RU2091779C1 - Method of determining combustion value of natural gas - Google Patents

Abstract

FIELD: physico-chemical analyses. SUBSTANCE: density of gas and quantity of combustion water are first measured, whereupon combustion value of gas is calculated from empirical relationship:

where Q $\genfrac{}{}{0ex}{}{\text{20}}{\text{min}}$ is lowest specific combustion value at 20 C and pressure 760 mm Hg, ρ density of gas at the same conditions, and

Description

 The invention relates to physical chemistry and can be used to analyze natural combustible gases, to determine their calorific values.

 Direct and indirect methods are known for determining the heats of combustion of combustible gases [1 3]. Direct methods include methods based on burning gas in a calorimeter (in a calorimetric bomb) and methods based on recording the heat flux from a mini-torch.

 Indirect methods include methods based on the functional relationships of any parameters characterizing the source gas or the process of its oxidation (combustion) with the heat of combustion of the gas.

 Direct (calorimetric methods allow determining the calorific value of gas with high accuracy (0.1 0.3%), but the duration of one measurement cycle, taking into account analyzes of combustion products for the formation and dissolution of sulfuric and nitric acids in water, is 16 hours. Direct methods of continuous monitoring the calorific value of gas by registering the heat flux from the mini-torch is less accurate due to the need to measure the volumetric flow rate of small quantities of gas (or maintain it at a given level).

 Indirect methods include the chromatographic method [3] the method for determining oxygen used for gas oxidation, and the method based on measuring the density of natural combustible gas [4] The chromatographic method consists in determining the chemical composition of the gas and the subsequent calculation of the calorific value from its known calorific values components. This method is characterized by a duration of a full measurement cycle of ≈ 2 hours and an error (1 1.5%). The “oxygen” method is based on the dependence (close to linear) of the calorific values of the combustible components of the source gas on the amount of oxygen necessary for its complete combustion and is characterized by an error of ≈ 0.75% (without taking into account the error of oxygen measuring instruments).

 Closest to the invention in technical essence is the method [4] which consists in measuring the density of the analyzed gas with subsequent calculation of the calorific value according to the dependence Q = f (ρ), where Q is the calorific value of gas; ρ is the gas density.

This method is based on an empirical linear dependence of the calorific values of the main combustible components of natural combustible gas (saturated hydrocarbons) on their densities (Fig. 1). Investigation of the correlation between Q $\genfrac{}{}{0ex}{}{\text{twenty}}{\text{n}}$ (lower calorific value of gas at +20 ^o C) and gas density for 13 different (by gas composition) fields showed that without taking into account non-combustible impurities (CO ₂ , N ₂ ), as well as impurities CO and H ₂ O, there is a linear dependence
Q $\genfrac{}{}{0ex}{}{\text{twenty}}{\text{n}}$ = 52.30 • ρ + 4.42, MJ / m ³ (1)
Moreover, the coefficient of variation characterizing the deviation of the experimental data from the calculated dependence is ≈ 0.08%
However, non-combustible impurities are always present in natural combustible gas, and CO and H ₂ O may also be present. The presence of these impurities leads to a “smearing” of the Q dependence $\genfrac{}{}{0ex}{}{\text{twenty}}{\text{n}}$ = f (ρ) and an increase in the corresponding coefficient of variation by ≈ 10 times (to ≈ 0.75%).

The objective of the invention is to increase the accuracy of determining the calorific value of natural combustible gases containing non-combustible impurities, as well as CO and H ₂ S.

The technical result obtained by using the invention is that:
the accuracy of determining the calorific value of natural combustible gas containing non-combustible impurities, as well as CO and H ₂ S, increases (by ≈ 10 times), while the coefficient of variation of the experimental data is 0.08%);
the time of one measurement cycle is ≈ 1 h.

где Q $\genfrac{}{}{0ex}{}{\text{20}}{\text{н}}$ относительная низшая теплота сгорания газа при +20^oC и 760 мм рт. ст;
ρ_отн относительная плотность газа при +20^oC и 760 мм рт. ст.To solve the problem of the invention in a known method, which consists in determining the density of the analyzed gas with subsequent calculation of the calorific value, additionally determine the amount of water generated by the combustion of gas, and the calorific value is calculated according to empirical dependence

where q $\genfrac{}{}{0ex}{}{\text{twenty}}{\text{n}}$ relative net calorific value of gas at +20 ^o C and 760 mm RT. Art;
ρ _{rel the} relative density of the gas at +20 ^o C and 760 mm RT. Art.

относительное количество образующейся при сгорании газа воды.

the relative amount of water generated during gas combustion.

An increase in the amount of noncombustible impurities (CO ₂ , N ₂ ), as well as CO and H ₂ S in the analyzed gas leads, as a rule, to an increase in its density, a decrease in the amount of water generated during gas combustion, and a decrease in its heat of combustion.

, можно количественно учитывать влияние вышеуказанных примесей на теплоту сгорания природного горючего газа. При этом зависимость между

является менее "размытой", чем зависимость между Q $\genfrac{}{}{0ex}{}{\text{20}}{\text{н}}$ и ρ_отн. Наглядно это демонстрируется графиком, представленным на фиг. 2. Исследования корреляционной связи между

для 17 различных месторождений газа, содержащего до 4,1% негорючих примесей и до 2,5% H₂S подтверждают существование точной линейной связи, выражаемой уравнением

При этом коэффициент вариации отклонения экспериментальных данных от расчетной зависимости составляет ≈ 0,08%
Если экспериментальные данные по этим же месторождениям (содержащим негорючие примеси CO и H₂S) обработать в координатах (Q $\genfrac{}{}{0ex}{}{\text{20}}{\text{н}}$ , ρ_отн), то коэффициент вариации отклонений экспериментальных данных от расчетной зависимости составит ≈ 0,75%
Таким образом, предлагаемый способ позволяет определять теплоты сгорания природных горючих газов в ≈ 10 раз точнее, чем способ-прототип.The authors found that introducing new parameters

, you can quantitatively take into account the influence of the above impurities on the calorific value of natural combustible gas. Moreover, the relationship between

is less blurry than the relationship between Q $\genfrac{}{}{0ex}{}{\text{twenty}}{\text{n}}$ and ρ _rel . This is clearly demonstrated by the graph shown in FIG. 2. Studies of the correlation between

for 17 different gas fields containing up to 4.1% non-combustible impurities and up to 2.5% H ₂ S confirm the existence of an exact linear relationship, expressed by the equation

In this case, the coefficient of variation of the deviation of the experimental data from the calculated dependence is ≈ 0.08%
If the experimental data for the same deposits (containing non-combustible impurities CO and H ₂ S) are processed in coordinates (Q $\genfrac{}{}{0ex}{}{\text{twenty}}{\text{n}}$ , ρ _rel ), then the coefficient of variation of the deviations of the experimental data from the calculated dependence will be ≈ 0.75%
Thus, the proposed method allows to determine the heat of combustion of natural combustible gases ≈ 10 times more accurate than the prototype method.

для основных компонентов природного горючего газа.In FIG. Figure 1 shows the relationship between the relative calorific value and relative density for the main components of natural combustible gas; in FIG. 2 relationship between parameters

for the main components of natural combustible gas.

The graphs show:
Q $\genfrac{}{}{0ex}{}{\text{twenty}}{\text{n}}$ relative net calorific value of the corresponding component of natural combustible gas at +20 ^o C and a pressure of 760 mm RT. Art.

ρ _{rel the} relative density of the corresponding gas component at +20 ^o C and a pressure of 760 mm RT. Art.

относительное количество образующейся при сгорании газа воды.

the relative amount of water generated during gas combustion.

параметр (Q $\genfrac{}{}{0ex}{}{\text{20}}{\text{н}}$ +ρ_отн) также увеличивается по линейному закону (для тех же основных компонент горючего природного газа). Но в отличие от первого случая (фиг. 1) отклонения параметра (Q $\genfrac{}{}{0ex}{}{\text{20}}{\text{н}}$ +ρ_отн) от линейной зависимости для тех же вышеуказанных примесей не превышают
1% для N₂
14% для CO
7% CO₂
2% H₂S
Предлагаемый способ определения теплот сгорания природных горючих газов реализован следующим образом.From FIG. Figure 1 shows that with an increase in the density of the combustible component of a gas, its heat of combustion increases linearly. Non-combustible impurities deviate (in Q $\genfrac{}{}{0ex}{}{\text{twenty}}{\text{n}}$ ) from this dependence by 70 100% From FIG. 2 shows that with an increase in the parameter

parameter (Q $\genfrac{}{}{0ex}{}{\text{twenty}}{\text{n}}$ + ρ _rel ) also increases linearly (for the same main components of combustible natural gas). But unlike the first case (Fig. 1), the deviations of the parameter (Q $\genfrac{}{}{0ex}{}{\text{twenty}}{\text{n}}$ + ρ _rel ) on a linear dependence for the same above impurities do not exceed
1% for N ₂
14% for CO
7% CO ₂
2% H ₂ S
The proposed method for determining the calorific value of natural combustible gases is implemented as follows.

где Δm_бл. привес массы блока после поглощения анализируемого газа, г;
V_емк объем емкости, см³;
P давление газа в емкости, мм рт. ст.When this selection is made the gas sample in a container with a calibrated volume V _EMK. The sampler contains a cartridge with phosphorus pentoxide to absorb moisture contained in the sample gas. The pressure P and the temperature T of the gas in the tank are measured. Then the gas is transferred from the tank to the absorption block containing activated carbon. The absorption block is equipped with two mini-taps to disconnect it from the unit (sometimes, depending on the type of impurities in the gas, the block must be cooled to completely absorb gas from the tank). The mass of the analyzed gas is determined by the weight gain of the absorption block on the laboratory balance (for example, VLR-200) with an accuracy of 10 ^-4 g. Based on the obtained measurement data, the absolute density of the gas is calculated, reduced to a temperature of +20 ^o C and atmospheric pressure of 760 mm Hg. st

where Δm _bl. weight gain of the block after absorption of the analyzed gas, g;
_V-cc tank volume, cm ^3;
P gas pressure in the tank, mm RT. Art.

T is the temperature of the gas in the tank, K;
The relative density of the gas is calculated by the formula
ρ _rel. = K ₁ -ρ _abs. , (4)
where K _{1 is an} empirical coefficient.

Then the gas from the absorption block is bypassed (by heating the block to ≈ 100 ^° C) into a preliminary combustion chamber with a calibrated volume V _k pre-evacuated by a forevacuum pump (to a residual pressure of not more than 10 ^-1 mm Hg). The pressure P and gas temperature T are measured. Then pure oxygen is fed into the chamber with an excess in volume of 3.5 times. The resulting gas mixture is ignited by an automobile spark type initiator. To guarantee the completeness of gas combustion, the combustion chamber is located at 250 - 300 ^o C. After the gas is burned, the combustion products are pumped by a foreline pump through an absorption block containing a substance that binds water (phosphorus pentoxide). The mass of water formed is determined by the weight gain of the absorption block.

где K₂ эмпирический коэффициент;
Δm_бл. привес массы блока, г;
R газовая постоянная;
T температура камеры сгорания перед опытом, K;
P давление анализируемого газа в камере сгорания перед опытом, мм рт. ст.The relative amount of water formed during gas combustion is calculated by the formula

where K _{2 is an} empirical coefficient;
Δm _bl. weight gain of the block, g;
R gas constant;
T is the temperature of the combustion chamber before the experiment, K;
P the pressure of the analyzed gas in the combustion chamber before the experiment, mm RT. Art.

подставляются в уравнение (2), из которого рассчитывается низшая теплота сгорания газа Q $\genfrac{}{}{0ex}{}{\text{20}}{\text{н}}$ (при температуре +20^oC и давлении 760 мм рт. ст.).The obtained values

are substituted into equation (2), from which the net calorific value of gas Q is calculated $\genfrac{}{}{0ex}{}{\text{twenty}}{\text{n}}$ (at a temperature of +20 ^o C and a pressure of 760 mm RT. Art.).

The proposed method allows to determine the calorific value of natural combustible gas containing non-combustible and combustible impurities, including N ₂ , CO ₂ , CO, H ₂ S.

 Using the proposed method increases the accuracy of determining the calorific value in comparison with the prototype ≈ 10 times. Moreover, the duration of one measurement cycle is ≈ 1 h.

 The proposed method is developed in laboratory use and is implemented on affordable equipment. In the future (when developing accurate sensors for gas density and humidity of its combustion products), the method can be implemented in a fully automatic device operating in continuous monitoring mode.

Claims (1)

The method of determining the calorific value of natural combustible gas, which consists in determining the density of the analyzed gas and the subsequent calculation of the calorific value, characterized in that it additionally determine the amount of water formed during gas combustion, and the calorific value of gas is calculated by an empirical linear relationship

where q $\genfrac{}{}{0ex}{}{\text{twenty}}{\text{n}}$ - the relative net calorific value of gas at +20 ^o C and a pressure of 760 mm Hg ρ _Rel - the relative density of the gas at +20 ^o C and a pressure of 760 mm Hg

the relative amount of water generated during gas combustion.

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