Effect of Changes in Producer Gas Composition on the Readings of an Automatic Gas Analyzer
Abstract Generated abstract
This study examines whether a GÉUK-21 automatic electric gas analyzer, originally intended for continuous carbon dioxide measurement in combustion products, can be recalibrated to determine hydrogen in producer gas. Using relative thermal conductivity data, the paper derives corrections for the influence of variable carbon dioxide and methane contents on hydrogen readings in a nitrogen and hydrogen calibration basis. The analysis indicates that carbon dioxide lowers the apparent hydrogen reading by about 0.06 percent per percent of carbon dioxide, while methane raises it by about 0.05 percent per percent of methane. After applying average composition corrections, comparison with VTI-2 analyses showed small differences, supporting the instrument’s practical use for hydrogen measurement in producer gas when composition corrections are included.
Full Text
Effect of Changes in the Composition of Producer Gas on the Readings of an Automatic Gas Analyzer
N. V. Podbornov
(Presented by the scientific seminar of the Faculty of Chemical Technology)
The objective of the present study is to determine the possibility of using an automatic electric gas analyzer for the analysis of hydrogen in producer gas, and also to determine the influence of the gas composition on the readings of this same gas analyzer. The operating principle of the electric gas analyzer is based on the difference between the relative thermal conductivity of certain gases and the thermal conductivity of air. The values of the relative thermal conductivity of gases are given in Table 1 [1].
Table 1
| No. | Gas | Relative thermal conductivity of the gas |
|---|---|---|
| 1 | Air | 1.000 |
| 2 | Oxygen | 1.015 |
| 3 | Nitrogen | 0.998 |
| 4 | Hydrogen | 7.130 |
| 5 | Carbon monoxide | 0.964 |
| 6 | Carbon dioxide | 0.614 |
| 7 | Methane | 1.318 |
| 8 | Water vapor at 100°C | 0.973 |
For gas mixtures consisting of gases that do not react with one another, the thermal conductivity, to a first approximation, may be determined as
\[ \lambda_{\mathrm{mix}}=\frac{\sum_{n=1}^{m} k_n \lambda_n}{100}, \tag{1} \]
where \(k_n\) is the percentage content of the \(n\)-th component in the gas mixture;
\(\lambda_n\) is the thermal conductivity of the \(n\)-th component;
\(m\) is the number of components in the gas mixture.
An industrial gas analyzer of the GÉUK-21 type, intended for the continuous automatic determination of carbon dioxide in combustion products, was recalibrated using nitrogen–hydrogen mixtures under laboratory conditions. Evaluation of the possibility of using it under plant conditions for the analysis of producer gas is connected with determining the effect of other components on the instrument readings. Naturally, it was first necessary to determine the effect of CO₂, since its content varies within considerable limits, from 4 to 8%.
Let us denote the hydrogen content in percent by H₂, the carbon dioxide content by CO₂, and all the remaining components by N₂. Their thermal conductivities will be denoted, respectively, by
\(\lambda_{\mathrm{H}_2}\), \(\lambda_{\mathrm{CO}_2}\), \(\lambda_{\mathrm{N}_2}\). A gas of composition \(\mathrm{H}_2+\mathrm{N}_2'=100\%\) has the thermal conductivity
\[ \lambda_{\mathrm{cm}} = \frac{\lambda_{\mathrm{H}_2}\cdot \mathrm{H}_2+\lambda_{\mathrm{N}_2}\cdot \mathrm{N}_2}{100}. \tag{1} \]
A gas of composition \(\mathrm{H}_2+\mathrm{N}_2' + \mathrm{CO}_2 = 100\%\)
\[ \lambda_{\mathrm{cm}}' = \frac{\lambda_{\mathrm{H}_2}\cdot \mathrm{H}_2+\lambda_{\mathrm{N}_2}\cdot \mathrm{N}_2' + \lambda_{\mathrm{CO}_2}\cdot \mathrm{CO}_2}{100}. \tag{2} \]
Subtracting (2) from (1), we obtain
\[ \Delta \lambda = \lambda_{\mathrm{cm}}-\lambda_{\mathrm{cm}}' = \frac{\lambda_{\mathrm{N}_2}(\mathrm{N}_2-\mathrm{N}_2')-\lambda_{\mathrm{CO}_2}\cdot \mathrm{CO}_2}{100} = \frac{\lambda_{\mathrm{N}_2}-\lambda_{\mathrm{CO}_2}}{100}\,\mathrm{CO}_2. \tag{3} \]
Since, from the initial conditions, \(\mathrm{N}_2-\mathrm{N}_2'=\mathrm{CO}_2\). For any three-component mixture \((\mathrm{CO}_2,\mathrm{N}_2',\mathrm{H}_2)\) of a given thermal conductivity, it is possible to select a two-component mixture \((\mathrm{N}_2'',\mathrm{H}_2'')\) with the same thermal conductivity, provided that the thermal conductivity of the mixture lies between the thermal conductivities of the components of the two-component mixture,
\[ (\lambda_{\mathrm{N}_2}<\lambda_{\mathrm{cm}}<\lambda_{\mathrm{H}_2}), \]
\[ \lambda_{\mathrm{cm}}' = \lambda_{\mathrm{cm}}+\Delta\lambda = \frac{\lambda_{\mathrm{H}_2}\cdot \mathrm{H}_2+\lambda_{\mathrm{N}_2}\cdot \mathrm{N}_2}{100} + \frac{\lambda_{\mathrm{N}_2}-\lambda_{\mathrm{CO}_2}}{100}\,\mathrm{CO}_2 \]
may be set equal to
\[ \lambda_{\mathrm{cm}}'' = \frac{\lambda_{\mathrm{H}_2}\cdot \mathrm{H}_2''+\lambda_{\mathrm{N}_2}\cdot \mathrm{N}_2''}{100}, \]
i.e.,
\[ \lambda_{\mathrm{H}_2}\cdot \mathrm{H}_2+\lambda_{\mathrm{N}_2}\cdot \mathrm{N}_2+ (\lambda_{\mathrm{N}_2}-\lambda_{\mathrm{CO}_2})\,\mathrm{CO}_2 = \lambda_{\mathrm{H}_2}\cdot \mathrm{H}_2''+\lambda_{\mathrm{N}_2}\cdot \mathrm{N}_2'', \tag{4} \]
\[ \mathrm{N}_2''+\mathrm{H}_2''=100, \tag{5} \]
\[ \mathrm{N}_2+\mathrm{H}_2=100, \tag{6} \]
from (5) and (6) it follows that \(\mathrm{N}_2''-\mathrm{N}_2=-(\mathrm{H}_2''-\mathrm{H}_2)\)
or
\[ \Delta \mathrm{N}_2=-\Delta \mathrm{H}_2. \tag{7} \]
Taking equality (7) into account, equation (4) takes the form
\[ \lambda_{\mathrm{H}_2}\cdot \mathrm{H}_2+\lambda_{\mathrm{N}_2}\cdot \mathrm{N}_2+ (\lambda_{\mathrm{N}_2}-\lambda_{\mathrm{CO}_2})\cdot \mathrm{CO}_2 = \lambda_{\mathrm{H}_2}(\mathrm{H}_2-\Delta \mathrm{H}_2) + \lambda_{\mathrm{N}_2}(\mathrm{N}_2+\Delta \mathrm{H}_2), \]
or
\[ \Delta \mathrm{H}_2 = \frac{\lambda_{\mathrm{N}_2}-\lambda_{\mathrm{CO}_2}} {\lambda_{\mathrm{N}_2}-\lambda_{\mathrm{H}_2}} \cdot \mathrm{CO}_2. \tag{8} \]
Dividing the numerator and denominator of equality (8) by \(\lambda_{\mathrm{air}}\), we obtain
\[ \Delta \mathrm{H}_2 = \frac{ \dfrac{\lambda_{\mathrm{N}_2}}{\lambda_{\mathrm{air}}} - \dfrac{\lambda_{\mathrm{CO}_2}}{\lambda_{\mathrm{air}}} }{ \dfrac{\lambda_{\mathrm{N}_2}}{\lambda_{\mathrm{air}}} - \dfrac{\lambda_{\mathrm{H}_2}}{\lambda_{\mathrm{air}}} } \cdot \mathrm{CO}_2 \simeq \frac{1-0.614}{1-7.130}\cdot \mathrm{CO}_2 \simeq -0.06\,\mathrm{CO}_2. \]
In an analogous way, for the other interfering component—methane—one can obtain
\[ \Delta \mathrm{H}_2 = \frac{ \dfrac{\lambda_{\mathrm{N}_2}}{\lambda_{\mathrm{air}}} - \dfrac{\lambda_{\mathrm{CH}_4}}{\lambda_{\mathrm{air}}} }{ \dfrac{\lambda_{\mathrm{N}_2}}{\lambda_{\mathrm{air}}} - \dfrac{\lambda_{\mathrm{H}_2}}{\lambda_{\mathrm{air}}} } \cdot \mathrm{CH}_4 \simeq \frac{1 - 1.318}{1 - 7.130}\cdot \mathrm{CH}_4 \simeq 0.05\,\mathrm{CH}_4 . \]
Thus, the presence of carbon dioxide in producer gas lowers the instrument reading by 0.06% for each percent of carbon dioxide, while methane raises it by 0.05% for each percent of methane.
A correction for the average content of carbon dioxide and methane was introduced into the readings of the instrument, which had been calibrated on a nitrogen–hydrogen mixture, after which a series of check analyses was carried out. The results of the check analyses are given in Table 2.
Table 2
| No. | Readings of the electrical gas analyzer with correction | H₂ content according to the results of analysis on VTI-2 | Difference in readings |
|---|---|---|---|
| 1 | 14.9 | 15.0 | 0.1 |
| 2 | 13.5 | 13.4 | 0.1 |
| 3 | 15.5 | 15.3 | 0.2 |
| 4 | 13.8 | 13.9 | 0.1 |
| 5 | 14.7 | 14.8 | 0.1 |
It is evident from this table that the instrument is sufficiently accurate.
Conclusions
-
The possibility has been demonstrated of using an electrical gas analyzer for the analysis of hydrogen in producer gas, with preliminary introduction of a correction for the gas composition.
-
A change in the content of carbon dioxide and methane in producer gas within the range of 4–5% does not introduce a significant error into the readings of the gas analyzer.
REFERENCES
- A. M. Turichin. Electrical Measurements of Nonelectrical Quantities. Gosenergoizdat, 1959.