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Text | Nan cat
Editor|Nancat
Gas turbine installations are an area where scientists are constantly innovating, and although originally developed for aero engines, gas turbines are now used in several applications, one of which is gas turbine cogeneration units, which can generate both electrical and thermal energy.
That's why aviation gas turbines are used in ground operation because of their reliability and superior performance. To meet the needs of ground applications, aeroderivative gas turbines have been modified to improve their efficiency and performance.
As a result, aeroderivative gas turbines offer the highest design efficiency in situations where fuel is limited during flight due to flight restrictions. The application of this technology brings more flexible, efficient and sustainable solutions for energy production.
Next, we will explore the advantages of gas turbine cogeneration groups in conventional and alternative gaseous fuel applications and the associated technical adjustments and readiness measures.
Gas Turbine Syndicate Group's solutions for the flexible use of gaseous fuels
The flexibility of gas turbine cogeneration units means that many stringent requirements need to be met, operating on conventional and alternative fuels and being able to easily transition from full load to partially full load and back. Maintain efficiency at full load and part load to maintain low emission levels even when operating at part load.
Internationally, many companies with outstanding performance in the field of aviation gas turbines are involved in aviation reform projects to facilitate the market demand for energy production equipment.
The most famous of these are: Rolls-Royce, Pratt & Whitney, General Electric, Motor Sich, Turbomeca, MTU and others. Rolls-Royce developed the RB 211-H211 gas turbine from the aviation RB 211, which increased its efficiency to 41.5% through a novel structure and technical modification.
The 38 MW version will be available in 2013 and is likely to be upgraded to 50 MW in the coming years. Many gas turbine manufacturers aim to reach full load within 10 minutes from the start. A Japanese project by Mitsubishi Heavy Industries Co., Ltd. (MHI) aims to build a gas turbine with an inlet temperature of 1700°C and an efficiency of 62%.
Pratt & Whitney developed the ST aeroderivative gas turbine series ST 18, ST 40, starting with the PW 100 turboprop. Research carried out at the COMOTI National Institute for Research and Development of Gas Turbines in Bucharest, by valuing obsolete or damaged aviation gas turbines that have exhausted flight resources, has allowed the acquisition of aeroderivative gas turbines in the range of 2,000 kW.
Therefore, the AI 20 GM turboshaft engine is an improved version based on the AI 20 turboprop engine, which uses natural gas as fuel. It is mainly used to drive the power pack of the standby compressor of the SC TRANSGAZ SA mainline natural gas pumping station.
The aeroderivative GTC 1000 is based on the Turmo IV C design and also uses natural gas as fuel. It was used in a power pack to drive two tandem centrifugal compressors that were used to compress the associated drilling gas at a SC OMV PETROM SA oil production project located in the ţicleni-gorje region.
We also conducted stabilization studies on landfill gas in aeroderivative gas turbine type GTE 2000. In the CHP application project, we started the CHP project using the GTE 2000 aeroderivative gas turbine in 2000.
The project features a cogeneration plant with two separate production lines that generate both electricity and heat. The plant is located in the city of Botosani and serves SC TERMICA SA (Figure 3, left).
The experience gained from the GTE 2000 CHP plant has been used in a new project for a medium-power aeroderivative gas turbine cogeneration plant using Pratt & Whitney's ST 18 A aeroderivative gas turbine.
The ST 18 A aeroderivative gas turbine is based on the aviation PW 100 by redesigning a series of components such as combustion chamber, receiver and air intake. The ST 18 A was designed and manufactured using a twin burner by water spray from the combustion chamber, which ensures a reduction in NO emissions.
The application is for a cogeneration plant with two separate cogeneration lines to generate superheated steam as well as electrical and thermal energy used in the oil extraction process.
The beneficiaries of the project are SC OMV Oil Company based in Sup Lacu de Barc Uu in Bijor County. The difference between aviation gas turbines and aeroderivative gas turbines is their operating conditions and reliability.
Aviation gas turbines require frequent starts and stops during their service life, which is relevant to the flight of the aircraft. The number of short operations between start and stop is high, and the operation time is usually several hours.
The operating conditions of the aeroderivative gas turbine are different, it can continue to operate after a short period of operation after each stop, and the total cumulative operating life can reach about 12,000 hours after a short period of operation of more than 1,000 hours between overhauls.
Aeroderivative gas turbines can operate continuously for up to 8,000 hours without orderly shutdown, with overhaul intervals of up to 30,000 cumulative operations, and for some brands, longer cumulative operating times.
Conventional and alternative combustion for gas turbine cogeneration groups
The performance, efficiency and emission levels of a gas turbine cogeneration group depend largely on the type of fuel used and its physical and chemical properties. The choice of fuel has an impact on the performance of the gas turbine, especially in terms of energy density. The low calorific value (LHV) of natural gas is typically between 30-45 MJ/Nm3.
Typical gaseous fuels can be classified according to their calorific value level:
The LHV of high calorific value fuels is between 45-190 mJ/nano3, including butane, propane, refinery exhaust, etc.
LHV of medium calorific value fuels between 11.2-30 MJ/Nm3, such as weak natural gas, landfill gas, coke oven gas; Fuels with lower calorific values (LHV<11.2 MJ/Nm3), such as blast furnace gas, refinery gas, petrochemical gas and gasification fuels. The choice of fuel and the level of calorific value have a significant impact on the performance and operation of the gas turbine.
Use of gas turbine fuel
General Requirements For gas turbines used in cogeneration groups, the most commonly used fuels for cost reasons are heavy oil and waste from various manufacturing or chemical processes using liquid fuel requirements.
Ensuring that no hot particles and deposits are deposited on the fire pipe and turbine during combustion can reduce the corrosive effect of combustion gases caused by the corrosive compounds sulfur, lead, sodium, vanadium, etc.
To solve the problem of pumping and atomization, a series of fuels such as filtration and heating must be well purified or filtered to remove water, solid particles or some impurities. Heavy liquid fuels must also be heated to the right temperature to allow them to be properly pumped and injected.
The coke and tar values are very important for the combustion process in a gas turbine. The coke value represents the carbon residue left over from the combustion of petroleum products, such as fuel oil and diesel.
The coke value plays a crucial role when these fuels are burned under special conditions such as enclosed spaces and limited air circulation. The tar value indicates the amount of resins, aromatic hydrocarbons, etc. present in liquid fuels. It should be noted that the coke value and tar value are for reference only.
In order to determine the combustion behavior of heavy liquid fuels, the coke value and tar value can be multiplied as an evaluation criterion. However, the possibility of using liquid fuels is decreasing when reusing aviation gas turbines for industrial purposes. Therefore, we need to consider coke value, tar value and other factors to find a more suitable fuel option for gas turbines.
For each application, we need to analyze the requirements of the beneficiary according to the fuel characteristics, such as density, molecular weight, evaporation limit, flammable temperature, volatility, viscosity, surface tension, latent heat of evaporation, calorific value and soot propensity. For the use of gaseous fuels, the solution to this problem is relatively simple due to its thermal stability, high calorific value and absence of soot and tar.
However, in order to ensure that the required pressure levels for equipment such as gas turbines and make-up are met, water and the removal of different impurities are required. Therefore, when using gaseous fuels, a control measuring station, such as the gas station at the 2xST 18 plant, must be provided to ensure the necessary handling and control of the gaseous fuels used.
While some alternative gaseous fuels produce biogas through gasification and biomass pyrolysis, residual gases from industrial processes rich in hydrogen can play an important role in the operation of gas turbine cogeneration groups, but they must meet some requirements regarding calorific value and composition.
Therefore, it is necessary to eliminate the impurity tar, limit sulfur and its compounds to 1 mg/nm3, and raise the alkali metal compounds to 0.1 mg/nano3, respectively.
Alternative fuels
The use of alternative fuels in gas turbine cogeneration groups has different characteristics and consequences. Biogas produced by anaerobic fermentation is an inexpensive alternative fuel and is a renewable energy source.
The combustion of biogas produces neutral CO2 and offers the possibility of treating and recycling residues, secondary agricultural products, various biological wastes, industrial organic wastewater, sewage and sewage sludge.
The nature and composition of biogas is influenced by factors such as the raw materials used, the treatment system and the temperature. When comparing the composition of natural gas and biogas, refer to Table 1.
The main component of these two fuels is methane CH4, and the main difference is that biogas contains a higher proportion of CO2 and hydrogen sulfide H2S. The right fuel needs to be selected on a case-by-case basis to achieve optimal combustion and performance.
Technically, the main difference is that the Woper index for natural gas is twice as high as for biogas. This leads to limited possibilities for replacing natural gas with biogas, as only gases with similar Wopo indices can replace each other. Biogas improvements can be achieved by replacing CO with 2 band CH4 and thus approaching the characteristics of natural gas.
However, water and hydrogen sulfide must be removed to avoid the harmful effects of sulfuric acid on different components of the cogeneration group, gas turbines, make-up devices, heat recovery steam generators, etc.
Landfill gas from waste deposition is a cheap energy source with a composition similar to biogas produced by anaerobic fermentation 45-60% methane, 40-55% carbon dioxide, when using biogas in a gas turbine cogeneration group or introducing it into the natural gas network, special treatment is required for condensate separation, drying, adsorption of volatile substances, etc.
DME, methane 3-O-CH3, is a clean alternative fuel that can be produced from fossil fuels, i.e. coal or plant biomass gasification. It can be transported and stored, similar to liquefied petroleum gas, and its physical and chemical properties relate to natural gas in Ardeal 99.8% CH4 and 0.2% carbon monoxide2.
The flame produced by burning DME is very similar to that produced by natural gas (Figure 5), which makes it suitable as a fuel in transportation, cogeneration groups, etc.
Oxidizing agents such as oxygen, air, steam, etc. for coal gasification through biomass. It can obtain hydrogen H2 and carbon monoxide CO as the main components. Syngas can be used to obtain methanol, hydrogen, methane, etc. Or it can be used as fuel in gas turbine cogeneration groups.
Since leaving the gas production unit, the gas contains ash particles and various compounds of chlorine, fluorine, alkali metals, etc., which must be removed to protect the cogeneration line.
By gasifying different kinds of biomass and utilizing different gasification technologies, the composition of the obtained gas and the lower calorific value LHV can vary according to the following factors.
Tables 1 and 3 show that biogas and syngas have lower calorific values than natural gas, requiring higher mass flow rates and minimal pressure loss in applications in cogeneration groups.
Therefore, the nozzles of the gas turbine and the burner of the make-up device must be designed to allow for a homogeneous mixing of fuel and oxides, as well as low pressure loss. Syngas contains a large amount of hydrogen, which affects combustion in the gas turbine cogeneration group in terms of flame stability, combustion efficiency, etc. The use of hydrogen as fuel and the introduction of diluting components such as steam and nitrogen will affect the operation of gas turbines.
Fuel interchangeability and verification standards
Interchangeability of gas turbine cogeneration groups refers to the ability to replace one gaseous fuel with another without affecting the combustion capacity of the application or equipment. The gaseous fuels used include the combustible gas methane and other light hydrocarbons, hydrogen, carbon monoxide and inert gases, mainly mixtures of nitrogen, carbon dioxide and water vapor.
The calorific value of gaseous fuels can be high or low depending on the proportion of methane in them, and the density and temperature of the fuel, as well as the ambient temperature, have an impact on the performance and lifetime of the equipment in the cogeneration group.
When considering these influencing factors, an important parameter to measure interchangeability is the Wopo index. This index is named after engineer and mathematician John Warper and is defined as the ratio of the square root of low calorific value LHV and fuel density to air density (d), indicating the rate of energy loss. By looking at the index, we are able to assess the interchangeability between different gaseous fuels.
While two gaseous fuels with different chemical compositions but the same index are interchangeable, the amount of heat delivered to the equipment is equal for the same fuel pressure. The table gives the Woper index values for several gaseous fuels, and in order to take into account the temperature of the fuel, the Woper index can be corrected according to the temperature, and the two fuels are interchangeable.
where δp1 and δp2 represent the overpressure 1 of fuels 1, 2 and Wo, respectively, as well as the Woper index 1 of Wo2 – fuel 1, 2 and A and the nozzle area of 2 – two fuels.
Therefore, the verification criteria for replacing fuel with equivalent fuel are given by the following equation:
Spontaneous ignition temperature and flame temperature have a great influence on NO formation, flame speed, backcombustion, efficiency, NOx and CO emissions, flue gas dew point, etc.
The spontaneous ignition temperature of air-mixed gaseous fuels is the temperature at which instantaneous and explosive spontaneous combustion occurs in the absence of an incandescent ignition source.
Turbulent flames are generally less stable than laminar flames, and the instability of the flame front rupture field is exacerbated by an increase in pipe diameter. Free-swirl turbulent flames are more prone to flame front rupture than laminar flames because of their higher jet velocities.
For turbulence angles greater than 30°, the stabilization zone is achieved only on the mixture-rich burner profile. In poorly mixed areas, reflux occurs without flame attachment to the burner edge due to reduced speed. The velocity distribution in the swirl determines the stability of the flame as a central suspended flame.
Fast-burning components, such as hydrogen, accelerate the flame velocity and have a tendency to reflux or extinguish. The tendency of the flame to return is proportional to the ignition velocity of the fuel gas, with high speeds leading to high effects.
It also depends on the proportion of primary air, components with reduced burning speed can cause the flame front to crack. To account for these factors, we establish the rupture index of the flame front when interchangeable.
In this equation, the symbol k represents the constant regarding the bursting limit of the flame front, the symbol f represents the coefficient with respect to primary air, the symbol lower represents the lower calorific value, and the symbols b and I represent the indices of the controlling fuel and alternative fuel, respectively.
When the calorific value of the fuel decreases, it causes a particular problem that the carbon monoxide content in landfill gas exceeds 40%. In order to achieve combustion, an appropriate fuel supply needs to be provided.
Fuels with reduced calorific value have a small flammability range and require the use of supplemental fuels such as propane during part-load or transient operation. Gas turbines running on low calorific value gas fuels require a higher mass flow rate to meet operational requirements than operating on natural gas.
It is important to note that we have neglected the injection of water or steam into the gas turbine. This fact changes the operating characteristics of the compressor. Therefore, when using low calorific value gas fuels, the corresponding gas turbine parameters need to be considered to ensure their normal operation.
By gasifying biomass and air, we can obtain syngas with a low calorific value of 4-6 MJ/Nm3. If vaporized by steam or oxygen, syngas with a low calorific value of 9-13 MJ/Nm3 can be obtained.
One alternative way to reduce the calorific value is to mix syngas with natural gas. For example, if the landfill gas has a low calorific value of 17-20 MJ/Nm3, mixing 60% of the gas with the low calorific value with 40% methane can obtain a fuel equivalent to a lower calorific value of 4.
This hybrid method provides the fuel performance required for applications or equipment that require lower calorific value fuels. By adjusting the mixing ratio, we are able to flexibly control the calorific value of the fuel to meet the needs of different applications. It is necessary to determine the most suitable mixing ratio according to the specific situation to obtain the desired combustion effect and performance.
Gas turbine cogeneration groups offer excellent flexibility in using conventional and alternative gaseous fuels. This advanced technology takes advantage of the advantages of aeroderivative gas turbines in ground energy production, while taking into account the efficient generation of electrical and thermal energy.
Aeroderivative gas turbines have been modified for use in cogeneration plants and have demonstrated high efficiency. These gas turbines feature fast start-up, part-load operation, low emissions, and the flexibility to use a variety of fuels such as natural gas, alternative gaseous fuels and waste.
The characteristics and composition of different fuels require proper adjustment and preparation of gas turbines to ensure their efficient and stable operation. Alternative fuels such as biogas, syngas, dimethyl ether, etc. are also being studied for use in gas turbine cogeneration groups, which need to meet specific quality and combustion requirements. Through interchangeability analysis and verification criteria, the suitability and substitution of different fuels can be ensured.