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Review of the impact of hydrogen addition to natural gas on gas turbine combustion

Bus, M.S. (2013) Review of the impact of hydrogen addition to natural gas on gas turbine combustion.

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Abstract:I In the future there will be a greater need to store energy as there will be more renewable energy which has a fluctuating output. One way of storing this energy is by producing hydrogen out of it and storing it in the gas grid, providing all users with a hydrogen enriched gas. This report will look at the effects of this hydrogen enrichment up to 20%vol on the combustion in gas turbines. II The current UK regulations do not allow hydrogen in the national gas grid. If hydrogen is to be injected into the gas grid for storage, new regulations will have to be made. The gas turbine manufacturers use less stringent limits for hydrogen; however the maximum amount differs per manufacturer. At the moment typically up to about 5%vol hydrogen is allowed in a gaseous fuel for a modern lean premixed combustion system. Some gas turbines allow higher levels, but this is rare for large utility power generation systems. III In the past various models have been developed to predict the interchangeability of different gases; three of these models have been used in the current work to investigate the impact of hydrogen addition on natural gas. The models used are the AGA Bulletin 36, Weaver and Dutton models. They all use indices based on flame behaviour to create a region of interchangeability. These methods were developed for use on domestic appliances and the limits are set based on experimental data. IV The literature review is split into four main categories: flame speeds, emissions, combustion dynamics and flashback. There are a number of relations that describe the laminar flame speed; the Bougrine relation appears to be the best one, however its limitations must be kept in mind when using it. The turbulent flame speed can be reasonably described with one of the Lipatnikov and Chomiak or Brower correlations. To calculate the turbulent flame speed some turbulent flame properties have to be known; as these are difficult to determine the turbulent flame speed will be hard to capture. The emissions of CO and NOx are the most important. Due to their opposite temperature dependence they create an operating window in which the temperature is limited by the level of emissions. The combustion dynamics can be investigated by looking at the turbulent source term, with the laminar flame speed as one of its parameters, and the flame transfer function, which depends on the expansion ratio and burning time. For flashback four different mechanisms are identified; the two most important are combustion induced vortex breakdown (CIVB) and wall boundary layer flashback (WBLF). V After the literature study the most promising methods found were reviewed for their suitability to get a better understanding of the effects of hydrogen addition. CHEMKIN PREMIX was used to simulate a freely propagating flame. This method gives information about the laminar flame speed, flow properties and species mole fractions. The sensitivity of the three turbulent flame speed correlations was investigated, focussing on the uncertainty in the velocity fluctuations, turbulent length scale and laminar flame speed. The Characteristic combustion time calculations give an indication about the time needed for the temperature to rise from its minimum to maximum or from 10% to 90% of the temperature change. The expansion ratio is calculated based on a basic approach to combustion. The Konle and Gradient models were reviewed for assessing the likelihood of CIVB and WBLF flashback respectively. VI After describing the methods used, their results have been shown. For CHEMKIN an indicative validation of the GRI 3.0 reaction scheme was shown next to the results of the species profiles, which included the emissions, laminar flame speeds and the burning time. The expansion ratio was calculated and the results presented in two ways. Finally the CHEMKIN laminar flame speeds were used to predict the likelihood of flashback with the Konle model. VII After performing the calculations the results were thoroughly analysed. The effect of hydrogen on the laminar flame speed was up to 10% at 20%vol hydrogen, which was less than expected. The turbulent flame speed predictions were not unambiguous, because the results strongly depended on the chosen correlation. Based on the Brower correlation there was an increase of about 22% at 20%vol hydrogen. The emissions did not change very much when hydrogen was added, the NOx remained the same and the CO may decreased slightly. The combustion dynamics would be unlikely to show major changes as the flame transfer function and the impact on the laminar flame speed depending turbulent source term do not change much. Based on the Konle model, only a minor increase in flashback risk was predicted when hydrogen was added, due to its dependence on the laminar flame speed. VIII The methods used indicate that low levels of hydrogen are unlikely to cause problems, but it is not possible to define a maximum limit based on this report. The analysis of the results indicates that only the increase turbulent flame speed may cause some problems as it increases by about 22% at 20%vol hydrogen addition; this prediction strongly depends on the correlation and estimations used. Since not all the calculations could be compared with results from literature, some level of uncertainty is still present in the results. It is recommended that new developments with regards to hydrogen addition to natural gas are followed. Further it would be recommended to perform experiments at conditions that are more relevant to gas turbine conditions, as this will give a good indication of the impact of hydrogen. The conditions used for the calculations are very similar to base load gas turbine conditions, it is likely that during start up the conditions differ, so the impact of hydrogen on the start up should also be reviewed.
Item Type:Internship Report (Master)
E.ON New Built and Technology, United Kingdom
Faculty:ET: Engineering Technology
Subject:52 mechanical engineering
Programme:Mechanical Engineering MSc (60439)
Keywords:natural gas, hydrogen gas, turbine combustion
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