Turbines dual-fuel

Methanol, a clean burning fuel relative to conventional industrial fuels other than natural gas, can be used advantageously in stationary turbines and boilers because of its low flame luminosity and combustion temperature. Low NO emissions and virtually no sulfur or particulate emissions have been observed (83). Methanol is also considered for dual fuel (methanol plus oil or natural gas) combustion power boilers (84) as well as to fuel gas turbines in combined methanol / electric power production plants using coal gasification (85) (see Power generation). 

Injection of Steam in the Combustor of the Gas Turbines Utilizing Present Dual Fuel Nozzles. Steam injection in the combustor has been commonly used for NO control as seen in Figure 2-43. The amount of steam, which can be added, is limited due to combustion concerns. This is limited to about 2-3% of the airflow. This would provide an additional 3-5% of the rated power. The dual fuel nozzles on many of the industrial turbines could easily be retrofitted to achieve the goal of steam injection. The steam would be produced using an HRSG. Multiple turbines could also be tied into one HRSG. 

The gas turbines major limitations on the life are the eombustor eans, first stage turbine nozzles and first stage turbine blades as seen in Figure 21-6. The effeet of dry Low NO eombustors have been very negative on the availability of Combined Cyele Power Plants, espeeially those with dual fuel eapability. Flash baek problems are a very major problem as they tend to ereate burning in the pre-mix seetion of the eombustor, and eause failure of the pre-mix tubes. These pre-mix tubes are also very suseeptible to resonanee vibrations. 

Prime movers are typically fueled by natural gas or diesel. Dual fuel turbine units exist that can run on natural gas and can automatically switch to diesel. So-called dual fuel reciprocating engines run on a mixture of diesel and natural gas. When natural gas is not available, they can automatically switch to 100% diesel. Most prime movers associated with producing facilities are typically natural gas fueled due to the ready availability of fuel. Diesel fueled machines are typically used to provide stand-by power or power for intermittent or emergency users such as cranes, stand-by generators, firewater pumps, etc. 

Gas turbines operate on natural gas, synthetic gas, landfill gas, and fuel oils. Plants are often designed to operate on gaseous fuel with a stored liquid fuel for backup, so as to obtain the less-expensive interruptible rate for natural gas. Dual fuel combustion capability is a purchase option on many gas turbines. The turbine is ideally suited as an integral component in the IGCC train. Further, modern gas turbines have proven to be reliable power generators, given proper maintenance. Time to overhaul is typically 25,000 to 50,000... 

The bio-oil used for the test programme was produced by BTG in Enschede (NL) with the Rotating Cone Technology from poplar as the bio-mass material. As alternative raw materials for pyrolysis wheat straw, Spanish thistle and other wood wastes have been investigated. The main objective of this paper is to demonstrate the power generation from bio-oil in a gas turbine, the optimisation of the combustion behaviour with respect to emissions and operation characteristics and the comparison of emission and operation data from bio-oil and conventional diesel fuel. For technical reasons the gas turbine had to be adapted to the operation in dual fuel mode with the possibility to switch between diesel and bio-oil. 

The gas turbine was started with diesel oil. Then followed a change over of the fuel through the main nozzle from diesel oil to bio-oil. Due to the lower heating value of the bio-oil it was only possible to operate the gas turbine at part load in the dual-fuel-mode. With the mass flows in Fig. 3 the following power levels are calculated and shown in Table 2. 

Table 2 Comparison of turbine power in diesel fuel and dual fuel operation...

The mass flow of bio-oil in relation to diesel fuel increases in the main nozzle with 6.5 GPH (gallons per hour) by up to 1.33. The power in the dual fuel mode is reduced to 73 % of the full power in the diesel fuel mode. The reduction is due to the specific limitations of the gas turbine used for the experiments and not inherent in bio-fuel operation. However, for later commercial use further alterations to the fuel system and the nozzle size would be required. 

When compared to diesel fuel the emissions of CO and HC are higher for bio-oil. The results indicate slightly lower NOx emissions for bio-oil. All measurements were taken at part load operation due to the limitation of the gas turbine. The initial tests in dual-fuel-mode were performed with a relatively cold combustion at part load which may be the reason for the incomplete combustion of the hydro carbons. Also the combustion air fuel ratio was not adjusted for bio-oil operation at part load. The combustion of bio-oil is clearly recognised by the exhaust gas odour and a light exhaust gas cloudiness. 

The most common fuels used are gas (natural gas, methane, propane, synthesis gas) and light fuel oils. Contaminants such as ash, alkalis (sodium and potassium) and sulphur result in deposits, which degrade performance and cause corrosion in the hot section of the turbine. Total alkalis and total sulphur in the fuel should both be typically less than 10 ppm. Gas turbines can be equipped with dual firing to allow the machine to be switched between fuels. 

Sulfur compounds pose a dual problem. Not only do their combustion products contribute to atmospheric pollution, but these products are also so corrosive that they cause severe problems in the operation of gas turbines and industrial power plants. Sulfur pollution and corrosion were recognized as problems long before the nitrogen oxides were known to affect the atmosphere. For a time, the general availability of low-sulfur fuels somewhat diminished the general concern... 

System component longevity Smooth (bumpless) synchronization of generators reduces wear factors on generators, couplings, and turbines. Smooth transfer of fuel types in a dual (gas/Uquid) or tri (gas/liquid/gas and liquid mixed) fuel system greatly compensates for the temperature bursts that take a severe toll on hot-section component lives. 

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