18. Global Gas Supply Base World energy reserves of hydrocarbons are approximately even divided between natural gas, oil, and coal. But if methane hydrates are added to the resource base, the global reserves in gas could represent more than 80% of the hydrocarbon resources. EU Proposed Range US Historical
38. Flame Types (Gas) Photos by Dr. F. Dinkelacker, Erlangen, 2005 Butane/Air Exit diameter 18 mm Fuel flow rate is hold constant Diffusion Flame Fully Premixed Flame Partial Premix Flame
45. Interchangeability: Geography Depending upon location on the pipeline grid, some users may experience more rapid shifts from domestic pipeline natural gas to imported LNG
46. Interchangeability: Geography Florida: 95%+ natural gas consumption is power generation, mostly gas turbines. Depending upon location on the pipeline grid, some users may experience more rapid shifts from domestic pipeline natural gas to imported LNG
49. NO x -Thermal NO x At lower combustion temperatures, less NO x is generated. But there is a limit to the minimum temperature. Combustion instabilities (dynamics) become important in lean combustion systems.
50. NO x -Thermal NO x Pressure Effect on NO x and CO Emissions in Industrial Gas Turbines Anuj Bhargava . , Donald W. Kendrick, Kent H. Casleton and Daniel J. Maloney U.S. Department of Energy Federal Energy Technology Center Meredith B. Colket, William A. Sowa United Technologies Research Center East Hartford, CT Source: ASME 2000-GT-97 (with permission)
51. NO x -Combustor Tests Wobbe Index variation, as well as combustor design can affect emissions. Chart shows the response of two different combustor systems to changes in gas composition.
52. Full Scale Engine Evaluation Heating Value Reduced Dynamics Increased Minor changes to the gas composition was coincident with a step change in the combustor dynamic pressure. INFLUENCE OF VARIATIONS IN THE NATURAL GAS PROPERTIES ON THE COMBUSTION PROCESS IN TERMS OF EMISSIONS AND PULSATIONS FOR A HEAVY-DUTY GAS TURBINE Lars Nord and Helmer Anderson IJPGC2003-40188 (with permission)
53. Fuel Effects-NO x Minor increase in NO x emissions with increase of Wobbe Index from 1335 to 1400, a range that would account for addition of LNG from offshore supplies. Full scale engine testing confirms rig testing that, without burner modifications or engine control enhancements, increasing Wobbe Index can result in increased NO x .
54. Fuel Effects-CO CO (and unburned hydrocarbon) emissions are affected as well, although not in the same manner as NO x .
55. Diffusion Combustor Model Calculated for diffusion combustion system. Water Injection for NO x control, and fixed firing temperature. Differences in NO x due to fuel quality changes (Wobbe Index) are indicated. . Based on: A Model for the Prediction of Thermal, Prompt and Fuel NO x from Combustion Turbines J. L. Toof Journal of Engineering for Gas Turbines and Power , Vol. 108, No. 4, 1986, pp. 340-347
56. Supply Comparison US Domestic gas index has varied by only +/-2%, or 1315 to 1369. Requirements that are too broad could be difficult to manage without special equipment modifications to adapt to changes. Requirements too narrow could leave a significant amount of gas unacceptable to US markets . Industry fuel specifications vary from +/-2% to +/-10%, depending upon design features and equipment capabilities.
57. LNG Source and Impact US Domestic gas index has varied by only by +/-2%, or 1315 to 1369. Requirements that are too broad could be difficult to manage without special equipment modifications to adapt to changes. Requirements too narrow could leave a significant amount of gas unacceptable to US markets . Industry fuel specifications vary from +/-2% to +/-10%. Tolerance requirements for fuel quality are related to combustor design, emissions, and contract requirements. Proposed EU Range
58. NO x Variation and Design Variability in NO x emissions of equipment in service can differ significantly among different gas turbine designs. Even within a specific engine model, significant variation is evident. Emissions for three different gas turbine models in power generation service, reported through EDR network.
61. LNG and Plume NO 2 production strongly dependent upon presence of specific hydrocarbons for reaction pathway. An Experimental and Kinetic Evaluation of the Promotion Effect of Hydrocarbons on the NO-NO 2 Conversion in a Flow Reactor Proceedings of the Combustion Institute, Volume 27. Hori, M., Matsunaga, N., Marinov, N., Pitz, W. and Westbrook, C., Proc. Combust.Inst. 27 (1998) 389-396
Background on Meeting The purpose of the meeting is to summarize some of the critical issues that have been brought to light in the last 13 months with the NGC+ task force. Specifically, today’s discussion will focus on the issues of gas fuel interchangeability with regard to gas fired turbines, and the role that LNG may play in that. The process began over a year ago, with the first meeting held by the FERC to address the subject of gas interchangeability. Out of that meeting, two subcommittees were established, one do address the hydrocarbon dewpoint, the other gas interchangeability. Ideally, both subjects should come under the general heading of interchangeability, for technical purposes, they have been dealt with in separate technical committees.
Some key industry statistics on the gas turbine industry.
A lot of different terms are used to describe what is interchangeable. Wobbe Index and Crocondentherm are two that will be reviewed in this discussion. Most of the discussion will bring up a several key parameters for interchangeability. The most common, and perhaps most widely used, is t Wobbe Index. This is the heating value of the fuel gas (volumetric) divided by the square root of the specific gravity of the gas. Gases with similar Wobbe Indices would be expected to deliver comparable amounts of energy through an orifice at a fixed pressure ratio. This is not to say that the flame temperatures are equivalent, or the emissions as well. But it helps to define the range of gas properties that might be interchangeable with a specific design burner.
Gas turbines evolved out of the Second World War. Their primary advantage was a tremendous degree of fuel flexibility. In the late 1940’s there were a number of industrial demonstrations of gas turbine technology in various industries (steel making form example). But through the period from the early 1900’s to the 1960’s the vast majority of power generation facilities were based on steam turbine technology—a predecessor to gas turbine power systems that would come later. But both steam and gas turbine technology share similar features. They are both fundamentally based on a rotating compressor/turbine that converts mechanical power into electricity.
There was a rapid expansion of the industry in the early 1980’s, which was followed by an even more dramatic expansion in the late 1990’s. In addition to the total number of gas turbines sold and installed, there was a general progression in both the size of the largest gas turbines, as well as their overall heat-rate improvement (efficiency). In the mid 1980’s, many large power plants may have been limited to less than 2000 F firing temperatures, by the late 1990’s this had increased to well over 2300 F.
Need to update these stats
Need to update these stats
This slide illustrates that even without the use of post combustion emission controls (SCR), gas turbines can routinely achieve NOx emissions that are below that of any competing technology. Adding SCR to the mix, and emissions (of NOx) are reduced to near background levels.
This is perhaps the most important aspect of the fuel specification. It establishes the boundaries of safe operation for the gas turbine. But the requirements to comply with a specific air permit or emission level is more complicated, and requires attention to more details than just found in the fuel specification.
Users at the terminal points of the pipeline system may have greater exposure to changes in the gas supply if the imports are brought into these endpoints. LNG imported into the Gulf Region, where significant volumes of domestic gas flow, has greater opportunity to blend with domestic supplies, averaging out the differences. However, as the volumes of imported gas increase, it will be harder to disguise the presence of the offshore gas in the supply chain.
Often the interchangeability limits are spelled out in the fuel specification or the contract. These are some of the most common fuel requirements noted in a gas turbine fuel specification. OEM requirements will also differ among the manufacturers.
This is a fairly simple adiabatic flame temperature curve (red line), for natural gas. The green line shows approximately the range of NOx production that might be measured at the different combustion temperatures, although the actual concentrations would be highly design specific. On the left half of the curve, the temperature, and NOx, are lowered as excess air is blended with the fuel. In contrast, on the right half, the temperature drops because there is too much fuel, and not enough air. But the flammability limit of most hydrocarbons is in the range of 0.50 to 0.54. The arrow on top attempts to illustrate that this is where combustion instability can initiate. The actual flammability limits at the combustor conditions may differ from those identified in the Bureau of Mines work, but there is an overall limit that exists somewhere in this area for all combustor designs.
The additional laboratory data shown here helps to define the shape of the NOx production curve, and its relative difference from the actual flame temperature.
Test results from a small industrial combustor. These are rig test results, and this curve is a composite that portrays the differences in response to two different types of combustor designs. There is a region of minimal change, but for rich or lean gases, it shows that there is an impact of fuel quality on NOx, and possibly other emissions as well.
This is data collected on a large frame gas turbine, with unheated gas. There was no attempt to mitigate the increased NOx using modifications to the fuel flow rates or splits. But it shows that, with no attempts to mitigate, there is an observable response to the unit to a change in the quality of the fuel mixture.
This is a computer modeled set of data only. There is no consideration of heat transfer, component life, or changes to combustion driven oscillations. The base case is the mid point, where the fuel is assumed to be 100% methane. In all cases, the firing temperature is fixed at 2350 F, and conditions are set at ISO (with a 750 F discharge temperature). This is a hypothetical diffusion flame combustor, where water injection is used to control NOx emissions. The top line shows emission rate of NOx (increase in mass rate), relative to the mid-point, for a fuel with a 4% increase in Wobbe (reached by blending propane and butane in the fuel). The bottom line shows emissions rate of NOx (decrease) with a much leaner fuel (-4% Wobbe). No predictions of CO, unburned hydrocarbons, opacity, or particulates are provided, since there are no reasonably well established models to project these emissions.
