Lars O. Nord

Weight and power optimization of steam bottoming cycle for offshore oil and gas installations

Offshore oil and gas installations are mostly powered by simple cycle gas turbines. To increase the efficiency, a steam bottoming cycle could be added to the gas turbine. One of the keys to the implementation of combined cycles on offshore oil and gas installations is for the steam cycle to have a low weight-to-power ratio. In this work, a detailed combined cycle model and numerical optimization tools were used to develop designs with minimum weight-to-power ratio. Within the work, single-objective optimization was first used to determine the solution with minimum weight-to-power ratio, then multi-objective optimization was applied to identify the Pareto frontier of solutions with maximum power and minimum weight. The optimized solution had process variables leading to a lower weight of the heat recovery steam generator while allowing for a larger steam turbine and condenser to achieve a higher steam cycle power output than the reference cycle. For the multi-objective optimization, the designs on the Pareto front with a weight-to-power ratio lower than in the reference cycle showed a high heat recovery steam generator gas-side pressure drop and a low condenser pressure.

HRSG Design for Integrated Reforming Combined Cycle With CO2 Capture

This article illustrates aspects of heat recovery steam generator (HRSG) design when employing process integration in an integrated reforming combined cycle (IRCC) with precombustion CO 2 capture. Specifically, the contribution of this paper is to show how heat integration in a precombustion CO 2 capture plant impacts the selection of HRSG design. The purpose of such a plant is to generate power with very low CO 2 emissions, typically below 100 g CO 2 /net kWh electricity. This should be compared with a state-of-the-art natural gas combined cycle (NGCC) plant with CO 2 emissions around 380 g C0 2 /net kWh electricity. The design of the HRSG for the IRCC process was far from standard because of the significant amount of steam production from the heat generated by the autothermal reforming process. This externally generated steam was transferred to the HRSG superheaters and used in a steam turbine. For an NGCC plant, a triple-pressure reheat steam cycle would yield the highest net plant efficiency However, when generating a significant amount of high-pressure steam external to the HRSG, the picture changed. The complexity of selecting an HRSG design increased when also considering that steam can be superheated and low-pressure and intermediate-pressure steam can be generated in the process heat exchangers. For the concepts studied, it was also of importance to maintain a high net plant efficiency when operating on natural gas. Therefore, the selection of HRSG design had to be a compromise between NGCC and IRCC operating modes.

A Study of Parameters Affecting the Combustion Stability and Emissions Behavior of Alstom Heavy-Duty Gas Turbines

A number of factors can influence the combustion instability region and emission behavior of a heavy-duty gas turbine. Changes in the composition of the natural gas supplied have an impact that was studied in a prior investigation, which focused on parameters such as fuel temperature and composition. To further investigate the fuel sensitivity additional plants were included in this study. In addition to the fuel properties the distribution of the fuel inside the combustor was examined. To expand the fuel properties study, additional parameters were examined. Ambient conditions were paid special interest, specifically ambient temperature and humidity. Included in this study was also the effect on combustion of changes in compressor discharge pressure. With the growing interest in inlet chilling a pulsation/emission study was included to specifically look for NOx and combustion instability effects due to inlet chilling. Also, influences from special occurrences such as on-line compressor washing were examined. The turbines in this study utilize a silo-type combustor with either the DLN [Dry Low NOx ] EV [EnVironmental] burners or with single diffusion burners using water or steam as NOx reduction medium. The rated power output of the gas turbines was in the range of 50–120 MW. The data acquired included frequency-analyzed combustion instabilities, various process data, as well as ambient conditions and fuel composition. The collected data shows the magnitude of the changes in the emissions and combustion noise with changes in the parameters studied. The conclusion is that some key parameters are very important for both the pulsations and the emissions, whereas others can be neglected. Some parameters affect the combustion instabilities only, without noticeable effect on emissions, and vice versa.Copyright

Influence of Variations in the Natural Gas Properties on the Combustion Process in Terms of Emissions and Pulsations for a Heavy-Duty Gas Turbine

The natural gas supply can vary significantly on a day-to-day or even hour-to-hour basis for a power plant equipped with gas turbines. The influence of such variations could potentially have an adverse effect on the combustion process in terms of emissions and acoustic pulsations, even if the fuel properties are within the original equipment manufacturer (OEM) guidelines. Since the operation of a gas turbine typically requires steady emissions within the air permit as well as low pulsations to limit mechanical damage on the unit, fuel variations could significantly affect how the unit can be operated. To investigate this matter, data from an ALSTOM GT11N1 gas turbine was collected and studied during a 6-month period. The data acquired included on-line gas chromatograph readings, frequency-analyzed combustion instabilities, various process data, as well as ambient conditions. The collected data shows the magnitude of the changes in the emissions and combustion noise with changes in the fuel. The conclusion is that normal day-to-day variations in the natural gas properties do not have a significant effect on the emissions and combustion instabilities; however, larger sudden changes, as exemplified in the paper, could lead to considerable changes in the combustion behavior of the unit.Copyright

A NOx Reduction Project for a GT11 Type Gas Turbine

A study was initiated to investigate the possibility of significantly reducing the NOx emissions at a power plant utilizing, among other manufacturers, ALSTOM GT11 type gas turbines. This study is limited to one of the GT11 type gas turbines on the site. After the initial study phase, the project moved on to a mechanical implementation stage, followed by thorough testing and tuning. The NOx emissions were to be reduced at all ambient conditions, but particularly at cold conditions (below 0°C) where a NOx reduction of more than 70% was the goal. The geographical location of the power plant means cold ambient conditions for a large part of the year. The mechanical modifications included the addition of Helmholtz damper capacity with an approximately 30% increase in volume for passive thermo-acoustic instability control, significant piping changes to the fuel distribution system in order to change the burner configuration, and installation of manual valves for throttling of the fuel gas to individual burners. Subsequent to the mechanical modifications, significant time was spent on testing and tuning of the unit to achieve the wanted NOx emissions throughout a major part of the load range. The tuning was, in addition to the main focus of the NOx reduction, also focused on exhaust temperature spread, combustion stability, CO emissions, as well as other parameters. The measurement data was acquired through a combination of existing unit instrumentation and specific instrumentation added to aid in the tuning effort. The existing instrumentation readings were polled from the control system. The majority of the added instrumentation was acquired via the FieldPoint system from National Instruments. The ALSTOM AMODIS plant-monitoring system was used for acquisition and analysis of all the data from the various sources. The project was, in the end, a success with low NOx emissions at part load and full load. As a final stage of the project, the CO emissions were also optimized resulting in a nice compromise between the important parameters monitored, namely NOx emissions, CO emissions, combustion stability, and exhaust temperature distribution.Copyright

Effective concepts for supplying energy to a large offshore oil and gas area under different future scenarios

Different possibilities were assessed to supply energy to a large offshore oil and gas area in the North Sea. The concepts studied involved: (i) onsite power generation by means of simple gas turbine cycles, (ii) full electrification of the plants with power taken from the onshore grid, and (iii) a hybrid solution where power can be either generated onsite or taken from the onshore grid. The analysis included 37 y of the facilities’ lifetime and was based on process simulations of the various concepts. The effect of the offshore area electrification was simulated through a model of the power system. The integration of process and power system modelling contributes to the originality and completeness of the analysis. The environmental impact of the concepts was evaluated in terms of cumulative CO2 emissions. The relative economic cost was also assessed to provide a complete picture. The results showed that the advantage of a specific concept over the others was significantly influenced by the future energy policies and the magnitude of the initial investment cost.