1. Field of the Invention
The present invention is in the field of electrical power production from steam flashed from high temperature geothermal brines which have high noncondensable gas content, most of the components of which are "acid gases."
2. Description of the Prior Art
Some high temperature geothermal brine resources are known which contain very large amounts of geothermal energy, but which until fairly recently had not been usable for the commercial production of electrical power because of a high dissolved silica content, and which also have a large noncondensable gas content, most of the components of which are acid gases such as carbon dioxide, boric acid, and hydrogen sulfide. Thus, the Salton Sea Known Geothermal Resources Area (KGRA), otherwise known as the Salton Sea Geothermal Anomaly, is estimated to have approximately 3,400 MW.sub.e of geothermal energy available for the generating of electrical power, which is believed to be self-regenerating by percolating waters. The KGRA geothermal resource is estimated to be a greater energy reserve even than the oil reserves on the north slope of Alaska. A similar geothermal resource is the Brawley geothermal field which is also in the Imperial Valley of Calif. Development of these large geothermal resources was almost completely blocked until relatively recently by the high dissolved silica content, which precipitated out in vessels and piping in power production plants to the extent of up to about 42 inches per year of scaling. This problem was resolved by Magma Power Company, now of Rancho Bernardo, San Diego County, Calif., by flashing the geothermal brine to steam for generating electrical power in a series of flash crystallizers in which the dissolved silica was precipitated out on a vast silica seed particle area rather than on surfaces of flash vessels and associated piping and valves. The use of flash crystallizers for this purpose was taught in the Featherstone U.S. Pat. No. 4,429,535, while retrieval of the silica seed particles useful for the purpose was taught in Van Note U.S. Pat. Nos. 4,302,328 and 4,304,666. Applicant's U.S. Pat. No 4,665,705 teaches an improvement over the Featherstone patent disclosure in which the flash crystallizers are made more effective by the use of external draft tubes which produce brine recirculation motive power for multifold recirculation of the geothermal brine through the flash crystallizers to provide time for silica precipitation on the silica seed particles.
Despite the foregoing solution to the silica precipitation problem, there nevertheless remained serious problems of inefficiency, corrosion, salt fouling, and undesirably high capital costs in geothermal electrical power production plants located in high temperature geothermal brine regions such as the Salton Sea Geothermal Anomaly, because of the presence of large quantities of noncondensable gases, most of which are acid gases. The specific problems caused by the presence of these noncondensable gases will be described in detail below.
Where high temperature geothermal brine is available as in the Salton Sea Geothermal Anomaly, at production well source temperatures on the order of 550.degree. F. or more, the conventional and most efficient practice is to flow the brine up through the production well and pressurize the brine through the plant under the power of its own flashing steam. For example, a geothermal brine well drilled and owned by Magma Power Company in the Salton Sea Geothermal field has a bottom hole temperature of approximately 550.degree. F., and with the brine flowing up through and out of the well under the power of its own flashing steam, about 11 percent of the total brine flow has flashed to steam by the time the brine reaches the wellhead, which represents about one-third of the steam power available from the brine. Almost all of the noncondensable gases (excepting only a slight amount dissolved in the liquid brine) are mixed with the flashed steam at the wellhead, the noncondensable gases amounting to more than 2.5 percent of the flashed steam at the wellhead.
Prior to the present invention, the conventional practice with such high temperature brines has been to allow the noncondensable gases to be carried through the geothermal production plant so that the noncondensables were mixed with the steam which drove the power generating turbine. This resulted in the noncondensables causing the numerous problems detailed below. These problems can be almost completely eliminated by removing the noncondensable gases at the high temperature end of the production plant, that is, removing the noncondensables from the brine feedline from the geothermal production well to the plant. However, prior to the present invention, there has been no satisfactory way known in the art to remove the noncondensables at the high temperature end of the plant, and hence to remove them from the steam which drives the turbine. To simply vent the noncondensable gases to the atmosphere at the high temperature end of the plant would necessarily also cause venting of the already-flashed steam, and hence loss of approximately one-third of the available steam energy, which would be an intolerable waste of available power. To vent only a fraction of the noncondensable gases and a proportionate fraction of the steam at the high temperature end of the plant would still waste a large amount of valuable high pressure, high temperature steam, and would leave the problems caused by the noncondensable gases still generally uncured.
Applicant is aware of three prior art patent disclosures of some noncondensable gas separation at the high temperature end of a geothermal electric power production plant. The first of these was the McCabe et al. U.S. Pat. No. 4,428,200, which showed a noncondensable gas separator in the production well outlet pipe in the forms of that invention shown in FIGS. 2 and 4. The purpose for separating noncondensable gases was to provide them as a replacement for more reactive atmospheric oxygen in the reinjection system part of the plant, and also to slightly pressurize the reinjection system. The amount of noncondensable gases separated was necessarily only a relatively very small amount for the purpose, and the heat energy of the accompanying steam and that of the noncondensable gases was totally wasted as far as power generation was concerned, since it was only applied to the reinjection system. Noncondensable gas separators at the high temperature end of the plants were also disclosed in the aforesaid Featherstone U.S. Pat. No. 4,429,535 and in applicant's aforesaid U.S. Pat. No. 4,665,705. While no purpose was stated in either of these patents for the presence of a noncondensable gas separator, it is applicant's understanding that in each case it involved separation of only a small amount of the noncondensable gases, and that the purpose was to help alleviate a foaming problem in the brine from the production well. In each case, the noncondensable gases, and necessarily some of the high temperature, high pressure steam, was simply vented to atmosphere, so that the heat energy of the accompanying steam as well as that of the noncondensable gases was totally wasted.
The problems which result from the current practice of allowing the noncondensable gases to flow through the plant with the brine and flashing steam and thereby pass through the generating turbine with the steam include the following:
1. The noncondensable gases in the turbine exhaust will accumulate in the exhaust condensor unless they are removed from the condensor by being compressed up to atmospheric pressure for rejection to the atmosphere. When the steam from the turbine exhaust condenses, the noncondensables remain in gaseous form, and they must be removed in order to maintain the highest possible condensor vacuum and corresponding maximum turbine efficiency. This requires a large amount of power which is generally derived from steam that would otherwise be available to the generating turbine. For example, in a geothermal generating plant having a net power output of approximately 34 megawatts, about 25,000 pounds per hour of motive steam would be required for pressurization and removal of the noncondensable gases from the turbine exhaust condensor, for brine produced by the aforesaid Magma Power Company well in the Salton Sea geothermal field. This represents at least about 1.2 megawatts of power, or at least about 3.4 percent of the available power from steam, which is a considerable power loss for the plant.
2. Additional generating energy is lost with the conventional practice of allowing the noncondensable gases to pass through the plant and be mixed with the steam applied to the turbine, because the presence of the noncondensable gases in the turbine exhaust prevents an optimum vacuum from being drawn in the exhaust condensor, even though the noncondensables are continuously being pressurized and removed. Maximum power output from the turbine is completely dependent upon the deepest possible vacuum being drawn in the condensor, and this is substantially impaired by the presence of noncondensables.
3. The presence of the noncondensable gases in the turbine exhaust in a conventional geothermal plant requires that the turbine exhaust condensor be considerably larger in size than it would need to be without the noncondensable gases, so that the condensor represents an undesirably high capital cost in the conventional plant.
4. Another undesirably high capital cost in the conventional geothermal power plant resides in the compression system of pumps and ejectors required to compress the noncondensable gases in the turbine exhaust back up to atmospheric pressure for ejection into the atmosphere.
5. A further undesirably high capital cost in the conventional geothermal power plant is caused by exotic and expensive metallurgy necessary for the turbine to combat acids formed in the condensing steam by the presence of the noncondensable gases, which are mostly acid gases such as carbon dioxide, boric acid, and hydrogen sulfide. The resulting turbine corrosion also results in continuing capital replacement costs over the life of the plant.
6. In a conventional geothermal power plant, the presence of the noncondensable gases in the turbine motive steam also creates a substantial salt fouling potential. Various constituents of the noncondensables have a high potential for chemically reacting with each other to produce a variety of salts, and also for reacting with plant equipment to produce ferrous and/or ferric salts.