Operating with varying fuel properties without additional Wobbe-Indexmeasurement

Transcription

1 Operating with varying fuel properties without additional Wobbe-Indexmeasurement on SGT-600 Authors: Mats Blomstedt Geir Nevestveit Per Johansson Siemens Industrial Turbomachinery AB Sweden

2 Introduction Operating a gas turbine with fuel properties (such as composition, heating value and density) varying over time is of increasing interest, for example from refineries but also if the fuels are supplied from different sources. This normally requires a separate hardware installation such as a Wobbe-index measurement. Such hardware not only increases the cost of the installation but also adds a component that may lead to increased risk of unavailability of the unit. Siemens have developed another solution, where this hardware is not necessary. A new loop has been introduced in the control system in order to take care of these variations in fuel properties, utilizing the normal installation of instruments for the gas turbine and the auxiliary equipment. With this control-loop it is now possible to run the SGT-600 (both conventional combustor type as well as the Dry Low Emission variant) with a fluctuating composition (and Wobbe index) over time without any additional installation. Nomenclature WI - Wobbe Index; normalized heating value: heating value/(square root of gas specific gravity). DLE - Dry Low Emission SGT - Siemens Gas Turbine MGT - Medium sized Gas Turbines within Siemens family (SGT-500/600/700/800) OEM - Original Equipment Manufacturer PG - Power Generation MD - Mechanical Drive LNG - Liquefied Natural Gas Fuel supply Historically the gas turbine business has mainly utilized standard natural gas fuel. This fuel has a quite narrow range in terms of variation of composition. Typically, a natural gas composition is dominated by methane (CH 4 ) and has a heating value in the range of 48 MJ/kg. Gas turbines have been optimized for this fuel, including minimizing the emissions. Today, interest in varying the fuel composition is increasing rapidly, driven by three major motivators: Environment - Minimize flaring by fuel utilization Economy - Use waste gas streams in the plant/source for increased profitability, e.g. end flash gas or heavy hydrocarbons. Reliability: Improve availability by avoiding complex and expensive fuel treatment equipment.

3 Picture 1: Flaring gas in the sunset - adding CO 2 to the environment. It can be noted that flaring & venting today corresponds to 150 billion m 3 gas annually. This is the same amount as 30% of the EU annual gas consumption. Flaring adds 400 million tons of CO 2 annually. This is more than targeted reductions for submitted projects under the Kyoto Protocol. Fuel properties The WI is used to compare the combustion energy output of different composition fuel gases. If two fuels have identical Wobbe Indices, given same pressure and valve settings, the energy output will also be identical. Variations of up to five percent are typical for a fuel supply and normally constitute the acceptance limit of combustion without some type of adjustment/compensation. Example of alternative gases for fuel supply: Inert (CO/N2) gases from processes, Coke Oven Gas (COG) and Syngas. Results in lower WI Liquefied Petroleum Gas (LPG) in natural gas. Results in a higher WI Hydrogen in refinery gas and COG Ethane from Liquefied Natural Gas (LNG) processing Heavy hydrocarbons (C 5 +) from LNG processing

4 Wobbe Index [MJ/Nm 3 ] Lower Heating Value [MJ/kg] Picture 2: WI versus heating value from customer s fuel specifications From the fuel specifications received from customers for evaluation it can be noted that the spread in fuel properties is high and it can also be noted that the spread is increasing over time. The picture above shows the fuel specifications received regarding the MGT-units during 2009 and 2010, where the high concentration of specifications in the middle mostly represents natural gases. SGT-600 core engine design The SGT-600 is a twin-shaft machine where the same design is used both for PG and MD. Compressor inlet 2 Variable guide vanes 2 Bleed valves Fuel Rod DLE combustor 2 stage compressor turbine 2 stage power turbine 10 stage compressor, EB welded rotor Picture 3: Cross section of the SGT-600 This machine has been on the market for more than 20 years with some 250 units sold and 7 million operating hours accumulated. Originally it was equipped with a conventional combustor but already in the early 90 s the DLE combustion chamber was introduced. The DLE share of the total operation experience is approximately

5 80%. The option between DLE and conventional type is cost-neutral and the DLE is therefore considered as the standard for this machine thanks to the lower emissions emitted. Conventional combustion can be offered on request for those customers looking for some wider fuel range (see picture below). Fully released; Conventional Fully released; DLE Sales approved case by case Low Calorific Value (LCV) Medium Calorific Value (MCV) Normal Pipeline NG High Calorific Value (HCV) Wobbe Index (MJ/Nm³) Picture 4 Fuel range definition for SGT SGT-600 fuel-control design The medium sized (15 50 MW) Siemens gas turbines are produced in Finspong, Sweden. This includes SGT-500/600/700/800. All these gas turbines have a common philosophy regarding the design of the fuel system where there are no individually controlled fuel injectors/burners. The burners are all supplied with fuel from a common manifold and are calibrated from the factory in order to have the same pressure drop over the burner and thus the same amount of fuel flow through all burners. The fuel supply to a manifold is controlled by one valve. This design philosophy applies both to the conventional combustor type as well as the DLE. M ain c o ntro l va lve Pilot control valve

6 Picture 5: Fuel supply system for the SGT-600 The fuel is supplied to two manifolds: pilot fuel which is used for start-up and will be reduced when load increases, and the main fuel. The ratio between the pilot and the main fuel is a function of the load and is controlled automatically. Combustion challenges Widening the range of acceptable fuels is a high priority challenge for all gas turbine suppliers: both conventional (non-dle) and DLE combustion chamber types. The parameters normally optimized for a specific fuel are flow number (area) of burner and combustion chamber and velocities of the air and the fuel entering the system. By optimizing these parameters the acceptable fuel properties may be changed considerably. The issues that may occur when stretching closer to the limits of a design include (but are not limited to): Flash-back the fuel ignites before this is intended and the flame gets closer to the burner tip (or even into the internals of the burner/injector). May lead to over-heating of hardware. Pulsations not optimum ratio between air and fuel injection causing vibration/humming of the combustion. May lead to cracking (high cycle fatigue) of components. Flame-out too lean mixture of fuel/air leads to a flame-out. Then there is a risk of re-ignition (explosion) further downstream of the installation where hot gas (including path) will be a source for ignition. Emissions non-optimal combustion. Combustion efficiency may be low and emissions of e.g. NO x and CO x will increase dramatically. Usually the combustion can be optimized for a specific, defined, fuel. The next level of challenge occurs if the fuel composition varies considerably (>5%) over time i.e. composition, heating value and WI. If a design allows such a variation, one way to handle these variations is to monitor the properties (WI-measurement is the most common way) and compensate for these variations - in the control system and/or valves for fuel and air operation - continuously. This causes an increased hardware cost as well as a reliability issue: the additional hardware (valves and measurement devices) may disturb the operation and there may be a delay in the control due to evaluation/compensation of the measured values. Operational load control For the MGT s the main parameter to control the load of the gas turbine is the fuel supply and the upper limit of the fuel supply is the maximum turbine inlet temperature. So increasing or reducing the load is as simple as opening/closing the two valves according to the specified ratio between them. No compensation applies for fuel composition, ambient conditions or other parameters. Conclusion is that, considering the control for the load, it is not necessary know the WI-value at all.

7 Necessity of defining the WI-value However, due to a few other reasons, the WI-value is of importance. Some examples for the MGT gas turbine control: Since there are no additional moving parts or control corrections, the load can be changed very rapidly: load rejection from 100% load is possible without flame-out, but in order to keep the machine running at idle after a load rejection it is necessary to have a proper minimum setting value of the valves when they are closing almost instantly. With a too-low valve setting (area) there will be a flame-out and with a too-high setting there is a risk of overspeed in the power turbine (too high fuel flow without load will increase the speed). During start-up of a unit a specific amount of heat shall be injected in order to get a proper start. If the WI-index is unknown, the start-up sequence may fail with an aborted start as a consequence. In those cases when the start-up is going to be made with the same fuel as was utilized in the most recent shutdown, a defined WI-value will conclude a proper valve setting in order to supply the right amount of fuel heat at start-up There are upper and lower limits to what fuel range is verified and accepted in the design. It is therefore necessary to keep track of the fuel supplied in order not to operate outside the defined limits. Determination of WI-value without a WI-measurement According to the presented control parameters of the SGT-600 there is a way to calculate the WI-value without introduction of any additional measurements. It is a well known fact that any gas turbine will have a varying output depending on the boundary conditions, e.g. lowering ambient temperature will increase the output. The basic for this calculation is the fact that the efficiency of the MGT gas turbines (and most of the other OEM s as well) is a function of the output independent of the boundary conditions, i.e. running at 20MW will give the same efficiency irrespective of whether it is a full load at hot ambient conditions or a part load at cold conditions. The WI-determination stepwise: 1. By monitoring the output 1 from the unit, the efficiency is known 2. The output divided by the efficiency will give the heat input to the unit 3. The feed-back signal of the position of the two fuel valves will give the flow number (area) of the valves according to defined calibration of the valves. 4. The defined heat input at the specified valve positioning (area) and the fuel supply pressure will result in a calculated WI-value. See chapter Fuel properties above. Verification & implementation Verification of this method has been carried out in the test stand at the Siemens facility in Finspong/Sweden. The demonstration was made on an engine equipped with a DLE combustion system. The way to demonstrate this was to inject/blend inert 1 Directly: at the generator terminals of a PG unit or a torque-meter for a MD unit. Alternatively: indirectly for an MD-unit utilizing characteristics for the driven equipment or the gas turbine.

8 gas (nitrogen) into the natural gas supply. This caused the WI to vary over time (see figure 6 below). The load was kept constantly at 20MW. The nitrogen content increased to 55% (by weight) in 40 minutes. In addition to the normal control parameters, the NOx-levels were measured as well as the combustor dynamics (pulsations). No abnormal values could be seen and the combustion was efficient without any disturbances. N2 content [wt%] and NOx [ppm] Combustion Dynamics [% of larm level] N2 NOx Comb Dyn Load Time [minutes] Load [MW] Picture 6: Measurements from test with nitrogen blending After 40 minutes the nitrogen tank was empty and the unit was shifting to run with 100% natural gas again. During these two minutes of shifting, the WI-index value changed 80% without any disturbances of the operation. A minor increase of NOx during the switch can be noted but the combustion dynamics are kept on the same level, indicating stable combustion. This method of operating units with varying WI-values has been implemented on a number of units (both SGT-500 and SGT-600) in Europe and Africa both with conventional combustion chamber and DLE. The installations of those units are in refineries and LNG-plants, where the WI-values are varying continuously due to the processes in which they are installed. Summary The demand for operation with varying fuel properties is already here and interest is expected to increase. For environmental, economical and reliability reasons it will be necessary to stretch the approved fuel properties and to have the flexibility to utilize different sources. It has now been demonstrated that the control principle developed for this machine is working as intended. The MGT gas turbines can operate successfully on fuels with varying properties that previously required further refining, were flared and/or required a continuous WI-measurement.