Combustion of hydrogen-containing gases in gas turbine plants

The graphs of Fig. 2 (a, b, c, d) show the dependences of the density, volume and mass heat of combustion and the volume Wobbe number of the hydrogen-containing gas on the volume concentration of hydrogen.

For each gas turbine installation, there is an acceptable range of changes in the heat of combustion and the Wobbe number of fuel gas. For example, for the SGT5-2000E gas turbine engine, the maximum value of the mass heat of combustion of the fuel gas is 50 MJ / kg ± 5 %.

Since the mass heat of combustion of VSG is higher than that of natural gas (49 MJ / kg), mixing hydrogen with natural gas will increase the mass heat of combustion of the fuel gas. With a maximum mass calorific value of 52.5 MJ / kg, the maximum permissible volume concentration of hydrogen in the fuel gas will be 0.27 (27 %).

When hydrogen is mixed with natural gas, the Wobbe number of the fuel gas will decrease, since the values of the volume Wobbe number are lower than those of natural gas (48.1 MJ / m3).

The minimum value of the volume lowest Wobbe number of hydrogen-containing gas for the SGT5-2000E gas turbine engine is 37.5 MJ / m3.

From the graph in Fig. 2g it can be seen that the dependence of the Wobbe number on the hydrogen concentration has a minimum when the value of the hydrogen concentration in the VSG is 0.84. The minimum value of the Wobbe number is 38.44 MJ / m3, which exceeds the minimum allowable value for a hydrogen-containing gas.

Therefore, for the SGT5-2000E GTU, the volume Wobbe number of the VSG is not a limiting factor. The limiting factor for it is the value of the maximum mass heat of combustion of the VSG.

The mass heat of combustion of VSG can be reduced by mixing nitrogen with the fuel gas. This will lead to a decrease in the mass heat of combustion, since nitrogen is an inert gas with a density of 1.25 kg / m3. For SGT5-2000E gas turbine units, the minimum mass heat of combustion of the fuel gas is 39.9 MJ/kg.

Table 4 shows the values of the mass heat of combustion and the volume Wobbe number from the volume concentration of hydrogen at the volume concentration of nitrogen in the VSG, varying in the range of 9.3…9.6 %. The table shows that the concentration of hydrogen in the fuel gas can be significantly increased when nitrogen is added to the VSG. The second important effect of nitrogen mixing is to reduce the proportion of natural gas in the VSG. Thus, when the nitrogen concentration in natural gas is less than 1 %, the permissible concentration of hydrogen is 27 %, and the share of natural gas is 72 %. When mixing 9% of nitrogen in the VSG, the permissible concentration of hydrogen increases to 31 %, and the proportion of natural gas decreases to 60 %.

The graphs of Fig. 3 (a,b,c,d) show the dependences of the density, volume and mass heat of combustion and the volume Wobbe number of hydrogen-containing gas on the volume concentration of hydrogen when mixing 10% (vol.) of nitrogen. Figure 3b shows that the mass lower heat of combustion of VSG varies in the range from 41 to 47 MJ / kg with a change in the hydrogen concentration from 0 to 100 %.

Therefore, at a concentration of 10% nitrogen in the VSG, the mass heat of combustion is in the permissible range when the hydrogen concentration changes from 0 to 100 %.

Fig. 3 shows that the presence of 10% nitrogen in the VSG, the minimum allowable Wobbe number is achieved at a hydrogen concentration of 32 %, which is 5% higher than the permissible WI value in the absence of nitrogen.Therefore, the presence of nitrogen in the VSG makes it possible to increase the permissible concentration of hydrogen in the fuel gas.

At 32 % hydrogen and 10 % nitrogen, the proportion of methane in the VSG decreases to 58 %.For comparison: in the absence of nitrogen in the VSG, the permissible concentration of hydrogen is 27 %, respectively, the proportion of methane will be 73 %.

Table 5 shows examples of the component composition of natural gas and hydrogen-containing gas with a hydrogen concentration of 60 %; Table 6 shows the values of the volumetric lowest calorific value and density of natural gas and VSG under normal conditions, as well as the values of the volumetric lowest Wobbe number of natural gas and VSG.

The mass lowest heat of combustion of VSG is 59.9 MJ / kg, which is significantly higher than the maximum permissible value of the Wobbe number for SGT5-2000E. To ensure the combustion of hydrogen-containing gas, which has the composition shown in Table 6, it is necessary to mix natural gas with the VSG.

The calculation shows that in order to reduce the mass heat of combustion of hydrogen-containing gas from 59.9 to 52.5 MJ / kg, it is necessary to mix such a volume of natural gas to the initial VSG that its share in the mixture is 55 % (vol.) of the total volume of fuel gas. As noted above, the share of natural gas in the fuel gas can be reduced if nitrogen is added to the VSG.

When calculating the geometric dimensions of the mixing device, it is necessary to take into account that the density of the VSG is significantly less than the density of natural gas due to the presence of hydrogen in the VSG, which has a low density. It should also be taken into account that the VSG is produced, as a rule, with a low pressure, so before the mixing device, the static pressure of the VSG and natural gas must be equalized, or reduce the natural gas having a higher pressure to the pressure in the VSG gas pipeline using a pressure regulator, or increase the pressure of the VSG using a gas-booster compressor unit.

To measure the flow rates of the mixed flows on each input gas pipeline (natural gas and the source WSS), it is necessary to install flow meters, and to regulate the flow rates of natural gas and WSS – flow regulators.

Mixing devices (SU) can have a wide variety of shapes-from simple tees to complex aerodynamic structures. When organizing the mixing process, it is important to determine the aerodynamic drag of the SU. In the event that after the mixing unit it is necessary to increase the pressure of the VSG using a gas-booster compressor unit, the decrease in pressure on the SU will have to be compensated by increasing the power of the CU, which will increase the own needs of the gas turbine power unit.

It is necessary, at a minimum, to control the following input and output parameters of VSG, as well as natural gas: density, lowest calorific value, Wobbe number, pressure, temperature.

Since turbulence increases during the mixing of two gas flows, which is usually accompanied by an increase in pressure pulsations, as well as other parameters listed above, it is necessary to install sensors to measure the parameters of the mixed gas at a distance from the SU, where the mixture of VSG and natural gas has a homogeneous character.

It is known that due to the artificial intensification of the mixing process with the help of flow swirlers, it is possible to reduce the distance from the mixing node to the place where the flow will be homogeneous, but, as a rule, the intensification of mixing leads to an increase in the aerodynamic drag of the SU. In this regard, it is necessary to search for optimal design and technological solutions when designing a SU for mixing two gas environments with different.

The ignition of the gas turbine is carried out on natural gas. Mixing of the VSG with the fuel gas is carried out by controlling the control valve, which is installed on the VSG line.

Depending on the requirements for the composition of the fuel gas of the GTU, several options for regulating the composition of the fuel gas can be offered:

Regulation of the percentage of VSG in the fuel gas.

This method is applicable if a sufficient indicator of the fuel gas composition for a gas turbine is the percentage of VSG in it. In this case, the required percentage of VSG in the fuel gas is set by the operator or the self-propelled gun of the gas turbine. The valve on the VSG line regulates the deviation of the actual VSG flow rate from the required flow rate. The required flow rate is determined depending on its specified percentage in the fuel gas as follows:

 

Fofg = Ffg (k/100), (1)

 

where Fofg is the required volume flow of VSG, m3 / h; Ffg is the measured volume flow of fuel gas (a mixture of VSG and natural gas), m3 / h; k is the specified percentage volume content of VSG, %.

 

Regulation of the Wobbe number by calculation.

It is used when a sufficient indicator of the fuel gas composition for a gas turbine is the calculated value of the Wobbe number of the fuel gas. A statistical error is allowed between the specified value and the actual value. In this case, the required number of Wobbe of fuel gas is set by the operator or the self-propelled gun GT.

The essence of the method is that, knowing a given Wobbe number of fuel gas, it is possible to calculate the content of VSG in the fuel gas, at which the Wobbe number will be equal to the specified value, taking into account the calculation error. The valve on the VSG line regulates the deviation of the actual VSG flow rate from the required one. The required flow rate is determined based on its percentage in the fuel gas, as indicated above. In turn, the percentage of VSG at which the specified WI value will be provided is determined from the formula for calculating the Wobbe number for a mixture of two gases:

 

WI = ,(2)

 

 

 

where WI is the specified Wobbe number (Wobbe Index), MJ / m3; Qofg is the lowest calorific value of WOBB, MJ/m3; Qng is the lowest calorific value of natural gas, MJ/m3; Gsofg is the relative density of WOBB (the ratio of WOBB density to air density under normal conditions); Gsng is the relative density of natural gas (the ratio of the density of natural gas to the density of air under normal conditions); k is the desired percentage of WOBB, %.

Solving the equation with respect to k, we get:

 

k = . 100, (3)

 

where a = (Qofg – Qng)2;

b = 2Qng(Qofg – Qng) – W2(Gsofg – Gsng);

c = Qng2 – W2Gsng.

Knowing the required VSG content required to provide a given Wobbe number of fuel gas, we determine the required VSG consumption (see Expression 1).

 

Regulation of the Wobbe number by calculation

with a correction based on its actual deviation.

This method is applicable if precise WI regulation is required (it complements option 2). The essence of the method is the introduction of a correction PI control loop based on the actual deviation of the Wobbe number. The contour is set up during the setup process. At the output of the correction circuit, an adjustment is formed to the calculated required value of the VSG flow rate, on the basis of which the control valve on the VSG supply line will maintain the measured Wobbe number of fuel gas within the specified limits.

The Wobbe number (WI) is an internationally recognized criterion for the interchangeability of gaseous fuels. It is defined as the ratio of the volumetric heat of combustion of a fuel gas to the square root of the relative density of the gas under normal conditions. The Wobbe number, lower or higher, characterizes the heat output and aerodynamic parameters of the burner device at constant pressure. This criterion was formulated in 1927 by the Italian engineer Alfredo Wobbe.

Since WI takes into account the change in the heat of combustion of the gas and the density when changing the composition of the gas that burns under atmospheric conditions, i.e. close to normal (0 °C) or standard conditions, it is a criterion for the interchangeability of gaseous fuels for devices operating at pressures and temperatures close to atmospheric.

Unlike boiler houses that burn gas at low pressures, gas is burned in GTU at elevated pressure and temperature values at the inlet to the CS. In modern gas turbine engines, the temperature of the fuel gas at the entrance to the combustion chamber reaches 250 °C, the pressure is 6 MPa.

The gas density at the entrance to the combustion chamber is proportional to the gas density under normal conditions, the gas pressure before the CS, and inversely proportional to the gas temperature:

 

r = wildebeest . (R/Rnu). (Tnu/T),

 

where gnu, Phu, Tnu are the density, pressure, and temperature (K) of the gas under normal conditions;

r, P, T– density, pressure and temperature of the gas at the inlet to the combustion chamber of the gas turbine engine.

The product of the gas flow rate by the lowest heat of combustion is equal to the amount of heat released during the combustion of gas in the CS per unit of time.

Let’s compare the parameters of two gases-No. 1 and No. 2, which have different component composition, respectively, different heat of combustion, when the same amount of heat is supplied to the gas turbine unit per unit of time. To account for the pressure and temperature of the fuel gas at the CS inlet, it is proposed to use the Actual Wobbe number, which is associated with the traditional WI by the following expression:

AWI = Qh (vol.)/(ggks/gwnu) 0.5, (4)

 

where ggks is the gas density at the entrance to the CS GTU, gwnu is the air density under normal conditions.

The actual Wobbe number is related to the traditional WI expression

AWI = WI / (ggks/ggnu)0.5. (5)

The amount of heat supplied to the CS per unit of time is equal to the product of the mass heat of combustion by the mass consumption of the fuel gas:

 

G1. Qh1 (mass) = G2 . Qh2 (mass), ( 6 )

 

where Qh1 (mass) is the mass lowest heat of combustion of gas No. 1 (MJ / kg); Qh2 (mass) is the mass lowest heat of combustion of gas No. 2 (MJ/kg); G1, G2 are the mass flow rates of gases No. 1 and No. 2 (kg/c).

Converting the expression (6):

(1/g1nu). G1 . Qh1 (vol.) = (1/g1nu). G2 . Qh1 (vol.), (7)

 

where g1nu, g1nu is the density of gases No. 1 and No. 2 under normal conditions (kg / m3).

Using the standard Wobbe number, the expression (7) can be written as:

(gwnu/g1nu)-0.5. G1. WI1 = (gwnu/g2nu)-0.5. G2. WI2, (8)

 

where, WI is the Wobbe number equal to Qn (vol.)/ (hgnu/gwnu)0.5; Qn – vol.) is the volumetric lowest heat of combustion; hgnu is the gas density under normal conditions; wgnu is the air density under normal conditions.

When the composition of the gas changes, its density and heat of combustion will change. Accordingly, in order to maintain a constant thermal power of the GTU, the fuel gas consumption must change.

If the Wobbe number of gases #1 and #2 is constant, then the ratio of the mass flow rate to the root of the gas density must be constant. In this case, the mixing processes of gas and air should be “similar” for both gases. The Gorenje Wobbe standard test works well in devices where the process of mixing and burning gas and air is carried out under conditions (temperature and pressure) close to atmospheric.

Using the Actual Wobbe Number (AWI), the expression (5) can be written as:

(gwnu/g1gks)-0.5. G1. WI1 = (gwnu/g2gks)-0.5. G2. WI2, (9)

where, AWI is equal to Qh (vol.)/(ggks /gwnu) 0.5.

It follows from (9) that the Wobbe number is proportional to the ratio of the square root of the relative density at the CS inlet to the mass flow rate of the gas. It can be shown that the ratio of the mass flow rate G to the root of the gas density r0. 5 is proportional to the dynamic pressure of the gas jets at the inlet to the CS.

The dynamic pressure of gas jets is equal to half of the product of the gas density by the square of its velocity, that is, Rdin = (ru2)/2, where r is the density and u is the average gas flow rate. The average gas flow rate is proportional to the mass flow rate of the gas (G) and inversely proportional to the gas density (r) and the cross-sectional area of the gas jets (F), i.e.

 

u = (1/r) . (G/F).

 

Let us express the dynamic pressure of gas jets in the form

 

Rdin = (1/2). (1/r). (G/F)2.

 

Given that the cross-sectional area of the gas distribution holes remains constant, the ratio of the dynamic heads of gas jets having different densities and calorific values and, accordingly, the volume and mass flow rates will be equal to

 

Rdin2/Rdin1 = (r1/r2). (G2/G1) 2 ~

~ (AW I1)2/(AW I2)2.

Thus, the square of the ratio of the Actual Wobbe Numbers of the compared gases is inversely proportional to the ratio of their dynamic heads.

Consequently, with an increase in the volumetric heat of combustion and the Wobbe number of the fuel gas, other things being equal (the supply of thermal energy, the pressure and temperature of the supplied gas), the dynamic pressure of the gas jets decreases, which leads to a change in the aerodynamics of the gas jets in the CS. This causes a change in the intensity and uniformity of mixing of the fuel gas and air in the combustion chamber of the gas turbine engine.

And it follows that when using a gas with a Wobbe number that differs from the original one, it is possible to preserve the value of the dynamic head of the gas jets in the CS by changing the pressure level or temperature of the gas in front of it so that the actual Wobbe number remains constant.

So, in the case of gas entering the GTU with a volumetric calorific value exceeding the volumetric calorific value of pipeline natural gas, it is necessary to: a) reduce the gas pressure before the CS; b) increase the gas temperature before the CS; c) do both at the same time. In the case of less caloric gas entering the GTU, you need to: a) increase the gas pressure before the CS; b) reduce the gas temperature before the CS; c) do both at the same time.

As a result of the study, the following parameters were determined:

n dependences of the density, volume and mass heat of combustion, volume Wobbe number of the fuel gas on the volume concentration of hydrogen in the absence of nitrogen;

n effect of nitrogen on the density, volume and mass heat of combustion, volume Wobbe number of fuel gas at different values of the volume concentration of hydrogen;

n factors limiting the concentration of hydrogen in the fuel gas.