On Regulating the Thermal Regime of a Gas Generator
Abstract Generated abstract
This paper examines why automatic fuel charging based only on outgoing gas temperature operates unreliably in gas generators. Using observations from industrial gasification of screened coal with a steam-air blast, the authors analyze how outgoing gas temperature depends on fuel bed height, gasification-zone temperature, and blast rate, and argue that the preparation-zone height should be the main load-control signal with process intensity as a corrective signal. They further contend that stabilizing the steam-air blast temperature alone cannot ensure stable operation under variable load, and propose that the temperature in the main process zone should serve as the principal indicator, with blast-temperature control adjusted according to this temperature or corrected by productivity.
Full Text
On the Regulation of the Thermal Regime of a Gas Generator
N. V. Podbornov, N. N. Norkin
(Presented by Prof. I. V. Gebler, Doctor of Technical Sciences)
Automatic regulation of the supply of fuel to a gas generator frees the operating personnel from the need to intervene in the control of the gasification process and helps maintain optimal loading conditions; consequently, the possibility arises of increasing the productivity of the gas generator. In practice, however, there is no reliably operating system of automatic regulation in gasification. The existing system for automatic charging of a gas generator according to the temperature of the outgoing gases does not operate reliably. Let us consider the causes that produce changes in the temperature of the gas at the outlet from the preparation layer. We shall assume that the moisture content of the fuel is constant and that the fuel is charged in small portions.
The gases, heated in the zone of the main process to a temperature \(t_g^{\,n}\), enter into heat exchange with the colder fuel layers of the preparation zone (Fig. 1), heat them, and themselves are cooled. It is obvious that the higher the layer of fuel above the zone of the main process, the lower will be the temperature of the outgoing gases, since the gas has to heat a layer of greater thickness. In addition to the height of the fuel layer, the temperature of the outgoing gases will be affected by the initial temperature of the gas (the temperature in the gasification zone) and by the velocity of gas motion (the productivity
Fig. 1. Change of temperatures over the height of the zone.
of the gas generator). This is confirmed by the results of experiments carried out on an industrial gas generator.
Coal of grade CC/14 from the Prokopyevsk-Kiselevsk deposit was subjected to gasification. Composition of the coal: \(C^{r}=89.5\%\); \(H^{r}=4.3\%\); \(N^{r}=2.2\%\); \(O^{r}=3.5\%\); \(S^{r}=0.5\%\); \(V^{r}=14\%\); \(A^{c}=11\text{–}21\%\); \(W^{p}=6\text{–}9\%\). The coal was screened to remove fines. Charging was carried out manually in portions equal to one charging box. The blast was steam-air. The blast temperature was regulated by an electric step controller. The gas station, consisting of two identical generators, was equipped with a capacity regulator. By acting on the set-point devices of these regulators during the experiments, the temperature of the steam-air blast and the capacity (blast rate) were changed. In addition, the temperature of the outgoing gas, the height of the fuel bed, the height of the slag cushion, the gas pressure, and the temperature in the combustion zone were monitored. The results of the experiments are entered in the table.
Table
| No. of experiments | Air pressure, mm water column | Gas pressure, mm water column | Temperature of steam-air mixture, °C | Gas temperature, °C | Temperature in combustion zone, °C | \(H_{T}\), mm | \(H_{\mathrm{ш}}\), mm |
|---|---|---|---|---|---|---|---|
| 1 | 310 | 175 | 47 | 490 | 1300 | 650 | 80 |
| 2 | 310 | 176 | 47.5 | 495 | 1270 | 500 | 70 |
| 3 | 280 | 150 | 48 | 525 | 1200 | 500 | 160 |
| 4 | 278 | 154 | 52 | 480 | 1190 | 600 | 190 |
| 5 | 280 | 152 | 52 | 455 | 1110 | 550 | 175 |
| 6 | 308 | 172 | 50 | 500 | 1260 | 550 | 120 |
| 7 | 290 | 154 | 54 | 510 | 1150 | 600 | 40 |
| 8 | 310 | 174 | 54 | 570 | 1210 | 500 | 150 |
| 9 | 316 | 190 | 53 | 550 | 1250 | 400 | 50 |
| 10 | 300 | 165 | 56 | 550 | 1150 | 500 | 60 |
| 11 | 292 | 154 | 58 | 560 | 1140 | 450 | 100 |
| 12 | 278 | 150 | 59.5 | 550 | 1000 | 425 | 0 |
| 13 | 278 | 150 | 62.5 | 570 | 1020 | 415 | 20 |
| 14 | 278 | 152 | 60 | 500 | 1050 | 400 | 130 |
Plotting the experimental points in the coordinates bed height—gas temperature (Fig. 2), we note that as the bed height increases, the gas temperature decreases. From the character of the arrangement of most of the points, it may be concluded that, in addition to height, the gas temperature is affected by the temperature in the combustion zone and by the blast rate. For normal operation of the generator, charging must be carried out in such a way that the fuel enters the zone of the main process sufficiently prepared, which can be achieved by maintaining the height of the preparation zone at an optimum value. When the height is reduced, the fuel enters the gasification process insufficiently prepared, while when it is exceeded a zone is created that does not participate in the process (an idle zone), which creates additional hydraulic resistances, reducing the capacity and efficiency of the generator. From the graph (Fig. 3), constructed on the basis of calculations [1], it is evident that as the intensity of the process increases, the optimum height of the preparation zone increases. Thus, the regulator of the ...
...load, it must operate according to a two-pulse scheme. The main pulse is the height of the preparation zone; the corrective one is the intensity of the process. An increase in the intensity of the process leads to a proportional increase in the gas velocity (Fig. 4). A regulator set to a definite temperature will change the height of the fuel depending on the velocity.
Fig. 2. Effect of the fuel-layer height on the temperature of the outgoing gases.
As the velocity, and consequently the intensity of the process, increases, the temperature of the outgoing gases rises; therefore the regulator will somewhat raise the fuel layer, and conversely, when the intensity decreases, it will lower it. Thus, the change in gas temperature with a change in intensity is a favorable phenomenon. An undesirable effect will be produced by fluctuations of the temperature in the zone of the main process. It is desirable to stabilize it also because it is the principal parameter determining the thermal operation of the generator. Plotting the experimental data in the coordinates “temperature in the zone of the main process—temperature of the steam-air blast” (Fig. 5), we note that the regulating parameters may be the temperature of the steam-air blast and the blast velocity. The blast velocity participates in regulating productivity. Gas production depends on its consumption: the greater the consum—
Fig. 3. Height of the preparation zone for Bogoslovsk coal as a function of gasification intensity and lump size. Calculated data.
the process, and consequently the greater the blast velocity must be. It remains to use, as the regulating action on the temperature in the zone of the main process, the temperature of the steam–air blast (in the case of using a heated blast—the steam–air ratio). In automation schemes it is not used as a regulator, but attempts are made to stabilize it, which somewhat improves the process, but only by reducing the chaotic effect on the temperature in the main zone. In the case of sharp changes in generator output, which occur when servicing open-hearth and glass-melting furnaces, it is necessary to change the temperature of the steam–air blast depending on the gasification temperature. In view of the fact that, as yet, no method has been found for reliably measuring the temperature in the zone of the main process, systems of automatic stabilization of the temperature of the steam–air blast must be built with a correcting impulse from the output.
Fig. 4. Change in the velocity of gases in the preparation zone with change in process intensity. Calculated data for a gas generator 3 m in diameter.
Fig. 5. Dependence of the temperature in the zone of the main process on the temperature of the steam–air blast and the blast velocity.
Thus, to stabilize the thermal operation of gas generators, the temperature in the zone of the main process should be taken as the principal indicator.
process. With such a formulation of the problem, the operation of the existing load regulator should improve.
Stabilization of the temperature of the steam-air blast, although it somewhat improves the conduct of the process, does not stabilize the operation of the generator, especially under variable operating conditions. Therefore, the set point for the steam-air blast temperature regulator must be changed as a function of the temperature in the main zone, or an automatic stabilization system for this temperature must be constructed with a correcting impulse from productivity.
REFERENCES
- B. N. Kataev, Heat Exchange in Shaft Furnaces, Metallurgizdat, 1945.