Thermo-Economic Modelling and Process Integration of CO2-Mitigation Options on Oil and Gas Platforms

The offshore extraction of oil and gas is an energy-intensive process associated with large CO2 and CH4 emissions to the atmosphere and chemicals to the sea. The taxation of these emissions has encouraged the development of more energy-efficient and environmental-friendly solutions, of which three are assessed in this paper. The integration of steam bottoming cycles on the gas turbines or of low-temperature power cycles on the export gas compression can result either in an additional power output, or in a greater export of natural gas. Another possibility is to implement a CO2-capture unit, which allows recovering CO2 that can be used for enhanced oil recovery. In this paper, a North Sea platform is considered as case study, and the site-scale retrofit integration of these three options is analysed, considering thermodynamic, economic and environmental performance indicators. The results illustrate the benefits of valorising the waste heat recovered from the exhaust gases, as the total CO2-emissions can be reduced by more than 15 %. Exploiting low-temperature heat seems feasible, although more challenging in retrofit situations. Integrating CO2-capture appears promising with a CO2-avoidance cost between 23 and 29 /tCO2 for the chosen economic assumptions.

CO2-mitigation options for the offshore oil and gas sector

François Maréchal, Tuong-Van Nguyen, Laurence Tock

The offshore extraction of oil and gas is an energy-intensive process leading to the production of CO2 and methane, discharged into the atmosphere, and of chemicals, rejected into the sea. The taxation of these emissions, in Norway, has encouraged the development of more energy-efficient and environmental-friendly solutions, of which three are assessed in this paper:. (i) the implementation of waste heat recovery, (ii) the installation of a CO2-capture unit and (iii) the platform electrification. A North Sea platform is taken as case study, and these three options are modelled, analysed and compared, using thermodynamic, economic and environmental indicators. The results indicate the benefits of all these options, as the total CO2-emissions can be reduced by more than 15% in all cases, while the avoidance costs vary widely and are highly sensitive to the natural gas price and CO2-tax. (C) 2015 Elsevier Ltd. All rights reserved.

2016

Thermo-chemical H2 production: Thermo-economic modeling and process integration

François Maréchal, Laurence Tock

Within the global challenge of climate change and energy security, hydrogen is considered as a promising decarbonized energy vector to be used in electricity production and transportation. In this paper, the thermo-chemical production of hydrogen by natural gas reforming and by lignocellulosic biomass gasification are analyzed, compared and optimized by developing thermo-economic models. Combining flowsheeting with process integration techniques, thermo-economic analysis and life cycle assessment (LCA), a systematic comparison of different process options with regard to energy, economic and environmental considerations is made. The choice of the technologies is optimized together with the operating conditions using multi-objective optimization. In both natural gas and biomass based H2 pathways, a CO2 removal step is included during the H2 purification which allows for CO2 capture and further sequestration. The potential for greenhouse gas mitigation is assessed and compared with conventional plants without capture based on the CO2 avoidance cost and the overall CO2 equivalent emissions computed from the life cycle chain. The system’s performance is improved by introducing process integration valorizing the waste heat by the combined production of heat and power. The H2 application purpose and the corresponding required purity are key factors defining the process performance. The trade-offs between competing thermoenvironomic (i.e. energy, economic and environmental) objectives are finally assessed using a multi-objective optimization. For natural gas based H2 production overall energy efficiencies up to 80% and production cost of 22-110 USD/MWH2 are computed compared to around 60% efficiency and 75-263 USD/MWhH2 for biomass based processes having the advantage of using renewable resources. The CO2eq emissions are reduced by more than 6.4kgCO2eq/kgH2 for NG and 20kgCO2eq/kgH2 for BM processes compared to the cases without CO2 capture. The competitiveness on the energy market depends strongly on the resource price and on the imposed CO2 taxes. Our study shows that the thermo-chemical hydrogen production has to be analyzed as a polygeneration unit producing not only hydrogen but also captured CO2 and electricity.

2011

Swiss Motor. Modification d’un moteur diesel pour le fonctionnement au gaz naturel en cogénération. Fonctionnement avec mélange stoechiométrique (l=1) et pauvre (l>>1).

Daniel Favrat, Geoffrey Basil Leyland, Roger Röthlisberger

The Swiss limit for NOx exhaust gas emissions from decentralised electrical power and heat cogeneration units using stationary I.C. engines operating on natural gas, despite a recent increase from 80 to 250 mg/mN3 (5% O2), remains the most severe in force in the world. The only technology currently able to meet this limit - the use of a stoichiometric mixture and a three-way catalyst - results in a relatively low global engine efficiency and suffers from a limited reliability of the exhaust gas treatment system. In this context, the Laboratory for Energy Systems of the EPFL has undertaken to develop new technologies (mixture, ignition,...) on the basis of a converted diesel engine for natural gas operation and to evaluate their reliability as well as their potential to reduce exhaust gas emissions, particularly NOx. The objective of this project is to perform an experimental study of the influence of the operating mode, either a stoichiometric mixture (l = 1) and a 3-way catalyst for exhaust gas treatment or a lean mixture (l >> 1) without catalyst, on the performance and emissions of a Liebherr G 926 engine, in order to establish a basis of comparison for future evaluation of new technologies. The realisation of this project required the construction of a new test bed especially developed for the study of gas engines. In the case of the stoichiometric operating mode, the results show that at an MBT spark timing of 20 °CABTDC, a mechanical power output of 106.3 ± 0.5 kW at 1502 ± 5 rpm and a global efficiency of 35.9 +1,7/-0.6% is achieved. A series of tests with different sparkplugs showed only a weak influence on the engine behaviour. The use of a 3-way catalyst for exhaust gas treatment reduces the NOX and CO emissions comfortably below the limits prescribed by the Swiss Federal Clean Air Act. In the case of the naturally aspirated engine, lean operation with an air to fuel ratio over 1.6 keeps the NOX emissions under the prescribed limit without a catalyst. However, the power output falls below 69 kW and the global efficiency below 35%. By turbocharging and intercooling the intake air and gas mixture the power output was increased from this low level to 150 kW (pme = 12 bar). As with stoichiometric operation, changing the spark plugs has only a weak influence on the engine behaviour. However, an increase of the ignition coil discharge time led to a perceptible reduction of the cycle-by-cycle variability. The results show that the best method of reducing the amount of energy dissipated by the exhaust manifold is to insulate the manifold, rather than water cool it. The insulation preserves more energy for the turbocharger and a high temperature for a possible catalytic exhaust gas after treatment. The reduction of the dead volume located between the cylinder liner, the cylinder head and the cylinder gasket led to a 40 % decrease of the THC emissions. Further, the use of a new piston, generating a higher level of turbulence during the combustion, raised the lean limit and thus reduced NOX emissions below 150 mg/mN3, 5% O2, while still keeping the global efficiency over 37%; these results are valid for a intake mixture temperatures of both 50 and 80 °C, however in the second case at a somewhat lower power output than 150 kW. Despite these improvements, and because of an extremely low CO raw emissions limit, the use of a oxidation catalyst is needed to fulfil the requirements of the Swiss Federal Clean Air Act. In order to reduce the raw CO emissions and to increase the engine global efficiency, this project will continue with a third part aiming to study the potential of an ignition system based on a prechamber.