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Fast On-Line Monitoring of Fuel Gases with the Thermo Scientific Prima

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Fast On-Line Monitoring of Fuel Gases with the Thermo Scientific Prima Fast On-Line Monitoring of Fuel Gases with Notes Application the Thermo Scientific Prima PRO Process Mass Spectrometer Daniel Merriman & Graham Lewis, Thermo Fisher Scientific, Winsford Cheshire UK Key Words • Calorific Value • Wobbe Index • Stoichiometric Air Requirement • Combustion Air Requirement Index • Magnetic Sector • Rapid Multistream Sampler Introduction Furnace or power plant fuel gases which have variable composition pose particular challenges for process engineers who need to achieve optimum efficiency and product yield. As the gas composition changes, the heating (calorific) value changes and the burner temperature can therefore change. So too can the amount of air that is required for combustion. Unless these properties are monitored and compensated for, the furnace or power plant will not be properly controlled, causing significant inefficiency, wastage and even burner damage. Normally furnaces are operated at constant temperature; to achieve this, a gas-fired furnace would ideally burn a gas of constant composition, delivered at a constant rate and using a constant intake of air at the correct rate for complete combustion. In practice, the gas supply is of varying composition, especially on a petrochemical complex or integrated iron and steel works, where fuel gases from a variety of processes are utilized. Thus in order to maintain a constant temperature, either the rate of the gas supply needs to be varied or, in the case of a mixing station mixing various gas streams, the relative flow rates of the different gases need to be varied. Also, the air supply rate needs to be varied, since there will be energy wasted if either too little air or too much air is used. If there is too little air, some of the fuel gas will be unburned and wasted. If too much air is added then heat will be wasted. In metallurgy processes, excess air can also cause surface defects on metal surfaces due to oxidation, and poor temperature control can have an adverse effect on product quality. Table 1 shows the energy parameters of some of the typical components of fuel gases; the wide range of energy values means that relatively low concentrations of some components can have a significant impact on the energy value of the gas stream. It is therefore imperative to provide a comprehensive analysis of the gas stream, not just measure the major components. Process Mass Spectrometry is the ideal technique for determining the properties of fuel gases, by virtue of its speed and accuracy. It is therefore becoming widely adopted as the preferred technique. 2 Calorific Value Air Requirement Component Density kg/m3 Specific Gravity MJ/m3 m3/m3 Table 1: Energy values of typical fuel gas Hydrogen 0.1079 0.0009 0.0238 0.0007 components. Methane 0.3588 0.0072 0.0956 0.0055 Water 0.0000 0.0083 0.0000 0.0064 Carbon Monoxide 0.1264 0.0125 0.0239 0.0097 Ethylene 0.5944 0.0126 0.1441 0.0097 Ethane 0.6436 0.0136 0.1686 0.0105 Oxygen 0.0000 0.0143 0.0000 0.0111 Hydrogen Sulfide 0.2337 0.0154 0.0723 0.0119 Propylene 0.8761 0.0191 0.2187 0.0148 Carbon Dioxide 0.0000 0.0198 0.0000 0.0153 Propane 0.9318 0.0201 0.2436 0.0155 Butane 1.2357 0.0270 0.3231 0.0209 Pentane 1.5663 0.0345 0.4093 0.0267 Benzene 1.5567 0.0385 0.3937 0.0298 Hexane 1.9000 0.0410 0.4900 0.0317 Fuel Gas Energy Parameters By analyzing the composition of the fuel gas, the precise heating properties can be predicted. This enables a ‘feed-forward’ method of control (rather than ‘feed-back’). The calorific value is calculated from the analyzed composition according to the sum of the pure component calorific values weighted on a mole fraction basis1, as below. Calorific Value = [Calorific Value of Pure Component A × Mole Fraction of Component A] of Mixture + [Calorific Value of Pure Component B × Mole Fraction of Component B] + [Calorific Value of Pure Component C × Mole Fraction of Component C] + etc In practice the fuel gas will flow through a restriction such as an orifice or valve and the actual flow is inversely proportional to the square root of the density of the gas, so the property known as the Wobbe Index is more useful than the Calorific Value. The Wobbe Index is defined as follows: Wobbe Index = Calorific Value √(Specific Gravity) The Wobbe Index indicates the effective heating value of the fuel gas. The specific gravity is equal to the density of the gas divided by the density of the reference gas (normally air). The density is calculated according to: Density of Mixture = [Density of Pure Component A × Mole Fraction of Component A] + [Density of Pure Component B × Mole Fraction of Component B] + [Density of Pure Component C × Mole Fraction of Component C] + etc To predict the required ratio of air/fuel required for complete combustion the property known as Stoichiometric Air Requirement is calculated as: Stoichiometric Air = [SAR of Pure Component A × Mole Fraction of Component A] Requirement (SAR) + [SAR of Pure Component B × Mole Fraction of Component B] + [SAR of Pure Component C × Mole Fraction of Component C] + etc 3 Calorific Value Air Requirement Again, because the actual flow of fuel gas through a restriction such as an orifice or valve is inversely Component Density kg/m3 Specific Gravity MJ/m3 m3/m3 proportional to the square root of the density of the gas, so the property known as CARI (Combustion Air Hydrogen 0.1079 0.0009 0.0238 0.0007 Requirement Index) is more useful than the Stoichiometric Air Requirement. The CARI is defined as follows: Methane 0.3588 0.0072 0.0956 0.0055 Water 0.0000 0.0083 0.0000 0.0064 CARI = Stoichiometric Air Requirement Carbon Monoxide 0.1264 0.0125 0.0239 0.0097 √(Specific Gravity) Ethylene 0.5944 0.0126 0.1441 0.0097 The ideal analyzer is capable of fast on-line determination of Wobbe Index and CARI, to enable feed-forward Ethane 0.6436 0.0136 0.1686 0.0105 control. Process Mass Spectrometry has established itself as the optimum technique for these measurements. Oxygen 0.0000 0.0143 0.0000 0.0111 Hydrogen Sulfide 0.2337 0.0154 0.0723 0.0119 Measurement of Fuel Gases by Process Mass Spectrometry Propylene 0.8761 0.0191 0.2187 0.0148 Process Mass Spectrometry is particularly suited to the measurement of fuel gas because the analysis is Carbon Dioxide 0.0000 0.0198 0.0000 0.0153 comprehensive, accurate and fast. The analysis of all the components present in fuel gas which typically Propane 0.9318 0.0201 0.2436 0.0155 contains many components (e.g. H2, CH4, CO, N2, O2, C2H4, Ar, C3H6, CO2, C3H8, C4H10 and C6H6) is completed Butane 1.2357 0.0270 0.3231 0.0209 in less than 30 seconds, with a precision of typically better than 0.1% relative or 0.01 mol% absolute. Mass Pentane 1.5663 0.0345 0.4093 0.0267 spectrometers function by ionizing the sample gas molecules and atoms, then using magnetic or electric Benzene 1.5567 0.0385 0.3937 0.0298 fields to separate the resulting characteristic ions according to their mass to charge ratio. The various ion Hexane 1.9000 0.0410 0.4900 0.0317 currents are measured with a detector2. An example of such an instrument designed for process measurements is the Thermo Scientific™ Prima PRO which is a compact and rugged unit using a scanning magnetic sector MS analyzer. This instrument, shown in Figure 1, provides exceptional reproducibility and linearity. Advantages of Prima PRO Process MS At the heart of the Prima PRO is a magnetic sector analyzer which offers unrivalled precision and accuracy compared with other mass spectrometers. Thermo Fisher Scientific manufactures both quadrupole and magnetic sector mass spectrometers; over thirty years of industrial experience have shown the magnetic sector based analyzer offers the best performance for industrial online gas analysis. Key advantages of magnetic sector analyzers include improved precision, accuracy, long intervals between calibrations and resistance to contamination. Typically, analytical precision is between 2 and 10 times better than a quadrupole analyzer, depending on the gases analyzed and complexity of the mixture. A unique feature of the Prima PRO magnet is that it is laminated. Our analyzer scans at speeds equivalent to that of quadrupole analyzers, offering the unique combination of rapid analysis and high stability. This allows the rapid and extremely stable analysis of an unlimited number of user-defined gases. The scanning magnetic sector is controlled with 24-bit precision using a magnetic flux measuring device for extremely stable mass alignment. Figure 1: Thermo Scientific Prima PRO magnetic sector mass spectrometer Sample gases, for example Natural Gas, Process gas used for on-line analysis of fuel gases. streams, Mixed gas • Gas composition • Calorific Value • Wobbe Index • Density • CARI Available as analog outputs, Modbus or Profibus 4 Rapid Multistream Sampling If the MS is to monitor all process streams then a fast, reliable means of switching between streams is required. Solenoid valve manifolds have too much dead volume and rotary valves suffer from poor reliability so we developed the unique RMS Rapid Multistream Sampler. It offers an unmatched combination of sampling speed and reliability and allows sample selection from 1 of 32 or 1 of 64 streams. Stream settling times are application dependent and completely user configurable. The RMS includes digital sample flow recording for every selected stream. This can be used to trigger an alarm if the sample flow drops- if a filter in the sample conditioning system becomes blocked, for example.
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