Patent Application
20090101822
The invention relates generally to monitoring and measuring fuel moisturization levels. The invention particularly relates to optical techniques for monitoring and measuring fuel moisturization levels.
Fuel moisturization systems have been used in combined cycle power plants in an attempt to increase power output and thermodynamic efficiency. One example embodiment is described in commonly assigned U.S. Pat. No. 6,389,794. In such systems, natural gas is saturated with water, and the moisturized fuel is heated to saturation conditions at the design gas pressure. The increased gas mass flow due to the addition of moisture results in increased power output from gas and steam turbines.
Natural gas fired combined cycle plants with Dry Low NOx (DLN) combustion systems impose strict requirements on the fuel gas saturation process due to tight fuel specification tolerances. These requirements relate to variables such as heating value, temperature, specific gravity and fuel composition. If fuel supply conditions deviate excessively from the designed fuel specifications, plant performance will degrade.
Lower heating value (LHV), specific gravity (SG), fuel temperature (Tf) and ambient temperature are important parameters that influence the energy of the fuel flowing in the system. Wobbe index (WI) numbers, defined as in Equation 1, provide an indication of energy flow in the system independent of gas pressure and gas pressure drops.
where reference temperature Tref=288 K. The WI number of the fuel gas supplied to the gas turbine tends to vary significantly in IGCC (integrated gasification combined cycle) plants because the fuel composition from the gasification system varies with load and feedstock to the gasifier. Water is added to the fuel gas to maintain a constant water-to-dry fuel ratio or fuel Wobbe index number to the gas turbine.
Moisturized fuel supply to a DLN gas turbine combustion system requires extremely tight control on the fuel saturation process in a moisturization column due to tight fuel specification tolerances, frequent load changes, and rapid load changes. Typically these DLN systems have at least two operating modes, one that provides robust performance from initial ignition through early loading, and another that provides optimized performance for base or high load conditions. Minimizing system emissions is desirable during operation at high load conditions.
Conventional fuel gas moisturization systems include a three-element control applied to a fuel gas saturation column. Such systems include measurements of the inlet fuel gas flow, make-up water flow, and exiting moisture content in moisturized fuel gas flow. The flow rate of water exiting with the moisturized gas from a moisturization column is measured using coriolis mass flow meters for dry fuel gas and moisturized fuel gas. The flow rate of water leaving the saturator mixed with moisturized fuel gas is given as,
Water component of outletflow=Wet outletfuelflow−Dry inletfuelflow. (2)
Because the fuel moisture in the moisturized stream is small in relation to the total flow, a small error in the total flow measurement may introduce a large error in the moisture content estimation. A more precise estimation of fuel gas composition may be performed using gas chromatography at a pressure of 14.73 psia (˜101.56 kPa) and a temperature of 60° F. (˜15.56° C.). Though accurate, the gas chromatography measurements are time consuming because the process involves sampling of the fuel gas and taking measurements at a reduced pressure and temperature. Further, the gas chromatography method is an off-line measurement of the constituent concentration. Therefore, the information about the components at high pressures and temperatures is not obtainable.
Thus, it would be desirable to have a sensor that can accurately measure on-line moisture content in fuel gas at high pressures and temperatures.
One embodiment disclosed herein is a fuel moisturization sensor system. The fuel moisturization sensor system includes a first light source configured for emitting light through a fuel and moisture flow path at a first wavelength, wherein the first wavelength is at least partially absorbable by the moisture when in a vapor phase and substantially not absorbable by the fuel, and a second light source configured for emitting light through the fuel and moisture flow path at a second wavelength, wherein the second wavelength is preferentially scattered by moisture when in a liquid phase and substantially not absorbed by the fuel or by the moisture when in a vapor phase, a detector system configured to detect light transmitted through the flow path at the first and second wavelengths and to generate a first data signal corresponding to the transmission at the first wavelength and a second data signal corresponding to the transmission at the second wavelength.
Another embodiment disclosed herein is a gasification system. The gasification system including a gasifier, a fuel moisturization system, a conduit for transferring a fuel and moisture mixture from the fuel moisturization system to the gasifier, and an on-line fuel moisturization sensor system disposed external to the gasifier, wherein the sensor system includes a first light source configured for emitting light through a fuel and moisture flow path at a first wavelength, wherein the first wavelength is at least partially absorbable by the moisture when in a vapor phase and substantially not absorbable by the fuel, a second light source configured for emitting light through the fuel and moisture flow path at a second wavelength, wherein the second wavelength is preferentially scattered by moisture when in a liquid phase and substantially not absorbed by the fuel or by the moisture when in a vapor phase, and a detector system configured to detect light transmitted through the flow path at the first and second wavelengths and to generate a first data signal corresponding to the transmission at the first wavelength and a second data signal corresponding to the transmission at the second wavelength.
Still another embodiment disclosed herein is a method for monitoring fuel moisturization levels. The method includes interrogating a fuel and moisture mixture with light at a first wavelength, detecting light at the first wavelength transmitted through the fuel and moisture mixture to generate a data signal corresponding to light absorbed at the first wavelength by the moisture in a vapor phase along a path of light transmittance through the fuel and moisture mixture, detecting a reference light signal at the first wavelength to generate a reference data signal corresponding to an intensity of the light at the first wavelength interrogating the fuel and moisture mixture, and determining a moisture level in vapor phase in the fuel and moisture mixture.
Yet another embodiment disclosed herein is a fuel moisturization sensor system. The fuel moisturization sensor system includes a first light source configured for emitting light through a fuel and moisture flow path at a first wavelength, wherein the first wavelength is at least partially absorbable by moisture when in a vapor phase and substantially not absorbable by the fuel, a second light source configured for emitting light through the fuel and moisture flow path at a second wavelength, wherein the second wavelength is preferentially scattered by particulate matter and substantially not absorbable by the fuel or by moisture when in a vapor phase, a third light source configured for emitting light through the fuel and moisture flow path at a third wavelength, wherein the third wavelength is at least partially absorbable by the moisture when in liquid phase and substantially not absorbable by the fuel or moisture when in vapor phase, and a detector system configured to detect light transmitted through the flow path at the first, second, and third wavelengths and to generate a first data signal corresponding to the transmission at the first wavelength, a second data signal corresponding to the transmission at the second wavelength, and a third data signal corresponding to the transmission at the third wavelength.
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:
As used herein the term “moisture” refers to both moisture when present in a vapor and moisture when present in liquid phase. Moisture in vapor phase is also interchangeably referred to herein as water vapor or steam.
As used herein the term “fuel” refers to gaseous phase natural gas or gasified coal suitable for combustion in industrial or power plant applications, for example. Non-limiting examples of molecular components of fuel include H2, H2O, N2, CO, CO2, C2H2, C2H4, C2H6, CH4, O2, COS, SO2, H2S, NO2, and NO.
As used herein particulate matter refers to solid and liquid particles entrained in the flowing fuel stream. Non-limiting examples includes liquid phase moisture particles and impurity particulates such as that of metals, hydrocarbons, dirt, and dust.
In the following specification and the claims that follow, the singular forms “a”, “an” and “the” include plural referents unless the context clearly dictates otherwise.
Embodiments disclosed herein include systems and methods for determining vapor and/or liquid phase moisture levels in a fuel and moisture mixture.
In one embodiment, a fuel moisturization sensor system includes a first light source at a first wavelength configured to interrogate a fuel and moisture flow path. The first wavelength is at least partially absorbable by the moisture when in a vapor phase and substantially not absorbable by the fuel. In one example, the first wavelength is selected to be in the infrared wavelength range. In another example, the first wavelength is selected to be in range from 925 to 975 nm. In one embodiment, “substantially not absorbed or absorbable” means that the absorption level is less than the noise level of the sensor system. In a specific embodiment, “substantially not absorbed or absorbable” means that the absorption is in the range of less than or equal to one percent of the initial intensity level of the moisture. In one embodiment, “at least partially absorbable” means that the absorption level is greater than the noise level of the sensor system. In a specific embodiment, “at least partially absorbable” means that the absorption is in the range of at least three percent of the initial intensity level of the moisture. In a more specific embodiment, particularly for elevated temperatures and pressures, “at least partially absorbable” means at least ten percent of the initial intensity level of the moisture is absorbed.
A second light source is at a second wavelength that is preferentially scattered by moisture when in a liquid phase and substantially not absorbed by the fuel or by the moisture when in a vapor phase. The second light source may be used to determine levels of particulate matter in the fuel and moisture mixture or in or on the chamber containing the fuel and moisture mixture. In one example, the second wavelength is selected to be in the visible wavelength range. In another example, a second wavelength is selected to be in range from 610 nm to 650 nm. As used herein, “preferentially scattered” means that the scattering cross-section is several times higher for the liquid phase as compared to the vapor phase.
A third light source may also be included in the system to interrogate the fuel and with a wavelength that is at least partially absorbable by moisture when in liquid phase and substantially not absorbable by the fuel or moisture when in vapor phase. In one example, a third wavelength is selected to be in a range from 1525 nm to 1575 nm.
The sensor system may further include detectors to detect the transmitted intensities of the interrogating light at one or more wavelengths and acquisition and analysis systems to determine parameters such as moisture levels and particulate matter levels based on the measured transmitted intensities.
The embodiment of
Although in some scenarios the moisture in a moisture and fuel mixture for gasification mostly includes vapor phase moisture, there are other scenarios wherein moisture in liquid phase may also be present in significant levels in the mixture and can be advantageously determined. In an alternate embodiment illustrated in
In another embodiment, a gasification system 130 includes a fuel moisturization system 132 as illustrated in
In one example, the fuel moisturization system in a gasification system is configured to receive a determined moisture to fuel ratio data and operable to modify the moisture to fuel ratio in the fuel and moisture mixture.
In one embodiment, a method for monitoring fuel moisturization levels includes interrogating a fuel and moisture mixture with light at a first wavelength to generate a data signal corresponding to light absorbed at the first wavelength by the moisture in a vapor phase along a path of light transmittance through the fuel and moisture mixture to determine a moisture level in vapor phase in the fuel and moisture mixture.
In a further embodiment, the method includes interrogating at a second wavelength to measure the presence of any particulate matter in the fuel and moisture mixture or along the transmission path, for example on a chamber containing the fuel and gas mixture. Light at the second wavelength, for example 633 nm is typically scattered by particulate matter. In a still further embodiment, light at a third wavelength, characteristic of an absorption peak of liquid phase moisture, is used to detect and measure the presence of liquid phase moisture in the fuel and moisture mixture.
In one embodiment, the moisture levels can be monitored on-line in real time in a system such as a gasification system to dynamically change moisture levels in the fuel and moisture mixture as desired.
Any appropriate technique may be used by the control and data acquisition system to analyze the detected data. In one embodiment, a method includes determining a molecular density of moisture in vapor phase by calculations including Beer-Lambert's relation, as discussed below. In a further embodiment, pressure and/or temperature corrections are applied to the absorption line width and shape calculations.
Typically in fuel, many different molecular species are found. Table 1 provides a list of the typical natural gas constituents and their percentage by molecular weight and by mole percent.
In
Similarly, in the comparative plot 168 of
Therefore, vapor phase moisture has an absorption characteristic that is not found in typical components in a fuel and hence can be used as a signature to detect the presence of vapor phase moisture and measure the vapor levels. Therefore, in one embodiment, the wavelength used to probe the vapor phase moisture is selected based on the absorption spectra of the other molecular species the vapor molecules that are present with in the mixture.
In one embodiment, the molecular density of moisture in vapor phase may be calculated using Beer-Lambert's relation given by:
where, I0 is the reference intensity, I is the transmitted intensity, Sθ″θ′ (T) is the line strength, f(v,vO, T, P) is the line shape function, Ni is the molecular density, and L is the path length of the beam. The line strength and the line shape functions of the interrogating laser radiation are dependent on the temperature and pressure as is well know in the art.
From Equation 3, the molecular density can be written as:
The above equation indicates that the molecular density is a function of reference and transmitted intensity. Using the above Equation (4), the specific volume can be calculated as:
v=N
av/(Ni*MWH
where Nav is the Avogrado number (molecules/mol) and MWH2O (gm/mol) is the molecular weight of water.
The density (ρ) of steam in the fuel gas mixture is then calculated using the relation
Multiplying the density with the volume of the vessel gives the mass of steam contained in the fuel gas mixture at any instant.
In some embodiments, the absorption line shape may be corrected to take into consideration the broadening of the absorption line due to elevated temperature and pressure conditions.
In one example, the operational pressure ranges from 500 psi (˜3450 kPa) to 600 psi (4140 kpa) and temperature is at least 400° F. (˜204° C.) At such a high pressure and temperature, there is a chance of absorption line broadening. Therefore knowledge of absorption line characteristics as a function of temperature and pressure is useful for the application of a spectroscopic based sensor in industrial environments.
The absorption line spectral shifts as a function of pressure and temperature is described in many references, such as Richard Phelan et al, “Absorption line shift with temperature and pressure: impact on laser-diode-based H2O sensing at 1.393 um,” Appl. Optics, Vol. 42, No. 24, pp. 4968-4974, 2003. The absorption line shapes have finite widths, which are mainly dependent on Doppler and collisional (pressure) broadening mechanisms. The absorption line width AVD at full width half-maximum (FWHM) in the Doppler limit is defined by
ΔV
D
=v
O
/c[(2kT ln 2/m)]1/2 (7)
where vO is the centre frequency, T is the temperature in degrees Kelvin, k is the Boltzmann's constant, m is the mass of the molecule, and c is the speed of light. The spectral shift and broadening of an absorption line, as a function of pressure and temperature is described by Equations (8) and (9), respectively,
V
P
=V
R
+δ*P, (8)
2γ(T)=2γ(TO)(TO/T)N, (9)
where VP and VR are the wavelengths of the peak absorption profiles at a pressure P and reference pressure R, respectively, and δ is the pressure-induced line-shift coefficient. In Equation (9), T0 is the reference temperature, 2γ(T0) is the broadening coefficient at the reference temperature, and N is the temperature-dependent exponent.
As reported in the aforementioned Phelan reference, the maximum measured spectral shift coefficient with pressure is 2.29×10−6 nm/mbar at room temperature. The variation of the shift coefficient as a function of temperature in the range of 300° K-1100° K is given in the same reference. Temperature broadening can shift the wavelength b plus or minus 0.03 nm/° C. In one example, a maximum shift at 600° F. (316° C.) is 9.46 nm. Pressure broadening can also shift the wavelength by plus or minus 0.0001 nm/Torr. This results in a shift of 2.75 nm at 550 psi (˜3.790 kPa).
Comparing line plots 152 (water vapor) and 158 (CO2) of
Without further elaboration, it is believed that one skilled in the art can, using the description herein, utilize the embodiments disclosed herein to their fullest extent. The following examples are included to provide additional guidance to those skilled in the art in practicing the claimed invention. The examples provided are merely representative of the work that contributes to the teaching of the present application. Accordingly, these examples are not intended to limit the invention, as defined in the appended claims, in any manner.
An experiment was carried out to measure moisture levels in steam mixed in nitrogen (N2) and carbon dioxide (CO2). A high temperature and pressure gas vessel with gas and steam connections was designed and fabricated for the purpose of conducting the experiments. The gas vessel was designed to withstand a pressure of 150 psia (˜1034.25 kPa) at 150° C. The maximum working pressure for this example was 80 psia (˜551.6 kPa). The windows in the gas vessel consisted of 6 mm thick quartz glass of 3-inch diameter. In order to remove moisture condensation on the windows, the windows were heated to 200° C. The temperature and pressure inside the gas vessel during the experiment were monitored using a thermocouple and a pressure gauge.
The interrogating laser radiation at 945 nm and 633 nm was divided into two parts each using a beam splitter. One part was incident on the gas vessel on one of the windows and the transmitted radiation detected through the other window. The second part was used as a reference for measurement of incident power.
The data was acquired at a 500 kHz rate and the data fed to algorithms written in MATLAB® to calculate the steam mass fraction in a gas mixture. The Beer's law and the steam line function were implemented in MATLAB in the algorithms used to calculate the steam levels.
The vessel was evacuated and filled with nitrogen up to a desired pressure. A data acquisition system acquiring the data from the moisturization sensor system was initiated and the transmitted and reference intensities were monitored. Steam was introduced in the vessel and the transmitted and reference intensities were monitored.
The absorption feature of light at 945 nm in
The steam mass was measured at different steam and N2 pressures. The results were validated using thermodynamic table based calculations and pressure, volume, and temperature (P, V, T) based calculations. For thermodynamic table based calculations, the steam temperature was measured using a k-type thermocouple inserted inside the steam chamber. For P, V, T based calculations, the steam pressure inside the chamber is calculated by the difference of chamber pressure with nitrogen plus steam mixture and nitrogen alone (before steam introduction in the chamber). Table 2 summarizes steam mass measurement at different steam pressures
A comparative table of average steam mass measured using the fuel moisturization sensor, thermodynamic table based calculations and P, V, T based measurements is given in Table 3. It can be noted that the average steam mass as detected by fuel moisturization sensor is very near to the average steam mass value estimated using thermodynamic table based calculation and P, V, T based measurements. This indicates that the sensor is capable of detecting moisture content in gas-steam mixtures.
The vessel was evacuated and filled with carbon dioxide up to a desired pressure. A data acquisition system acquiring the data from the moisturization sensor system was initiated and the transmitted and reference intensities were monitored. Steam was introduced in the vessel and the transmitted and reference intensities were continued to be monitored.
Line 202 marks the baseline intensity level. The absorption feature of light at 945 nm in
In one embodiment, the discussed examples 1 and 2 demonstrate the ability of the moisturization sensor to monitor and track the transients in steam mass due to steam introduction in the chamber containing either N2 or CO2.
While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.
