example, geothermal steam, after removal of particulate matter and polluting gases, such as hydrogen sulfide and ammonia, can be used in the manner of boiler-generated steam to operate steam turbine generators.
Naturally pressurized geothermal brine or water having a temperature of over about 400.degree. F. can be flashed to a reduced pressure to convert some of the brine or water to steam. The steam produced in this manner can then be used to drive steam turbine generators. The flashed geothermal liquid and the steam condensate obtained from power generation are typically reinjected into the ground to replenish the aquifer and prevent ground subsidence. Cooler geothermal brine or water can often be used to advantage in binary systems in which a low-boiling point, secondary liquid is vaporized by the hot geothermal liquid, the vapor produced being used to operate gas turbine generators. The cooled brine is typically reinjected into the ground.
As might be expected, use of geothermal steam is preferred over use of geothermal water or brine for generating electric power because the steam can be used more directly, easily and cheaply. Consequently, where readily and abundantly available, geothermal steam has been used for a number of years to generate commercially important amounts of electric power at favorable costs. For example, by the late 1970's, geothermal steam at The Geysers in Northern California was generating about two percent of all the electricity used in California.
While energy production facilities at important geothermal steam sources, such as at The Geysers, are still being expanded, when not already at capacity, the known number of important geothermal steam aquifers is small compared to that of geothermal brine or water. Current estimates are, in fact, that good geothermal brine or water sources are about five times more prevalent than are good sources of geothermal steam. The potential for generating electric power is, therefore, much greater for geothermal brine and water than it is for geothermal steam. As a result, considerable current geothermal research is understandably directed towards the development of economical geothermal brine and water electric generating plants, much of this effort being expended towards use of vast geothermal brine resources in the Imperial Valley of southern California.
Although, as above mentioned, general processes are known for using geothermal brine or water for production of electric power, serious problems, especially with the use of highly saline geothermal brine, have often been encountered in practice. These problems have frequently been so great as to prevent the production of electric power at competitive rates and, as a consequence, have greatly impeded the progress of flashed geothermal brine power plant development in many areas.
These severe problems are caused primarily by the typically complex composition of geothermal brines. At natural aquifer temperatures in excess of about 400.degree. F. and pressures in the typical range of 400 to 500 psig, the brine leaches large amounts of salts, minerals and elements from the aquifer formation, the brine presumably being in chemical equilibrium with the formation. Thus, although brine composition may vary from aquifer to aquifer, wellhead brine typically contains very high levels of dissolved silica, as well as substantial levels of dissolved heavy metals such as lead, copper, zinc, iron and cadmium. In addition, many other impurities, particulate matter and dissolved gases are present in most geothermal brines.
As natural brine pressure and temperature are substantially reduced in power plant steam conversion (flashing) stages, chemical equilibrium of the brine is disturbed and saturation levels of impurities in the brine are typically exceeded. This causes the impurities and silica to precipitate from the brine, as a tough scale, onto surrounding equipment walls and in reinjection wells, often at a rate of several inches in thickness per month. Assuming, as is common, that the brine is saturated with silica at the wellhead, in high temperature portions of the brine handling system, for example, in the high pressure brine flashing vessels, heavy metal sulfide and silicate scaling typically predominates. In lower temperature portions of the system, for example, in atmospheric flashing vessels, amorphous silica and hydrated ferric oxide scaling has been found to predominate. Scale, so formed, typically comprises iron-rich silicates, and is usually very difficult, costly and time consuming to remove from equipment. Because of the fast growing scale rates, extensive facility down time for descaling operations may, unless scale reducing processes are used, be required. Associated injection wells may also require frequent and extensive rework and new injection wells may, from time to time, have to be drilled at great cost.
Therefore, considerable effort has been, and is being, directed towards developing effective processes for eliminating, or at least very substantially reducing, silica scaling in flashed geothermal brine handling systems. One such scale reduction disclosed in U.S. Pat. No. 4,370,858 to Awerbuch, et al, involves the induced precipitation of scale-forming materials, notably silica, from the brine in the flashing stage by contacting the flashed brine with silica or silica-rich seed crystals. When the silica saturation level in the brine is exceeded by the brine being flashed to a reduced pressure, supersaturation amounts of the silica leaving solution in the brine deposit onto the seed crystals. Not only do the vast number of micron-sized seed crystals introduced into the flashing stage provide a very much larger surface area than the exposed surfaces of the flashing vessels but the silica from the brine tends to preferentially deposit onto the seed crystals. Substantially all of the silica supersaturation, therefore, precipitates onto the seed crystals instead of precipitating as scale onto vessel and equipment walls and in injection wells.
Preferably, the seed crystals are introduced into the high pressure flashing vessel, which may then be referred to as a high pressure flash crystallizer, wherein the brine first becomes supersaturated in scale-forming materials. The crystallization process, while starting in the high pressure flash crystallizer, continues in successive, lower pressure flashing vessels in which the brine typically again becomes supersaturated with silica. In a downstream reactor-clarifier, the silicious precipitate is separated from the brine as a slurry which contains about 30 percent by weight of silica. According to known processes, a portion of this silicious slurry from the reactor-clarifier stage is recirculated back upstream into the high pressure flash crystallizer, whereby the silica material in the slurry acts as seed material.
After subsequent filtering to remove fine silicious particles not removed in the reactor-clarifier stage, the "clarified" brine is commonly reinjected into the ground in an injection stage.
As above-mentioned, geothermal brines used for electric power generation are, at wellhead temperature and pressure, frequently saturated with silica. As a consequence, substantial amounts of silica must be precipitated from the brine onto the seed material in the flash crystallization stage in order to prevent silica scaling in downstream brine handling equipment. Such removal of silica from the geothermal brine requires, particularly for high brine flow rates associated with production of reasonably large amounts of power, effective and rapid silica precipitation so that brine residence time in the flash crystallizer vessels, as determined by vessel capacity, can be maintained within acceptable and practical limits.
Such known silica seeding processes which use the silicious slurry from reactor-clarifier stage as seed material have, however, substantial disadvantages. A major disadvantage is that because of the large volume of seed material required for rapid, effective silica precipitation and because of the lower temperature of the seed slurry compared with that of the main flow of brine into which the slurry is introduced, steam production in the flash crystallizer is significantly reduced by quenching action of the slurry. The amount of steam "lost" as a result of such quenching may be about 5 percent. For an exemplary 10 megawatt power plant a 5 percent steam loss is equivalent to about 0.5 megawatts power loss, having a current value of about $33 an hour or about a quarter of a million dollars for a typical, 10 month operational year. Another disadvantage is that substantial costs are associated with the purchase, installation, operation and maintenance of the pipes, fittings and relatively large pumps required for pumping the seed slurry from the reactor-clarifier stage upstream to the high pressure flash crystallizer.
It is therefore, an object of the present invention to provide a process for removing silica from flashed geothermal brine by in-situ seed material formation in the flash crystallization stage of a geothermal brine handling system.
Another object of the present invention is to provide a process for removing silica from flashed geothermal brine by in-situ seed material formation in the flash crystallization stage of a geothermal brine handling system by reacting a base with naturally present heavy metals dissolved in the flashed brine so as to form insoluble heavy metal compounds which function as seed material.
Other objects, advantages and features of the present invention will become apparent to those skilled in the art from the following description, when taken in conjunction with the accompanying drawing.