Enhanced geothermal systems (EGS) using CO2 as working fluid—A novel approach for generating renewable energy with simultaneous sequestration of carbon

Responding to the need to reduce atmospheric emissions of carbon dioxide, Brown [Brown, D., 2000. A Hot Dry Rock geothermal energy concept utilizing supercritical CO₂ instead of water. In: Proceedings of the Twenty-Fifth Workshop on Geothermal Reservoir Engineering, Stanford University, pp. 233–238] proposed a novel enhanced geothermal systems (EGS) concept that would use carbon dioxide (CO₂) instead of water as heat transmission fluid, and would achieve geologic sequestration of CO₂ as an ancillary benefit. Following up on his suggestion, we have evaluated thermophysical properties and performed numerical simulations to explore the fluid dynamics and heat transfer issues in an engineered geothermal reservoir that would be operated with CO₂. We find that CO₂ is superior to water in its ability to mine heat from hot fractured rock. Carbon dioxide also offers certain advantages with respect to wellbore hydraulics, in that its larger compressibility and expansivity as compared to water would increase buoyancy forces and would reduce the parasitic power consumption of the fluid circulation system. While the thermal and hydraulic aspects of a CO₂-EGS system look promising, major uncertainties remain with regard to chemical interactions between fluids and rocks. An EGS system running on CO₂ has sufficiently attractive features to warrant further investigation.     

Influence of different fracture morphology on heat mining performance of enhanced geothermal systems based on COMSOL

Formation of enhanced geothermal systems (EGS) is the necessary approach to obtain geothermal energy efficiently. In-situ stress, nature of reservoir physical properties and fracturing methods will affect the artificial fracture morphology after reservoir stimulation. A three-dimension thermal coupled seepage model of fractured media was established to simulate the influence of fracture morphology on heat mining performance of EGS, considering the pressure- and temperature-dependent physical properties of working medium. The results indicate that formation of complex fracture network is favorable for heat mining. Production mass flow in Case1 with complex fracture network enhances nearly 2.5 times comparing to the unenhanced model at exploitation beginning. The total net energy rate will up to 44 MW and be maintained above 10 MW for 5 years. The system impedance can be effectively reduced, however the sustainable heat mining duration decreased to 30 years. The increase in length and number of branch fractures is expected. While increasing the width of branch fractures deliberately has little effect on the exploitation of EGS. Finally, we investigate the adaptability of employing supercritical CO₂ in EGS with complex fracture network. Production mass flow will be enhanced 3–5 times compared with water, but the stability is poor, total net energy rate decrease from 90 MW to 3 MW over the 10-year operation period.     

热储分层影响EGS采热的数值模拟研究

由于岩石构造不同、天然裂隙的差异以及压裂过程的随机性等因素,增强型地热系统（EGS）人工热储通常具有较强的非均质性。探究热储的非均质性对EGS热开采过程的影响,对EGS性能预测与分析评价有重要意义。论文考虑到热储沿深度方向的非均质性,基于等效分层多孔介质物理模型,并使用自主开发的EGS数值模型,模拟了多个具有分层热储EGS的长期运行过程,发现热储深度方向上非均质性对热能的开采影响显著,而流量分布的不均匀性是导致系统采热性能下降的主要原因。为了方便分析和评价,我们建立一种新的定量化描述热储非均质性的方法,然后基于更多的非均质热储EGS算例结果,拟合得到EGS采热性能与热储非均质性的定量关系式。     

Electricity generation using a carbon-dioxide thermosiphon

There is an opportunity to expand the baseload geothermal electricity generation capacity through the development of engineered geothermal systems (EGS). Carbon dioxide (CO₂) could be used as an alternative to water to extract heat from these systems considering its advantages of ease of flow through the geothermal reservoir, strong innate buoyancy that permits the use of a thermosiphon rather than a pumped system over a large range of fluid flow rates, and lower dissolution of materials that lead to fouling. However, the thermodynamics of EGS using CO₂ to extract heat from subsurface rock masses is not well understood. Here we show that the wellbore frictional pressure losses are the dominant factor in CO₂-based EGS. Wellbore friction is the major limiter on the amount of energy that can be extracted from the reservoir by CO₂, as measured by the exergy available at the surface. The result is that CO₂ is less effective at energy extraction than water under conditions similar to past EGS trials. Nevertheless, CO₂ can perform well in lower permeability reservoirs, or if the wellbore diameter is increased. Our results demonstrate that CO₂-based EGS need to be designed with the use of CO₂ in mind. We suggest this work to be a starting point for analysis of the surface infrastructure and plant design and economics of CO₂-based EGS.     

增强型地热系统水力压裂与双井连通实验仿真研究

增强型地热系统（Enhanced Geothermal System, EGS）作为未来新能源和清洁能源利用的一个重要方向,受到了世界各国的广泛关注。一直以来,野外试验场的工程实践和数值模拟分析是进行EGS研究的两种主要方式。本文通过实验室规模的小型试验系统,对EGS的水力压裂、裂隙监测、生产井定位和注水测试进行了仿真,成功实现了注入井−热储层−生产井的水力连通,分别以定井口压力和定注水流量进行水力测试。试验结果表明,热储层的裂隙开度会随着水力特性而发生变化,注水压力较大时热储层的裂隙具有更大的开度和渗流能力。从提升热储层经济性的角度考虑,实践中应当在较大注水压力时对热储裂隙结构进行加固处理。     

增强型地热系统中液-岩化学作用数值模拟研究

增强型地热系统（Enhanced Geothermal System, EGS）利用深层岩石中连通的裂隙网络进行流体工质循环,从而实现地热能的持续开采。EGS运行时循环流体工质会与深层岩石产生化学反应,引起岩石中矿物的溶解/沉积,使热储中的裂隙网络形貌产生动态变化,对地下流动与传热过程造成影响。本文分析了EGS中液–岩化学作用特点,详细阐述了在多孔介质热流动模型中耦合入液–岩化学反应的方法,基于已开发成功的EGS传热传质数值模型初步建立了传热–流动–化学（Thermal-Hydraulic-Chemical, THC）多场耦合数值模型,并使用该模型对五井布局EGS的长期运行过程进行了模拟分析,模拟时仅考虑方解石在水流体中溶解和沉积。模拟结果显示,循环流体的注入温度以及注入流体中的矿物离子浓度的设定十分重要。如果二者没有达到较为合适的“平衡”,就会导致注入井附近渗透率和孔隙率的持续变化,对EGS的导流能力造成极大影响。     

西藏羊易EGS开发储层温度场与开采寿命影响因素数值模拟研究

西藏羊易地区具有丰富的地热能,单井开发潜力接近10 MW,对其深部热储进行EGS开采,可缓解西部能源紧缺问题。本文建立二维理想EGS开发模型,探讨深层地热开采过程中开采流量、注采方式、注入温度等参数对热储温度场分布及开采寿命的影响。基于羊易温度信息设计了12个数值模型,对比研究发现,开采流量对EGS开采的影响较大,为保证开采50年内的商业利用价值,最大开采流量应控制在0.028 kg/s以下;考虑到钻井成本,注采方式的选择以高注高采和中注高采为最佳;注入温度对热储开采影响较小,可选择40℃ ~ 80℃之间任意温度的地热尾水进行回灌,实现地热资源梯级利用。     

Impact of choice of CO2 separation technology on thermo‐economic performance of Bio‐SNG production processes

Three different CO₂ separation technologies for production of synthetic natural gas (SNG) from biomass gasification – amine‐based absorption, membrane‐based separation and pressure swing adsorption – are investigated for their thermo‐economic performance against the background of different possible future energy market scenarios. The studied scale of the SNG plant is a thermal input of 100 MW_th,LHV to the gasifier at a moisture content of 20 wt‐% with a preceding drying step reducing the biomass' natural moisture content of 50 wt‐%. Preparation of the CO₂‐rich stream for carbon capture and storage is investigated for the amine‐based absorption and the membrane‐based separation technology alternatives. The resulting cold gas efficiency η_cg for the investigated process alternatives ranges between 0.65 and 0.695. The overall system efficiency η_sys ranges from 0.744 to 0.793, depending on both the separation technology and the background energy system. Amine‐based absorption gives the highest cold gas efficiency whereas the potential for cogeneration of electricity from the process' excess heat is higher for membrane‐based separation and pressure swing adsorption. The estimated specific production costs for SNG c_SNG for a process input of 90.3 MW_th,LHV at 50 wt‐% moisture vary between 103–127 €₂₀₁₀/MWh_SNG. The corresponding production subsidy level c_subsidy needed to achieve end‐user purchase price‐parity with fossil natural gas is in the range of 56–78 €₂₀₁₀/MWh_SNG depending on both the energy market scenario and the CO₂ separation technology. Sensitivity analysis on the influence of changes in the total capital cost for the SNG plant on the production cost indicates a decrease of about 12% assuming a 30% reduction in total capital investment. Capture and storage of biogenic CO₂ – if included in the emission trading system – only becomes an option at higher CO₂ charges. This is due to increased investment costs but, in particular, due to the rather high costs for CO₂ transport and storage that have been assumed in this study. Copyright © 2013 John Wiley & Sons, Ltd.     

A novel two‐layered glass‐ceramic sealant design for solid oxide fuel cells

A thermo‐mechanical stress develops on the sealant of an solid oxide fuel cell (SOFC) stack because of thermal expansion coefficient (TEC) differences among ceramic electrolyte, metallic interconnector, and glass‐ceramic sealant. The stress initiates cracks and usually results in a sealing failure. This study offers a thermo‐mechanically stable two‐layered sealant to decrease thermo‐mechanical stresses among the SOFC components. The novel sealant involves two layers in which TEC of the first layer is close to that of the interconnector, while TEC of the second layer is similar to that of the electrolyte. SiO₂, CaO, Al₂O₃, and Na₂O glass‐ceramics are selected as a sealant material, and five cases are prepared by changing the amount of Al₂O₃ to obtain the appropriate properties close to both the interconnector and the electrolyte, separately. Tensile strength, wetting angles, shrinkages, and electrical resistances are measured to determine the sealant properties. To see effect of the reaction on the microstructure, scanning electron microscopy (SEM) tests are conducted for the samples exposed both in air and in crofer 22 APU. Long‐term sealing performance of the novel sealant is compared with the traditional designs. While the two‐layered sealant keeps 35 kPa pressure for 500 h after 100 thermal cycles, test pressure with traditional design reduces to 31–32 kPa after 50 h. Copyright © 2016 John Wiley & Sons, Ltd.     

Characteristic evaluation of a CO2‐capturing repowering system based on oxy‐fuel combustion and exergetic flow analyses for improving efficiency

A CO₂‐capturing H₂O turbine power generation system based on oxy‐fuel combustion method is proposed to decrease CO₂ emission from an existing thermal power generation system (TPGS) by utilizing steam produced in the TPGS. A high efficient combined cycle power generation system (CCPS) with reheat cycle is adopted as an example of existing TPGSs into which the proposed system is retrofitted. First, power generation characteristics of the proposed CO₂‐capturing system, which requires no modification of the CCPS itself, are estimated. It is shown through simulation study that the proposed system can reduce 26.8% of CO₂ emission with an efficiency decrease by 1.20% and an increase power output by 23.2%, compared with the original CCPS. Second, in order to improve power generation characteristics and CO₂ reduction effect of the proposed system, modifications of the proposed system are investigated based on exergetic flow analyses, and revised systems are proposed based on the obtained results. Finally, it is shown that a revised proposed system, which has the same turbine inlet temperature as the CCPS, can increase power output by 33.6%, and reduce 32.5% of CO₂ emission with exergetic efficiency decrease by 1.58%, compared with the original CCPS. Copyright © 2010 John Wiley & Sons, Ltd.     

Hydrogen production and carbon sequestration by steam methane reforming and fracking with carbon dioxide

An opportunity to sequester large amounts of carbon dioxide (CO₂) is made possible because hydraulic fracturing is used to produce most of America's natural gas. CO₂ could be extracted from natural gas and water using steam methane reforming, pressurized to its supercritical phase, and used instead of water to fracture additional hydrocarbon-bearing rock. The useful energy carrier that remains is hydrogen, with carbon returned to the ground. Research on the use of supercritical CO₂ is reviewed, with proppant entrainment identified as the major area where technical advances may be needed. The large potential for greenhouse-gas reduction through sequestration of CO₂ and avoidance of methane leakage from the natural gas system is quantified.     

Experimental and numerical study of oxy‐methane flames in a porous‐plate reactor mimicking membrane reactor operation

The work investigates the reacting flow field, oxy‐methane flame characteristics and location, and the species distributions in a porous‐plate reactor mimicking the operation of oxygen transport membrane reactors (OTMRs). The study was performed experimentally and numerically considering ranges of operating equivalence ratio, from 0.5 to 1.0, and CO₂ concentrations in the total oxidizer flow (O₂ and CO₂), from 0% to 55% (by Vol). Oxygen was supplied through a slightly pressurized top and bottom chambers to cross the two porous plates to the central chamber, where a premixed mixture of CH₄ and CO₂ is introduced. ANSYS Fluent 17.1 software was used to solve for conservation and radiative transfer equations in the full three‐dimensional (3‐D) domain. The modified Westbrook‐Dryer (Oxy‐WD) two‐step reduced mechanism for oxy‐methane combustion was adapted for the calculations of chemical kinetics. The captured flame shapes using a high‐speed camera were compared with the calculated ones, and the results showed good agreements. At fixed equivalence ratio, elongated flames were obtained at higher CO₂ concentrations due to the increase in the mainstream Reynolds number and reduction in reaction rates, which delays the completeness of combustion. At fixed CO₂ concentration, the increase in equivalence ratio resulted in more compact and intense flames. The effective mixing and flame stability resulted in complete fuel conversion under stoichiometric condition. Stable flames were located between the two porous plates at reasonable distance. This perfect flame location prevents the thermal fracture of the membranes and improves their oxygen permeation flux, resulting in better combustion characteristics when the results are projected on the case of OTMRs. This implies efficient and safe applicability of the OTMRs by the condition that membranes can provide sufficient oxygen flux for complete combustion. A warm outer recirculation zone (ORZ) was created beside each porous plate, which helps anchoring the flame at the leading edge of the porous plate. The range of temperature within the ORZ was 800 to 1600 K, which lies in the operability limits of membranes for the case of OTMRs. The effective complete mixing and flame stability resulted in complete fuel conversion under stoichiometric condition. The temperature and species distributions within the reactor are presented in detail over wide ranges of operating conditions. The results recommended the reactor operation under stoichiometric combustion condition based on performance and economic points of views. The results are promising when projected on the application of the OTMRs under oxy‐combustion conditions for clean and efficient energy production.     

河北献县东部雾迷山组碳酸盐岩热储酸化压裂试验研究

通过对河北献县东部雾迷山组碳酸盐岩进行大尺寸高地应力酸化压裂物理模拟试验与小尺寸温度应力耦合环境下酸化压裂试验,讨论地应力、温度、酸液排量以及压裂模式等因素与碳酸盐岩压裂效果之间的关系,找到碳酸盐岩储层压裂裂缝的生长规律。研究表明：将裂缝发育与裂缝不发育储层碳酸盐岩压裂曲线对比发现,储层岩石裂缝发育程度可明显降低破裂压力;压裂试验中储层岩石内裂缝激活对破裂压力具有显著影响,现场压裂过程中应当考虑到储层工程地质中裂缝发育程度的问题;酸液处理可显著降低破裂压力,更有利于形成复杂裂缝网络,破裂过程中诱发更多声发射事件,同时储层岩石裂缝发育程度直接影响到压裂效果。     

Effect of Fe infiltration to Ni/YSZ solid‐oxide‐cell fuel electrode on steam/CO2 co‐electrolysis

This study proposes a novel methodology for controlling syngas production from high‐temperature CO₂/steam co‐electrolysis. The co‐electrolysis of CO₂/steam mixtures is one of the most promising methods to reduce CO₂ emissions and mitigate climate change. CO₂ and steam are reduced to produce synthetic gas (H₂ and CO) through thermo‐electrochemical reactions occurring in a solid‐oxide‐cell fuel electrode. To make this technology viable, it is essential to improve electrochemical cell performance and obtain controllability of gas conversion and product gas selectivity. In this study, Fe infiltration to the Ni/YSZ fuel electrode and subsequent in situ alloying of Ni‐Fe is used to enhance the cell performance and syngas productivity. Impregnation of Fe‐oxide nanoparticles on the fuel electrode support of solid oxide cells and subsequent in situ alloying Ni‐Fe is obtained. Their homogeneous morphology and distribution are obtained by using an advanced infiltration technique. Results show that the Ni‐Fe/YSZ fuel electrode enhances CO selectivity and lowers an overvoltage imposed on the cell. This may result in syngas production with higher carbon contents and a higher co‐electrolysis system efficiency. In addition, its long‐term durability for 500‐hour operation is also evidenced with stable syngas production and negligible cell degradation.     

Performance analysis and optimization design of an axial‐flow vane separator for supercritical CO2 (sCO2)‐water mixtures from geothermal reservoirs

Supercritical CO₂ (sCO₂) has been proven to be a promising working fluid for geothermal heat mining, and the produced hot sCO₂ can be directly used for power generation. However, the sCO₂ produced from a brine‐based reservoir may contain a certain amount of water, preventing direct power‐cycle utilization. In this paper, an axial vane separator was designed to address the separation problem of sCO₂ and water produced from geothermal reservoirs. First, the influences of operational and structural parameters on the separation performance were analyzed through numerical simulations. Five factors were selected to develop separation performance regression models by the response‐surface method (RSM). Finally, geometrical parameter optimization was applied to these RSM models. The results show that the guide vane area and the exhaust inlet are the main locations impacting the system pressure drop. The separation performance can be affected by many factors, including the guide blade outlet angle, number of vanes, hub diameter, length of the vortex tube, droplet size, and inlet velocity. The water‐droplet size and the number of vanes are the most critical factors affecting the separation efficiency. The inlet velocity, the number of vanes, and the hub diameter have a larger influence on the pressure drop of the separator. The optimization results indicate that the separation efficiency can reach 100% under certain operating conditions with a pressure drop no greater than 100 kPa.     

Thermodynamic analysis of the CO2‐based Rankine cycle powered by solar energy

Using carbon dioxide as working fluid receives increasing interest since the Kyoto Protocol. In this paper, thermodynamic analysis was conducted for proposed CO₂‐based Rankine cycle powered by solar energy. It can be used to provide power output, refrigeration and hot water. Carbon dioxide is used as working fluid with supercritical state in solar collector. Theoretical analysis was carried out to investigate performances of the CO₂‐based Rankine cycle. The interest was focused on comparison of the performance with that of solar cell and those when using other fluids as working fluids. In addition, the performance and characteristics of the thermodynamic cycle are studied for different seasons. The obtained results show that using CO₂ as working fluid in the Rankine cycle owns maximal thermal efficiency when the working temperature is lower than 250.0°C. The power generation efficiency is about 8%, which is comparable with that of solar cells. But in addition to power generation, the CO₂‐based solar utilization system can also supply thermal energy. Copyright © 2007 John Wiley & Sons, Ltd.     

The potential contribution of geothermal energy to electricity supply in Saudi Arabia

With increase in demand for electricity at 7.5% per year, the major concern of Saudi Arabia is the amount of CO₂ being emitted. The country has the potential of generating 200×10⁶ kWh from hydrothermal sources and 120×10⁶ terawatt hour from Enhanced Geothermal System (EGS) sources. In addition to electricity generation and desalination, the country has substantial source for direct application such as space cooling and heating, a sector that consumes 80% of the electricity generated from fossil fuels. Geothermal energy can offset easily 17 million kWh of electricity that is being used for desalination. At least a part of 181,000 Gg of CO₂ emitted by conventional space cooling units can also be mitigated through ground-source heat pump technology immediately. Future development of EGS sources together with the wet geothermal systems will make the country stronger in terms of oil reserves saved and increase in exports.     

Implications of system expansion for the assessment of well‐to‐wheel CO2 emissions from biomass‐based transportation

In this paper we show the effects of expanding the system when evaluating well‐to‐wheel (WTW) CO₂ emissions for biomass‐based transportation, to include the systems surrounding the biomass conversion system. Four different cases are considered: DME via black liquor gasification (BLG), methanol via gasification of solid biomass, lignocellulosic ethanol and electricity from a biomass integrated gasification combined cycle (BIGCC) used in a battery‐powered electric vehicle (BPEV). All four cases are considered with as well as without carbon capture and storage (CCS). System expansion is used consistently for all flows. The results are compared with results from a conventional WTW study that only uses system expansion for certain co‐product flows. It is shown that when expanding the system, biomass‐based transportation does not necessarily contribute to decreased CO₂ emissions and the results from this study in general indicate considerably lower CO₂ mitigation potential than do the results from the conventional study used for comparison. It is shown that of particular importance are assumptions regarding future biomass use, as by expanding the system, future competition for biomass feedstock can be taken into account by assuming an alternative biomass usage. Assumptions regarding other surrounding systems, such as the transportation and the electricity systems are also shown to be of significance. Of the four studied cases without CCS, BIGCC with the electricity used in a BPEV is the only case that consistently shows a potential for CO₂ reduction when alternative use of biomass is considered. Inclusion of CCS is not a guarantee for achieving CO₂ reduction, and in general the system effects are equivalent or larger than the effects of CCS. DME from BLG generally shows the highest CO₂ emission reduction potential for the biofuel cases. However, neither of these options for biomass‐based transportation can alone meet the needs of the transport sector. Therefore, a broader palette of solutions, including different production routes, different fuels and possibly also CCS, will be needed. Copyright © 2009 John Wiley & Sons, Ltd.     

Evaluation of Mg‐MOF‐74 for post‐combustion carbon dioxide capture through pressure swing adsorption

This paper presents a computational study of an energy‐efficient technique for post‐combustion CO₂ capture using novel material, namely, Mg‐MOF‐74, using pressure swing adsorption (PSA) processes. A detailed one‐dimensional, transient mathematical model has been formulated to include the heat and mass transfer, the pressure drop and multicomponent mass diffusion. The PSA model has been further extended by incorporating a heat regenerating process to enhance CO₂ sequestration. The heat dissipated during adsorption is stored in packed sand bed and released during desorption step for heating purpose. The model has been implemented on a MATLAB program using second‐order discretization. Validation of the model was performed using a complete experimental data set for CO₂ sequestration using zeolite 13X. Simulation of the PSA experiment on fixed bed has been carried out to evaluate the capacity of Mg‐MOF‐74 for CO₂ capture with varying feed gas temperature of 28 and 100 °C, varying pressurization and purge times and heat regeneration. It was discovered that the PSA process with heat regeneration system might be advantageous because it achieves equivalent amount of CO₂ sequestration in fewer PSA cycles compared with PSA without heat regeneration system. Based on the simulated conditions, CO₂ recovery with Mg‐MOF‐74 gives high percentage purity (above 98%) for the captured CO₂. Copyright © 2015 John Wiley & Sons, Ltd.     

Thermochemical CO2 splitting using a sol‐gel–synthesized Mg‐ferrite–based redox system

The synthesis, characterization, and application of the Mg_xFe_3?xO₄ (MF) redox materials towards conversion of CO₂ via thermochemical redox reactions are reported. Sol‐gel method was utilized for the synthesis of MF materials. The derived MF materials were characterized to determine the physicochemical properties using various analytical techniques. PXRD results authorized the phase pure composition, and the SEM analysis designated nanoparticulated morphology of all the synthesized MF materials. The MF materials were further tested to estimate their O₂ releasing and CO production ability in multiple thermochemical cycles using a high‐temperature thermogravimetric analyzer (TGA). Attained outcomes specify that the MgFe₂O₄ (MF10) was superior in terms of the thermal reduction and CO₂ splitting capacity as compared with the other MF materials. For instance, MF10 liberated 58.7 μmol of O₂/g·cycle and produced 79.6 μmol of CO/g·cycle with an average O₂ recovery of 67.7%/cycle in nine thermochemical cycles.