The Kårstø gas processing plant at the south-western coast of Norway was built in the early 1980s to receive natural gas from the northern part of the North Sea. The plant was extended in 1993 to receive condensate from the Sleipner field and in 2000 to receive gas from the Åsgard field in the Norwegian Sea. Moderate extensions were also made in 2003 and 2005. The plant distills raw natural gas and condensate into methane-rich sales gas and ethane, propane, iso- and normal-butane, naphta and condensate. The sales gas is compressed and exported through subsea pipelines. The other fractions are delivered in liquid state by boat.
The Kårstø plant has a nominal capacity to handle 88 million standard cubic meter rich gas per day [1], which is one-third of the Norwegian natural-gas production. This corresponds to approximately 1.2 EJ annually, which is 1.5 times the domestic Norwegian end use of energy (excluding the oil and gas sector). The dry-gas fraction, pipeline sales gas, from the oldest part of the plant (Statpipe) is compressed in three parallel compressors each powered by a Rolls-Royce Avon gas turbine (GT). The gas is further compressed by three electrically driven booster compressors. As this equipment has been in operation since 1985, one is discussing possibilities for replacement or upgrading.
The operation of a processing plant like Kårstø consumes considerable amounts of energy. The plant “feeds” on the hydrocarbon flow, and saved energy can be sold to the customers. With increasing energy prices, the economic potential for improvements is increasing. Moreover, the oil and gas industry contributes a considerable share of the Norwegian CO2 emissions. Thus, efficiency improvements in this industry are likely to be required to comply with the obligations of the Kyoto protocol on greenhouse-gas emissions.
This study focuses on the drivers of the sales gas compressors. The aim of the study was to evaluate the existing system and the relevant alternatives on the basis of exergy conversion and CO2 emissions. Three main alternatives to the existing GTs and heat-recovery steam generators (HRSGs) are studied with respect to energy and exergy utilization. The alternatives are to replace the existing turbines with new and retain the HRSGs, to electrify the compressors and build an on-site combined heat and power (CHP) plant, or to purchase electricity from outside. The alternative with a new CHP plant may produce additional electricity replacing purchased electricity for other purposes at the processing plant.
The alternatives will result in different types and quantities of input and delivery: fuels, mechanical work, electric energy and steam. Hence, a common metric for comparison is needed. Exergy is regarded as such a metric, as it accounts for the “quality” of the energy. Hence, the delivered exergy as a fraction of input exergy can be used for comparing the alternatives.
Emissions of CO2 are considered for the alternatives. The cases can be compared in terms of emissions per unit of delivered exergy. Furthermore, the different cases can lead to different consequences with respect to international agreements (e.g. the Kyoto protocol) on emissions reduction. Use of purchased electricity may cause emissions outside the borders of the country and hence, be assigned to the CO2 “account” of another country. Further details of the investigations can be found in [2].
There seems to be a general public notion that CHPs are beneficial to power plants and that waste heat is utilized. This is reflected in the fact that several countries have implemented legislation to encourage installation of CHP. There is, however, no similar notion that CHPs are beneficial to separate production of heat, although this benefit is even larger in the terms of thermodynamics. For instance in Norway, gas-fired power plants cause heated public discussions, whereas installation boilers with the similar amounts of fuel consumption at refineries and gas-processing plants is hardly mentioned in the public debate.
Thermodynamic analysis of CHP and other thermal plants is well established in textbooks and research literature (cf. Section 3). In spite of this, evaluation based on exergy analysis is not much used in industry or by regulatory public administration even for thermal plants with different deliveries (electricity, steam). The performance of CHP systems are often evaluated according to certain indicators as reviewed by [3]. Most of these indicators are based on the 1st law and do not fully reflect the thermodynamics of heat and electricity delivery [4]. The knowledge and understanding of the exergy method (or other 2nd-law methods) and its results are often limited.
There is a challenge in educating both the engineers and the public, including politicians and civil officers, about thermodynamic evaluation of thermal plants. The idea of “energy quality”, which is a simplified notion of exergy, has gained some interest among technologists, environmentalists and politicians. A part of the challenge is to set up readily presentable examples or showcases. A real case, as analyzed here, with realistic alternatives, tends to gain more interest than an exemplary calculation from a textbook.
The existing plant was originally designed with respect to economical conditions as they were known at that time. Since then, the regular operation and all suggested modifications have been evaluated by economic analysis. In recent years, increasing energy prices have changed the limitations of such plants. In near future, possibilities for pricing of CO2 (tax, tradable quotas, and national limits) may cause even greater changes in the economics of the plant. The objective of the study was to take a step back, and compare the alternatives on a thermodynamic basis while economical considerations are left out.
The process will be described in Section 2, theory and methods for the exergy analysis in Section 3. Subsequently, the details of the predictions and results will be presented and discussed. This result in a comparison of the alternatives for work and heat production. An important issue will be to compare and discuss different indicators for the performance of the plant. Furthermore, the accounting of the electric energy that has to be purchased in the third alternative will be discussed. Some considerations on CO2 emissions within or outside country borders are also given