Development of a Sub-Surface Burner Technology for In-Situ Heating

Paper from the AFRC 2015 conference titled Development of a Sub-Surface Burner Technology for In-Situ Heating

There are large deposits of oil shale and inaccessible coal in the US and around the world. If; these deposits can be heated in situ then it is possible that liquids and gases can be produced; without mining. The purpose of this work is to develop a system for in situ heating of oil shale; and other energy rich deposits. Two fuel-lean, natural-gas-fired burner concepts have been; examined. One involves flameless catalytic oxidation and the other homogeneous oxidation.; Pressures of 10-30 bars and temperatures of 700-1000 K are being considered. A preliminary; study included calculations of overall energy balance, heat transfer and pressure drop, and; detailed bench-scale experiments examined the catalytic activity of several metals for dilute; methane oxidation. A new flameless burner configuration has been proposed and is being; evaluated by looking at mixing of fuel and oxidant over extended distances. A set of cold-flow; mixing experiments will be used to validate design calculations. Technical, economical and; operational concerns for the proposed burner are being considered.

DEVELOPMENT OF A SUB-SURFACE BURNER TECHNOLOGY FOR IN-SITU HEATING Fatemeh Babazadeh Shareh, Geoffrey Silcox, Eric Eddings* Department of Chemical Engineering and Institute for Clean and Secure Energy, University of Utah, Salt Lake City, Utah Leonard Switzer American Shale Oil, LLC, Rifle, Colorado Abstract There are large deposits of oil shale and inaccessible coal in the US and around the world. If these deposits can be heated in situ then it is possible that liquids and gases can be produced without mining. The purpose of this work is to develop a system for in situ heating of oil shale and other energy rich deposits. Two fuel-lean, natural-gas-fired burner concepts have been examined. One involves flameless catalytic oxidation and the other homogeneous oxidation. Pressures of 10-30 bars and temperatures of 700-1000 K are being considered. A preliminary study included calculations of overall energy balance, heat transfer and pressure drop, and detailed bench-scale experiments examined the catalytic activity of several metals for dilute methane oxidation. A new flameless burner configuration has been proposed and is being evaluated by looking at mixing of fuel and oxidant over extended distances. A set of cold-flow mixing experiments will be used to validate design calculations. Technical, economical and operational concerns for the proposed burner are being considered. Introduction There are large underground oil shale deposits in the U.S. and around the world. Heating oil shale deposits to a certain temperature, about 300 °C, results in shale oil and natural-gas-like production via pyrolysis of kerogen within the rock. There are generally two ways to heat the oil shale and extract the hydrocarbons: surface retorting and in-situ retorting. The first method * Corresponding author Email Address: [e-mail] basically has three steps: 1) mining the oil shale, 2) thermal processing to obtain a refinery feedstock, and 3) disposal of the spent shale. The second method involves heating the oil shale underground over a period of time to extract the hydrocarbons directly. There has been increasing attention given to the second method, in-situ retorting, especially for deep formations or other situations where mining is less suitable. In certain cases, in-situ retorting doesn't have the disadvantages of surface retorting, and it provides the opportunity to recover deeply deposited oil shale [1]. The Conduction, Convection, Reflux (CCRTM) retorting process is one of the in-situ heating and extraction processes proposed by AMSO, LLC [2]. Figure 1 is a schematic of the CCRTM process. There is a boiling pool of shale oil in the bottom of the retort, in contact with a heat source. Hot vapors from the boiling pool recirculate through the retort by natural convection and heat the surrounding oil shale. It is hypothesized that heating the shale may cause it to fracture and further enhance the effective permeability of the retort. Thus more oil shale is exposed to the hot vapors. The hot vapors condense on recently exposed oil shale and drain to the boiling pool. As the oil shale reaches temperatures between 300 °C and 350 °C, the pyrolysis rate of the kerogen becomes high enough to yield oil and gas in a reasonable time frame. Figure 1: Schematic of the CCRTM process [2] In this paper, a downhole burner is proposed as an efficient method and apparatus to provide the heat required to boil the pool. The schematic of the method is shown in Figure 2. Fuel and oxidizer pass through the 2000-ft vertical well to the mostly horizontal bore hole where the boiling pool is distributed. The downhole burner is placed in the horizontal section where there is a counter-current heat exchange between the incoming fuel/oxidizer and the outgoing flue gas. Two burner concepts are examined here: one involving flameless catalytic oxidation and one using homogeneous oxidation. Both would rely on oxidation of dilute natural gas/oxygen mixtures at high pressures and low temperatures (P ~ 10-30 bars and T ~ 7001000K). The advantages and concerns regarding both approaches are discussed. The flow patterns in the homogeneous burner were investigated by simulating a one meter section using ANSYS simulation software. Bench-scale experiments and CHEMKIN calculations were used to explore the flameless catalytic approach. In addition to the in-situ oil shale retorting, there are other applications such as gasification of underground coal and recovery of underground heavy hydrocarbons, for the above mentioned downhole burners. Down-hole Burner Figure 2: Schematic of a generalized down-hole burner Flameless Catalytic Oxidation Burner A schematic of the catalytic burner is presented in Figure 3. The heater has two main pathways, one for the inner flow of oxidant and one for the outer flow of exhaust gases. The inner passageway has several reaction zones. Each zone has a mixing section where the oxidizer and the fuel mix, and a reaction section which is a catalytic bed where the oxidation reaction occurs. The flue gases pass through the outer flow passageway, preheat the fuel and oxidizer and then return to the surface. Figure 3: Flameless Catalytic Oxidation Heater Configuration [3]. Although the flameless catalytic oxidation concept can provide the desired temperature and the heat needed to retort the oil shale, there are some concerns regarding this design that should be addressed; - Controlling the dilution of the oxygen along the extended length of the heater - Having sufficient mixing of fuel and oxidizer - Preventing pre-ignition of the fuel/oxidizer mixture before reaching the catalyst bed - Preventing channeling of the gas mixture in the catalytic beds - Controlling the pressure drop in catalytic beds - Finding a catalyst with a reasonable lifetime and thermal stability for long-term operation To address some of the concerns, detailed bench-scale experiments examined the catalytic activity of several metals and a Pd-coated catalyst for dilute methane oxidation. Commercial chemical kinetics software, CHEMKIN, was also used to model homogenous methane oxidation under a range of conditions to compare with the experimental results. In addition, kinetic parameters were derived from the experimental data to start engineering calculations for burner design. Both experimental and modeling results indicate that Pd-coated alumina catalyst is suitable for downhole burner use and promotes complete methane oxidation at temperatures as low as 300 ºC. A kinetic model that describes this reaction was developed; however, it will not be presented here. Homogeneous Oxidation Burner The homogeneous oxidation heater is shown in Figure 4. The heater includes two separate pipes carrying fuel and oxidizer. Each pipe has several holes acting as small nozzles to introduce the non-premixed fuel and oxidizer along the length of the heater. The fuel and oxidizer would be injected into the reaction chamber in a manner that would create sufficient mixing. The mixing would also be controlled by the relative locations of the fuel and oxidizer nozzles to produce a dilute combustion environment so as to control peak temperatures and NOx formation. Flue gas or CO2 may also be used as a diluent to assist with mixing, to control the rate of the combustion reactions and thus keep the flame temperatures low. The fuel and oxidizer mix and auto-ignite inside the burner. Combustion of fuel and oxidizer takes place in the reaction chamber that has holes to allow for flue gas to escape into an annular exhaust pathway. The ideal situation is to have a complete reaction within each mixing zone so that there would be no significant unburned fuel within the chamber. Figure 4: Homogeneous Oxidation Heater configuration This configuration was proposed to eliminate some of the issues in the flameless catalytic oxidation heater. The following lists some of the advantages of this configuration, some of which are shared with the flameless catalytic heater: - Reducing safety concerns in handling the gases (non-premixed gases) - Preheating the fuel and oxidizer by effective heat transfer between exhaust gas, oxidant and fuel lines - Having a uniform temperature along the burner - Control of temperature and NOx formation due to dilution of fuel and oxidizer - Eliminating concerns due to deactivation and poisoning of the catalyst along the burner (less problem in long term operation) - May be easier to assemble, install and operate relative to other designs But there are still some concerns that should be addressed. Some of the issues regarding this configuration are listed below: - Maintaining desired flow rate of fuel and oxidizer along the full length of the burner - Controlling the ignition - Minimizing pressure drop across fuel and oxygen nozzles - Optimizing relative position of fuel and oxidizer nozzles to address hot spots, mixing, NOx - Controlling the flame stability - Subsurface pipe alignment Detailed engineering calculations along with CFD simulations are being performed to address the above-mentioned issues and to find the optimum design, pipe sizing, nozzle sizing, and nozzle spacing and orientation. Preliminary results are presented in this paper. CFD simulations The flameless burner configuration is being evaluated by simulating the mixing of fuel and oxidizer over a 1 meter length of the heater. A set of cold-flow mixing experiments are planned. Preliminary CFD modeling (ANSYS/Fluent, version 15.0) has provided mixture fraction profiles for fuel and oxidizer for various relative positions of nozzles in the pipes. The calculations will guide the design and optimum position and orientation of fuel and oxidizer nozzles to achieve the desired level of mixing vs. dilution. The results of the simulations presented here do not include reactions at this time. Two perforated pipes are placed in a pipe that is also perforated 10 cm from the end as shown in Figure 5. The whole system is located within a fourth pipe. Figure 5 shows the three cases studied. In cases 1 and 2, the nozzles of the feeder pipes are delivering fuel and oxidant in adjacent or different axial locations, respectively, but have the same circumferential orientation. In case 3, the nozzles are at the same axial location, but are directed away from each other. In case 1 and 3 the nozzles are 25 cm from the gas inlet and in case 2 the position of one nozzle is 25 cm and the other is 50 cm from the gas inlet. Figure 6 shows the boundary conditions. CH4 and O2 are injected into the pipes, pass through the nozzles, mix inside the perforated annulus, and escape into an annular exhaust pathway. The pipe walls are insulated. The system was modeled at 300K and 1 atm, as these calculations were focused on non-reacting mixing behavior. Figure 5: Schematic of Cold Flow Study Figure 6: Inlet and Outlet boundary conditions; walls are treated as insulated (adiabatic). Results Figure 7 shows a plane at x = 0 where the calculated values will be presented. Figure 8 shows the O2 mole fraction at plane x = 0 for the three cases introduced above. In Case 1, two streams are well mixed in the perforated chamber. In case 2, there are 2 distinct regions, a region where the concentration of O2 is high (CH4 is low) and a second where two gases are well mixed. In Case 3, the two streams are partially mixed. The results show that the axial positions of the nozzles have a more significant role in the mixing behavior than the radial orientations of the nozzles. Case 2, where the axial positions of the nozzles are different, shows less mixing than cases 1 and 3 where the radial positions of the nozzles are the same. All cases would be suitable depending on the purpose of the design, the desired level of mixing/dilution and the desired temperature. Moreover, Case 2 seems more suitable for the purpose of dilute combustion. In order to see how well the gas is distributed in the cylinder surrounding the oxygen and the methane lines, the gas velocity is compared at two openings in the perforated cylinder. Figure 9 illustrates the positions of the oxygen and methane nozzles and the openings in the perforated cylinder. The two above mentioned openings are showed in red circles. Figure 10 shows the velocity magnitude along lines 1 and line 2. The velocity magnitudes are the same for both, which means the gas flowrate is well distributed in the combustion zone. The results shown are for Case 2, but the same results are obtained with both Case 1 and Case 3. Case 1 and 2 X=0 Case 3 Figure 7: Schematic showing the plane x=0 and the nozzle locations Case 1 Case 2 Case 3 Figure 8: Mole Fraction of O2 at plane x = 0 for all 3 cases. Nozzles Figure 9: Position of the nozzles on iso surface x = 0. ...Line 1 … Line 2 Figure 10: velocity magnitude along line 1 and line 2 in Figure 9, for Case 2. Conclusions and Future Work Two configurations for a downhole burner, a homogenous oxidation burner and a flameless catalytic oxidation burner, are proposed. A suitable catalyst, Pd-coated alumina, for methane oxidation was found and a kinetic model was derived based on the experimental results. Engineering calculations and cold flow CFD modeling have been conducted and the appropriate nozzles sizes, radial orientations and axial positions have been identified. Although many concerns regarding the configurations have been addressed, there are still some concerns that require further study: - Since the simulations presented here do not include reactions, it is necessary to perform combined mixing and reaction simulations to verify optimal design. - A detailed literature review was performed to identify suitable operation conditions regarding ignitability and flame stability; however, it is still critical to perform experiments to verify the ignitability and stability of the fuel/oxidizer/diluent mixtures, to refine the optimal design of the burner. - Since the heater operating temperature depends heavily on the effectiveness of heat transfer to the surrounding oil and/or rock, the design and control of the burner must address the potential for operation under variable oil conditions outside of the burner, which may be different at different locations along the length of the burner. Thus, it is also essential to perform detailed engineering-level heat transfer calculations and heat transfer experiments with a representative oil to address the heat transfer issue and to provide accurate assessment of heat transfer rates. Acknowledgement This research was sponsored by American Shale Oil, LLC (AMSO). Their support is gratefully acknowledged. References [1] AMSO, Oil Shale Extraction Methods, (2015). http://amso.net/about-oil-shale/oil-shaleextraction-methods. [2] A.K. Burnham, R.L. Day, P.H. Wallman, J.R. McConaghy, H.G. Harris, P. Lerwick, et al., In situ method and system for extraction of oil from shale., US patent, US 8,162,043 B2, American Shale Oil, LLC, 2010. [3] A.K. Burnham, H. Wallman, J. McConaghy, R.L. Day, Heater and method for recovering hydrocarbons from underground deposits., Patent, WO 2010/053876 A2, American Shale Oil, LLC, 2010.