The heating process

Heat exchangers

Hitaveita Reykjavikur operated a pilot heating plant at Nesjavellir during 1974 - 1990. Various types of heat exchangers have been tested. Conventional plate heat exchangers are used for the condensation of steam from the separators and to cool the condensate. They are equipped with EPDM-gaskets and made of titanium plates to avoid stress corrosion, as it is not possible to guarantee problem free operation if stainless steel plates are used.
Conventional heat exchangers cannot be used for the separated water due to the high content of dissolved solids (TDS 1200 PPM) which would cause severe scaling of silica. A new type of heat exchanger, in the geothermal context, has been tested successfully in the pilot plant. These are the so-called "fluidized bed heat exchangers", or FBHX made by Eskla Heat exchangers BV in the Netherlands. They are shell and tube heat exchangers operating in a vertical position . Stainless steel balls, 1.5 mm in diameter, circulate in the flowstream of the separated water. They impact continuously against the pipe surfaces and remove any scaling that may form. A mechanical device is fitted to the inlet and outlet of the heat exchangers to keep the steel balls evenly distributed in the flow stream. The FBHX heat exchangers make possible the direct utilization of the heat in the water from the separators and contribute to the overall economy of the heating process.

Deaeration

The cold ground water is saturated with dissolved oxygen and becomes very corrosive when heated. A conventional thermal deaeration method is used where the ground water is boiled under vacuum after heating to remove the oxygen. The cold ground water has a pH-value of 7.5-8.5. It is partially degassed through boiling after heating. This raises the pH-value to 9.0-9.5 and the oxygen content is reduced down to about 50 ppb. The remaining dissolved oxygen is removed through injection of small amounts of geothermal steam that contains acid gases (H2S and CO2). Hydrogen sulfide gas reacts rapidly with the dissolved oxygen. The final water product then has a pH-value of 8.5-9.0. It is free of dissolved oxygen, and contains 0.5-2.0 PPM of H2S. The remaining H2S gas reacts against any oxygen absorption in accumulators and ensures that the "pleasant smell", which the users of geothermal water in Iceland have become accustomed to, is retained.
Amorphous Mg-Si scaling was formed in the distribution system in Reykjavik during the first months of operation of the plant, due to the high pH-value of the mixture of the geothermal water from the low-temperature fields and the heated ground water from Nesjavellir. Different ratios of these two water types control the pH-value. Scaling can only be avoided by reducing the amount of geothermal water in the mixture below 10-15%. Therefore the original plan of mixing these two water types in the distribution network has been abandoned. They will be used separately.

The waste geothermal water

Geothermal heating plants in high-temperature fields only utilize the thermal energy of the geothermal fluid, which, after use in heat exchangers. must be disposed of with minimum risk to the environment. This disposal can be performed in two different ways, i.e. at surface or into subsurface aquifers. Surface disposal can be carried out in a similar way to the natural disposal of flow from the hot springs, i.e. into the brook in the Nesjavellir valley, which disappears into a lava field before reaching Lake Thingvallavatn. Subsurface disposal requires that the waste water is pumped back into the geothermal reservoir. This latter method is obviously more friendly to the environment but more expensive. It can also be more difficult to operate due to scaling in the reinfection wells and their aquifers.
There are two important features of the waste water from high-temperature fields that may have a negative effect on the environment. These are the raised temperature of surface waters and ground water aquifers and the presence of hazardous chemicals in the waste water, i.e. arsenic, mercury, boron, etc. Extensive research has been carried out at Nesjavellir with respect to disposal of the waste water. Chemical and biological measurements have been carried out at Lake Thingvallavatn since 1979 to define the pre-exploitation value for future reference. All the wells at Nesjavellir were flow tested in 1984-1987 as a part of the exploration program. Large amounts of geothermal water were disposed of at the surface during these tests without any apparent effects on water chemistry at the shoreline of the lake. This is in agreement with the prediction of a ground water model that simulates fluid flow and distribution of chemicals in the ground water system at Nesjavellir.
Chemical analysis of the geothermal fluid show that dangerous chemicals, which may be expected from the condensate of the steam phase, are almost absent.
All arguments seem to indicate that surface disposal of the waste water can be used for the geothermal power plant at Nesjavellir.

THE GEOTHERMAL POWER PLANT

General outline

Due to scaling, the geothermal fluid from the Nesjavellir field cannot be used directly in the space heating distribution network. The power plant therefore uses the geothermal energy to heat cold ground water indirectly in heat exchangers. The heated water is treated so that it can be used directly in the network.
The geothermal power plant at Nesjavellir consists of the following five sub-systems all of which have separate functions:

• Cold water supply
• Geothermal fluid supply
• Heating and treatment of cold ground water
• Transmission pipeline to Reykjavik
• Electricity co-generation

The cold water supply

Cold ground water (4°C) is pumped from 30 m deep wells, at Gramelur, 6.2. km north of the power house, in a lava field at the shore of Lake Thingvallavatn. The nominal capacity of each pump is 278 kg/s, but larger pumps can be installed. Four wells have been drilled so far with only 5 m spacing.
Pumping tests of up to 600 kg/s have confirmed a very high permeability of the lava formation. The pumping station is designed so that it can be enlarged for future developments and house additional wells.
The cold water is piped 6.2 km through a DN 900Æ mm pipe from Gramelur to the power house. The pipe is made of ductile iron and has the same capacity as the transmission pipeline to Reykjavik, i.e. about 1900 kg/s. The water is piped to an 1000 m3 storage tank by the power house, before entering the heat exchangers and deaerators.

The geothermal fluid supply

The geothermal fluid supply system gathers the fluid from the production wells, separates water and steam and then pipes them individually to the power house.
Figure 15 shows a schematic flow diagram of the system. It includes two phase pipes from the production wells, separators, pressure control valves and the mist eliminators by the power house.
The wells discharge a mixture of water and steam, which is transported along the two-phase pipes to a central separator station close to the power house instead of a number of separators nearer to the wells. The two-phase pipes are therefore relatively long, which is made possible by the high enthalpy of the well fluid and favorable topography.
The dissolved solids are largely confined to the separated water phase, as steam and water are almost completely (over 99.9%) separated in the separators. The separator station is situated 400 m away from the power house. The separated steam pipeline is constructed so that some condensation occurs in the pipe. The condensate washes out remaining dissolved solids in the steam. It is drained through control valves on the pipe and the remaining droplets are removed in the mist eliminators.
Three wells (no. NJ-11, 13 and 16) are connected to the separator station for the first phase, with well NG-6 as a reserve. These wells have a very high steam fraction (enthalpy 2000 kJ/kg). It was therefore decided to operate the steam separators at 15 bara, which is an unusually high pressure for a geothermal power plant. The advantages are smaller pipes and more efficient electricity generation. Power station II will utilize wells with lower fluid enthalpy and a lower separator pressure will be more practical, probably 8 bara.
Vertical separators have hitherto been chosen for steam separation in geothermal power plants. Nesjavellir is the first one to operate conventional horizontal separators with Chevron-filters, their main advantages being less height, hence lower cost for separator building and much easier water level control. The capacity of each separator equals 35 kg/s or about 50 MWt (at 15 bara).
The mist eliminators are in principle of the same size and type (horizontal) as the main separators but are fitted with "wire-mesh" filters.
No steam turbine is installed in the first phase of the power plant. The steam pressure must therefore be lowered in control valves from 15 to 2 bara before entering the heat exchangers. This causes superheating of the steam and very high noise level due to sonic flow. The control valves are therefore placed in a separate building. Here, condensate is injected into the superheated steam to cool it to saturation conditions to protect the gaskets in the plate heat exchangers.
Electricity generation is planned in phases 2 and 3. The high pressure steam will then expand in back pressure turbines, down to 2 bara, relieving the control valves of the high flow load. The exhaust steam from the turbine will be piped directly to the heat exchangers.

The heat exchangers

The indirect heating of the cold ground water takes place in the heat exchangers. About 82% of the heat is transferred in the steam heat exchangers. The condensate heat exchangers cool the condensate from the steam heat exchangers down to 20°C and add about 14% to the heating process, whereas the heat exchanger for the separated geothermal water finally contributes only 4% to the heating in the first phase of the power plant.
Steam heat exchangers. Three out of four of the installed steam heat exchangers have titanium plates, but one of the heat exchangers has plates of ANSI 316 stainless steel for testing the long-term corrosive resistance of this material. The steam temperature is kept below 120°C (2 bara), the maximum temperature that the EPDM material in the gaskets can withstand for a longer period. They are manufactured by REHEAT in Sweden. Each titanium heat exchanger is composed of 329 plates with a total heat exchange surface area of 280 m2, whereas the stainless steel heat exchanger has 367 plates and heating surface of 312 m2. The heat transfer coefficient is stated to be 4300 W/(m2K) for clean plates.
Condensate heat exchangers. The condensate heat exchangers are of the conventional plate type. They extract the heat of the condensate from the steam heat exchangers through cooling from about 90 to 20°C. Two heat exchangers connected in parallel were installed in the first phase of the power plant, one acts as a reserve. They are manufactured by REHEAT of Sweden. The plates are made of the ANSI 316 stainless steel and have a heat exchange surface area of 190 m2
Heat exchangers for the separated water. There are two fluidized bed heat exchangers (FBHX) connected in series, which transfer heat from the separated water to the cold ground water. Each FBHX is equipped with 19 steel pipes (ID 50 mm, length 9 m). These heat exchangers contribute only 4% to the heating process of the first phase, as stated earlier. They are therefore installed mainly to obtain operational experience, as they will play an important role in power station II when the low enthalpy wells will be connected. The separated water is cooled down to 20-35°C before entering the waste water system.

Deaeration of the heated water

The main role of the two deaerators installed in the plant is to remove oxygen from the heated fresh water. It enters the vessel at the temperature of 85-88°C and is deaerated through boiling by vacuum pressure down to 83°C. The main flow enters the central part of the deaerators. The water boils vigorously as it sprays over the filling material. Steam and gas rise to the top. There the steam is condensed through injection of cold water before the gas is ejected. At the bottom of the vessel a small amount of geothermal steam is injected into the deaerated water to dissolve hydrogen sulfide. This lowers the pH, rids the water of any remaining oxygen and acts against oxygen absorption.
The deaerators are made of stainless steel. They are 2.5 m in diameter and 11 m high. The nominal capacity of each is 278 kg/s of heated water or 50% of the first phase.

The control system

The computerized control system for the geothermal power plant at Nesjavellir is identical to the one that is used to supervise and control the pumping stations for the low-temperature fields and the distribution network in Reykjavik. This control system was tailor-made for Hitaveita Reykjavikur [Magnusson and Gunnarsson, 1989]. The advantage of using the same type of control system is to reduce the investment, training and maintenance costs.
The data processors. The control system is built around process computers of the Texas Instruments 565 PC type. They take care of sequence and closed loop controls. They are situated in the cold water pumping station, in the transformer station and in the power house. Two process computers are connected together in the power house in a hot back-up configuration, as they control the most critical part of the heating process.
The SCADA System. The process computers are connected to a SCADA system (Supervision Control and Data Acquisition). The SCADA system is based on a PDP 11/83 computer in the control building at Nesjavellir . Peripherals such as color screens and printers are located in the control rooms at Nesjavellir and in Reykjavik . The power plant is operated round the clock from Reykjavik as the control room at Nesjavellir is usually unmanned. The peripherals in Reykjavik are connected to the PDP 11/83 computer through a 64 kbit/s data multiplex and a fiber-optical data link.
The color VDU display system used is of the ABB Tesselator type. One of the advantages of this system is that it can be connected to the SCADA computer by modem. The Tesselator system can therefore easily be moved around, which facilitates all process tests and remote monitoring.
Should the SCADA system fail, the power plant can be controlled from switch boards that are connected directly to the PC's processors.
The operation of the power plant is fully automated. It can run all day without any manual intervention, except during breakdowns. A closed loop control is used at all stages and reserve pumps take over automatically in case of pump failure. Restarting of the plant after shut-down is at present done manually but automation will be gradually increased as experience is gained in the operation.
The process simulator. A computer model has been developed for the dynamic behavior of the plant. It runs on a PC computer and the whole process is incorporated into the model. A lot of effort was made to make the program code as effective as possible for real time simulation.
The simulator consists of a PC computer that simulates the process, a process computer with the same software as is used for the control system of the plant and a SCADA system similar to the one used in the power plant. The process computer program had to be modified to communicate with the PC computer instead of the sensors and control system of the power plant.
The main advantages of the simulator are expected to be:
· The designers and operators can optimize regulation and control strategies in a simple way. Tests that are either too risky or time consuming can be simulated.
· Training of personnel can be carried out without disturbing the operation of the plant. Both normal operation and various types of breakdowns can be simulated.
· Development of an expert system for operation and maintenance of various parts of the system.

Ventilation of buildings

The atmosphere at Nesjavellir is contaminated with H2S gas from the geothermal field. Its concentration varies depending on weather conditions, but is estimated to be around 100 ppb on average. H2S is especially corrosive for copper and silver, materials that are common in electrical and electronic equipment. Instruments are therefore largely situated in air-tight buildings . These buildings are fitted with airlocks and an independent ventilation system where the hydrogen sulfide gas is absorbed in active carbon filters. The requirements for the indoor conditions are that the H2S content is below 3 ppb and relative humidity around 40%.
This is expected to be achieved with the ventilation system by pressurizing the building up to 100 kPa, recirculating about 85% of the air inside the building and placing the fresh air intake about 6 m above the roof of the power house (20 m above ground level).
The power plant buildings have two additional ventilation systems installed. One is for the visitors' reception hall and the other for the process halls. Both of these are conventional systems without gas purification but they use the same fresh air intake.