Low Emission Conversion of Fossil Fuels with Simultaneous or Consecutive Storage of Carbon Dioxide

Abstract

This thesis evaluates the possibility of using underground coal gasification with a low CO2 footprint. The thesis consists of two parts. In the first part, by using the concept of exergy, a framework was constructed through which the practicality (feasibility) of an energy conversion/extraction method can be systematically evaluated. This framework, based on exergy analysis and cumulative degree of perfection, is described by analyzing a low emission underground coal gasification (UCG) process. For the evaluation of energy conversion processes we introduce a new concept, viz. recovery factor, which is a better indicator of the exergetic viability of a conversion process than the traditionally used efficiency factors. In the second part, various issues related to the aquifer storage of CO2 are studied. Aquifer storage is considered as an option for low emission fossil fuel utilization. Each chapter is summarized as follows: In chapter 2, various options are considered to reduce CO2 emissions when utilizing deep coal by applying UCG, i.e., (1) in combination with carbonation of synthetic minerals (CaO), (2) conventional UCG followed by ex-situ separation of CO2 and (3) upgrading the product gas using naturally occurring minerals (wollastonite). A chemical equilibrium model was used to analyze the effect of the process parameters on product composition and use it for an exergy (useful energy) analysis. The result is presented in terms of theoretical (ideal unit operations), practical (state of the art technology), and zero-emission (applying current CO2 capture and sequestration (CCS) to all sources of CO2 emission) recovery factors. The results show that underground gasification of deep coal can optimally extract 52-68 % of the coal chemical exergy, but zero-emission extraction gives a negative recovery factor, indicating that it is not practical with the current state of the art CCS technology. Using in-situ CaO, which will enhance the H2 production, is theoretically feasible with a recovery factor around 80%, but is not exergetically feasible with the current state of technology, i.e. with a negative practical recovery factor. Ex-situ upgrading of the conventional UCG product gas with wollastonite is exergetically feasible for both practical and zero-emission cases according to the equilibrium model. Slow attainment of chemical equilibrium makes its application questionable. In chapter 3, based on recent successful low-pressure underground coal gasification pilot experiments that use alternating injection of air (oxygen) and steam, a mathematical model is written to evaluate the potential of alternating injection UCG in large scale hydrogen production. This chapter extends an existing steady state model to a transient model that can describe an alternating injection of air and steam for deep thin coal layers. The model includes transient heat conduction, where the produced heat during the air injection stage is stored in the coal and surrounding strata. The stored heat is subsequently used in the endothermic gasification reactions during the steam injection. Comparison of the results with field data show that product composition and temperature oscillation can be predicted with a reasonable accuracy. The stored heat can deliver additional energy that can maintain the gasification during the steam injection period for a limited time. During the steam injection cycle, at low pressure the volumetric flow and the hydrogen content of the product gas are both high, but at higher pressures while the hydrogen composition is still high, the coal conversion rate decreases considerably. The exergy analysis confirms that alternating injection of air/steam describes a practical process for UCG at low pressure. However, injection of a mixture of steam and oxygen results in a practical recovery factor of 50% and produces 0.15 kg CO2 per MJ of exergy, which is higher than the practical recovery factor (40%) of the alternating injection process, which produces 0.12 kg CO2 / MJ of exergy. In the second part of the thesis, two issues related to aquifer storage of CO2 are discussed: injectivity problems due to salt precipitation, and storage capacity and long term storage due to dissolution of CO2 in water. In chapter 4, the negative saturation (NegSat) method, which is a combination of negative flash and multicomponent single/two-phase flow in porous media, is studied. It has been shown to be beneficial in numerical simulations of phase appearance/disappearance for mixtures that consist of volatile components, i.e., components that appear in both liquid and gas phases. The method is extended to a three phase system of CO2 -water-NaCl, in which NaCl appears as a nonvolatile dissolved component (NaCl) and as an immobile precipitated solid phase. The extended method is of practical use to assess carbon dioxide storage options. A detailed thermodynamic analysis of the NegSat method is given and the possibility to extend it to injection in brine aquifers is demonstrated. Precipitation of salt occurs due to evaporation of water into supercritical CO2 . Precipitation decreases the permeability near the injection well forming a dried-out zone. With the ensuing permeability change, the injection pressure needs to be increased to maintain the CO2 injection rate, which requires more compression energy and hence influences the exergetic viability of the carbon dioxide sequestration process.. To address this issue, first a thermodynamic model is optimized to predict the phase behavior of the CO2 -water-NaCl system with reasonable accuracy. Then the NegSat method for two-phase flow is modified to include salt precipitation. The model is solved to analyze the effect of various physical parameters on the injectivity of CO2 . Finally an exergy analysis is performed to quantify the effect of salt precipitation on the compression power requirement for CO2 injection into high pressure-high temperature-high salinity aquifers. Exergetic applicability of carbon capture and sequestration for low emission carbon dioxide fuel consumption, can presently only be achieved if the energy-intensive step of nitrogen-CO2 separation prior to injection can be avoided. In chapter 5, the enhanced mass transfer of CO2 in water for a CO2 saturated layer on top of a water saturated porous medium is studied experimentally and theoretically. Dissolution of carbon dioxide in water has a large effect on the capacity of an aquifer for carbon dioxide storage. Without the dissolution effect the storage capacity of aquifers is low. A high pressure cylinder with a length of 0.5 m and a diameter of 0.15 m is used in pressure decay experiments. The relatively large size of the vessel minimizes the pressure measurement errors that can happen due to temperature fluctuations and small leakages. The experimental results were compared to the theoretical result in terms of onset time of natural convection and rate of mass transfer of CO2 in the convection dominated process. In addition a non-isothermal multicomponent flow model in porous media is solved numerically to study the effect of the heat of dissolution of CO2 in water on the rate of mass transfer of CO2 . The effect of the capillary transition zone on the rate of mass transfer of CO2 is also studied theoretically. The simulation results including the effect of the capillary transition zone show a better agreement with experimental results compared to the simulation result without considering a capillary transition zone. The simulation results also show that the effect of heat of dissolution on the rate of mass transfer is negligible. The overall conclusion is that, for the current state of technology, use of underground coal gasification with a similar carbon foot print as the use of natural gas is not possible. It is to be expected that technological developments will make it possible in the future to use coal with a low carbon footprint.