Applied Thermal Engineering 58 (2013) 564e569
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Concept for production of chemicals and power using geothermal energy Bassam J. Jody*, Richard D. Doctor, Tawatchai Petchsingto, Seth W. Snyder Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439, USA
h i g h l i g h t s Co-production of power and chemicals using geothermal energy is discussed. Process captures energy more efficiently as chemical, sensible and latent heat. The co-production process can improve the economics of geothermal energy. Novel designs are required for insure safety and guard against contamination.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 November 2012 Accepted 26 April 2013 Available online 13 May 2013
This paper presents a concept for conducting commercial chemical reactions and production of power using geothermal heat. The high pressures (Ps) and temperatures (Ts) that fluids attain in deep reservoirs can be used to manufacture chemicals or decontaminate wastes. High P reactions which can be expensive and/or unsafe to conduct above ground can be conducted in geothermal reservoirs using closed designs. We present examples of reactions that could benefit from Enhanced Geothermal Systems (EGS) including production of ammonia (NH3), supercritical oxidation of wastewater contaminants, production of hydrogen (H2) by steam reforming of methanol (CH3OH) and partial oxidation of methane (CH4) to produce CH3OH. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Geothermal EGS Chemical reactions NH3 Wastewater treatment CH3OH production
1. Introduction Geothermal energy is a huge domestic energy resource that is environmentally friendly and can provide baseload capacity to meet customer demands around the clock. It is estimated that geothermal power plants emit only between 19 and 103 g of CO2equivalent per kWh of electricity [10]. By comparison, coal-fired power plants emit about 1235 g of CO2-equivalent per kWh of electricity, and natural-gas-fired power plants emit about 487 g of CO2-equivalent per kWh of electricity [10]. A recent study by the Massachusetts Institute of Technology [6] estimated that EGS could provide >100,000 MWe in 50 years. At present, geothermal energy is being used for power generation and for industrial and domestic heating applications. Technology to recover energy from EGS is under development. * Corresponding author. Tel.: þ1 630 252 4206; fax: þ1 630 252 1342. E-mail addresses:
[email protected] (B.J. Jody),
[email protected] (R.D. Doctor),
[email protected] (T. Petchsingto),
[email protected] (S.W. Snyder). 1359-4311/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.applthermaleng.2013.04.047
Presently, many challenges are facing the development of a successful EGS industry including: Drilling EGS wells that are deeper than 3000 m is in the experimental stage even though the oil and gas industry has been able to drill deeper wells but in lower temperature environments. Exxon Neftegaz Limited has drilled a well over 11 km deep [3]. Fracturing of the rock and injecting a fluid to capture the heat are necessary results in excessive water use and loss. Hundreds of millions of gallons of water are required, which will stress competing water demands. For example, a plant that pumps 10 gal/s (38 kg/s) of water and have a 10% water loss will require over 31 million gallons of water per year to compensate for water loss only. This amount is in addition to the water consumed by the power plant. Water at high T and P, especially supercritical water, is a good solvent for some rock minerals. Dissolved minerals could reprecipitate as the T and P fluctuate in the EGS and in the
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pipes, thereby reducing the permeability of the EGS. Fluids that do not interact with the rock minerals could have advantages. Seismic activity associated with hydraulic fracturing of EGS reservoirs could be a showstopper in some localities. Deep EGS projects, where the T exceeds about 375 C, can cause softening of the rocks which causes the rocks to creep and deform, which make it difficult and very costly to maintain the fractures. Therefore, new concepts and designs may be necessary for more efficient utilization of this energy resource. The temperatures (Ts) and high pressures (Ps) required to drive many commercial chemical reactions increases costs and decreases sustainability. Fluids injected in deep and hot Enhanced Geothermal Systems (EGS) reservoirs can reach their supercritical state without expensive and energy intensive mechanical pumping or compression. Supercritical fluids are excellent solvents and efficient media for conducting chemical reactions [4,7,8]. This makes the EGS reservoirs suitable for conducting chemical reactions while at the same time transporting the EGS heat for power generation. Even exothermic reactions require heating to increase their reaction rates. In addition, because the reactions take place underground the cost of the system could be reduced because of less stringent criteria for high P reactors. Co-production of power and chemicals using EGS reservoirs present a unique business opportunity. Because potential contamination of the underground or of the surface is a major concern both by the chemicals as well as contamination of the reactants and products by materials that may exist in the EGS and to prevent loss of reactants or products, closed system designs are necessary. In this paper we selected high temperature and high pressure processes that could be considered for use with EGS reservoirs. These are discussed below. 2. Production of NH3 Ammonia is produced commercially via the reaction (3H2 þ 2N2 / 2NH3) at Ps > 150 bar and Ts > 400 C. NH3 synthesis is favored at low T and high P. However, high Ts are used in order to increase the reaction rate. Fig. 1 shows the stoichiometric equilibrium composition for the reaction at different Ts and Ps. The process can be carried out at the bottom of the injection pipe of the EGS. A supercritical solution of H2 and N2 (at w200 bar and w27 C,
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density w62 kg/m3) can be pumped down the injection well of the EGS e supercritical fluids are miscible with each other. Injecting the mixture into a 10 km deep reservoir increases the P from 200 bar to w260 bar which increases the NH3 production by w15%. (The 10 km depth is used for illustration purposes. The United States Department of Energy is looking at EGS as a significant, low-carbon long term domestic energy source in order to tap into high temperature heat which will result in more efficient power generation.) The products that exit the production pipe of the EGS will be cooled with heat recovery to condense the NH3. The unreacted H2 and N2 are mixed with makeup H2 and N2 and returned to the EGS. About 31 kWh/kmol of NH3 will be required for producing the supercritical N2 and H2 feed. Because less compression is required and heating is supplied by the EGS the cost of NH3 could be reduced. Fig. 2 is a conceptual design of the process. The heat supplied from the EGS and the heat of the exothermic reaction is recovered by heating the fluid used in the power plant. The choice of the power plant depends on many factors including the temperature at which the heat is available for power generation and the pressure of the heat carrying fluid and its composition. Conventional steam Rankine power plants, including supercritical plants can be used when the temperature of the available heat at the surface is >375 C. When hydrogen is a byproduct of the reaction it can be separated and for an energy source or for hydrogenation (e.g. at an oil refinery). Hot and high pressure ammonia can be used as the working fluid in the power cycle such as using the Kalina cycle. 3. Oxidation of contaminants in wastewater Oxidation of hazardous contaminants in recalcitrant wastewater at high Ps and Ts is a known technology [9,11,15]. Supercritical water is known to be a good solvent for materials and a good medium for chemical reactions [11]. Wastewater can be pumped to the required P and contacted with air or O2 to absorb enough O2 to oxidize the contaminants and then injected in the geothermal reservoir. Solubility of O2 in fresh water at 25 C is w207, mg/L at 25 bar and increases at the rate of 8.27 mg/L for each 1 bar increase in P. Oxygen can also be added as liquid H2O2 at ambient T and P. As the water travels down the injection pipe its P and T increase at the rate of w96 bar and <10 C per 1 km of depth. This will keep the O2 in solution. The water is treated as its T and P increase and the treated water which carries the heat from the EGS and the heat generated by the oxidation reaction exits the production well in the supercritical state. The oxidation process converts the organic carbon in the contaminants to CO2, the H2 to H2O and the halogens to acids such as HCl. After neutralizing the HCl, the supercritical water can be used to drive the turbines of the power plant. This process conserves water because the treated water can be used in the power plant and then can be used for other applications after it is analyzed for residual hazardous materials. 4. Production of H2 via steam reforming of CH3OH
Fig. 1. Equilibrium composition of NH3, N2 and H2 at different temperatures and pressures.
Methanol is synthesized by reacting H2 and CO produced by steam reforming of CH4. The EGS Ts are low for steam reforming CH4. The reforming reaction (CH3OH(liq) þ H2O(liq) þ Heat 4 CO2(gas) þ 3H2(gas)) can be used for capturing and transporting the EGS heat while producing H2 [1,12,13]. The standard enthalpy of the reaction is about 130,000 kJ/kmol of CH3OH e over 2.5 times the heat that water can capture when it boils. This reaction can occur at Ts as low as 200 C using catalysts [13]. Fig. 3 shows the equilibrium composition for the reforming reaction for a CH3OH/H2O mole ratio of 1:2. A liquid mixture of CH3OH/H2O can be injected into the EGS at room T and P. The products exiting the
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Fig. 2. Conceptual design for the production of NH3 using a 3000 m deep EGS.
EGS can be cooled with heat recovery to produce power. The H2 can then be separated from the products. The unreacted water and methanol will be condensed after separating the H2 and CO2 and both CH3OH and H2O can be injected as a liquid solution after adding the required makeup amounts. The unreacted condensed CH3OH/H2O can be mixed with a fresh supply of reactants and returned to the EGS. The remaining stream will contain mainly CO2. Fig. 4 shows the equilibrium conversion ratio of CH3OH at different Ts in a 10 km deep EGS where the P will be about 670 bar. Fig. 4 shows that at Ts above about 500 C over 90% conversion can be expected. Fig. 5 shows the equilibrium composition of the decomposed mixture in this EGS at different T and Fig. 6 shows the enthalpy and exergy (ideal work) gained by the steam reforming of CH3OH. At about 600 C each kmol of reformed CH3OH results in a gain of about 150,000 kJ of exergy. At a second law efficiency of 60%
Fig. 3. Equilibrium compositions of CH3OH, H2O, CO, CO2 and H2 at different temperatures and pressures starting with CH3OH/H2O mole ratio of 1:2.
this corresponds to about 25 kW of power. This output will be reduced by the exergy of the products recovered as chemicals. By comparison, one kmol/h of H2O, when heated to 600 C and assuming it reaches the surface at a pressure of 250 bar, would have gained 52,600 kJ of exergy. At a second law efficiency of 60% this corresponds to about 8.8 kW of power. Therefore, depending on what the reactants and products are, similar flow rates as for water are expected to be required to produce the same amount of power in addition to the chemicals that can be produced. 5. Production of CH3OH via partial oxidation of methane Methanol synthesis starts with steam reforming of CH4 to produce H2 and CO. About 20% of the methane is used to supply heat for the process. The process consumes water and produces CO2. Partial oxidation of CH4 at high Ts and Ps can produce CH3OH (CH4 þ 1/2O2 / CH3OH) [2,5,14]. Because the combustion of
Fig. 4. Steam reforming of the methanol in a 10,000 m deep EGS reservoir.
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Fig. 5. Equilibrium composition of the steam reformed methanol.
methane is thermodynamically more favorable the ratio of CH4:O2 must be kept high (>10). The process can also utilize natural gas where the primary reaction is with methane with assistance on initiation of the reaction from the heavier trace hydrocarbons. However, the conversion of methane to CH3OH is low but the selectivity toward CH3OH can be high. Formaldehyde (CH2O) and formic acid (HCO2H) are the other products. Because of the hazards associated with employing O2 and CH4 together this reaction is generally avoided. Careful design of this system is necessary. The following points should be addressed in the design of such a system: (a) O2 and the CH4 should be fed through separate pipes to prevent the combustion of the methane in the feed pipes, (b) mixing in the hot zone should be controlled such that the O2/CH4 mole ratio is low enough so that combustion does not occur, and (c) the products contain CH3OH, CH4, CH2O, HCOOH (formic acid) and some residual O2. Separation of these species adds to the complexity of the process design. 6. System design for implementing the chemical reactions These processes can be carried out using closed double concentric pipe arrangements such as that shown in Fig. 7. The annulus of the double pipe can be fitted with an appropriate catalyst in the hot zone and/or with membranes to separate some of the products, such as H2, in order to speed up the reactions. The inner pipe will be insulated. The reactants are injected into the
Fig. 6. Enthalpy and exergy gain as the methanol is reformed.
Fig. 7. Conceptual design for the production of NH3 and power using the EGS heat.
annulus of the double pipe system and the products rise in the inner pipe and get processed at the surface. Pressurized water is injected through a separate pipe to fill the fractured EGS and to keep the fractures open. This water stays in the fractured rocks. The water facilitates the conduction of the heat from the rocks to the reaction zone. When large flow rates of the reagents are required the wells may experience rapid cooling in the short term due to rapid thermal drawdown. Induced forced convention in the pressurized water will be necessary in order to enhance the heat transfer rate to the reaction zone and to maintain an adequate temperature distribution in the EGS for a lengthy period of time. The design presented in Fig. 7 illustrates how the heat is transported from the fractured rocks to the reaction zone. Water is circulated internally through the fractured rocks to capture the heat from the rocks and then delivers the heat to the chemically reacting materials. The difference between this design and the conventional design, as far as heat transfer is concerned, is that in the design shown in Fig. 7 the heat is recovered from the water injected into the fractures by the reaction in the EGS (underground) instead of bringing the hot water to the surface and then recover the heat from the reaction products above ground. Therefore, the heat transfer characteristics from the EGS to the chemically reacting working fluid should not differ much from the conventional case. The chemically reacting fluid captures the heat from the water that is injected in the EGS as chemical energy, sensible heat and latent heat. Therefore, more heat is transferred per unit mass or mole to the surface that what water can transfer. The flow rate will be determined by the amount of heat required to produce the required electrical power and/or the necessary chemical production rates. The double pipe design will ensure that the chemicals will always be contained and will not be allowed to leak out of the exterior pipe. Corrosive materials in a high temperature and pressure environments dictate that corrosion resistant casing materials
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be used in order to prolong the service life of the pipes. In this application the casing material has to also have good heat transfer materials. Research is underway to develop such materials at reasonable cost. Such materials will also be required in deep high temperature EGS reservoirs regardless of what the working fluid is. As it is well known separating one of the products will result in speeding the reaction rate. Products such as H2 can be separated insitu at the bottom of the inner pipe of the double pipe by incorporating an appropriate membrane sleeve (such as palladium membranes for H2 separation). Catalysts can also be incorporated in the design (Fig. 8). Both of these will require more frequent maintenance. This may require bringing the inner pipe (in sections) to the surface to maintain the membrane and the catalyst when required. 7. Economic analysis Detailed economic analysis to establish economic feasibility will be necessary before this concept can be implemented. However, reliable economic analysis can be conducted when the design and operation specifics of the EGS and the chemical and power processes are known. At this stage this paper presents a concept. A brief discussion of its potential economic benefits is given below. In the conventional case two wells will have to be drilled: an “injection” well and a “production” well. Water is injected into the EGS through the injection well and is circulated through the fractured EGS in order to get heated. The heated water reaches the “production” well and rises in it to the surface. In the proposed concept (Fig. 7) two wells are also needed. However, one of the two wells is a smaller diameter well for injecting water into and circulating it through the fractured zone. The water will be kept underground and therefore only a small diameter pipe will be necessary for injecting this water. The second will be fitted with a double pipe. The outer pipe will be closed at the bottom and will be used for injecting the reactants through the annulus of the double pipe. The internal pipe is open at the bottom and will be used for bringing the products of the reaction to the surface. The reacting fluids will capture the heat from the circulating water underground. Because the reacting fluids will capture
the heat as chemical energy in addition to sensible and latent heats the flow rate of the reacting fluids for the same amount of heat capture can be smaller than that for water. Therefore, the diameter of this well may also be smaller than the conventional well and may cost less. The cost of the internal pipe in the double pipe will be relatively very small. Therefore, overall the drilling and completion costs can be lower than the costs for a conventional system. Heat is recovered from the hot products at the surface and is used for power generation and the chemical products are recovered for use as chemicals. The products which will reach the surface as hot and at high pressure gases can also be used directly as the working fluids in the power cycle. For example, when NH3 is the product of the reaction of N2 and H2, the products can be expanded to produce power and the low pressure ammonia that exits the expander can be recovered as a product. Therefore, the cost of the wells will be split between the power plant and the chemical process. Because some reacting chemicals can capture significantly more heat per unit mole or volume than water, the system can be designed to produce the required power and chemicals with the same flow rate or even less. In addition, the reactions will be carried out underground without having to pump or compress the reactants to very high pressures and no expensive high pressure reactors or heat exchangers will be required. Another benefit is that high pressure safety issues associated with above ground operation which will require special containment structures will not be required and this will reduce the cost also. 8. Conclusions Combined power generation and production of useful chemicals using EGS reservoirs present an opportunity for more efficient utilization of geothermal energy. These reactions capture EGS heat as chemical energy as well as sensible and latent heat and therefore, can capture significantly more heat per unit mass of flow than water. Costs, safety and environmental impacts present distinct challenges for the deployment of these fluids. Other renewable heat sources, such as concentrated solar, could also provide the energy for driving chemical reactions. Developing a more detailed model of the overall process performance and economics will assist the industry in realizing this opportunity. Acknowledgements The submitted manuscript has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357. The U.S. Government retains for itself, and others acting on its behalf, a paid-up nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government. This work is sponsored by the United States DOE e Geothermal Technologies Program e Enhanced Geothermal Systems. References
Fig. 8. A closed geothermal heat capture and transport system with reactionenhancing mechanisms.
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