UTILIZATION OF THE HEAT OF AIR-BLAST PRODUCTS AT WATER GAS STATIONS

Ya. A. Landa

I. Statement of the Problem

Water gas is used for the following purposes: a) for heating, b) for obtaining domestic gas (water gas serves as an “additive” to illuminating gas), c) for obtaining hydrogen, synthetic fuels, etc.

As is known, the name “water gas” is connected with the fact that the principal reactions in the process of obtaining this gas are the reactions between water vapor and incandescent carbon (fuel). These reactions are endothermic, and in order to carry out the process it is necessary to supply heat to the fuel bed. Depending on the method by which heat is supplied to the bed, one or another production method is obtained.

The production of water gas may be continuous or intermittent. The former is based on the application of a number of methods for supplying heat to the bed, in particular the chemical and the physical methods. The chemical method consists in feeding a steam–oxygen blast. In this case the heat of the exothermic reactions of carbon combustion compensates for the heat consumption in the endothermic reactions of water vapor and carbon. The physical method consists in accumulating the heat required for the endothermic reactions in the gasifying reagents by means of high superheating of the steam, heating the circulating gas to a high temperature, etc. The intermittent method of water-gas production is associated with the cyclic character of the blast feed. In the first part of the cycle, air (cold or hot) is fed into the fuel bed, ensuring the occurrence of the exothermic reactions forming CO₂ and CO. During the air blast the bed is heated, i.e., it accumulates heat and producer gas is formed. In the second part of the cycle, saturated or superheated steam is fed into the bed and takes part in the endothermic reactions. During the steam blast, water gas is formed and the fuel bed is cooled.

At present the intermittent method is the most widespread. Installations operating by the latter method are found at a number of plants. The fuels used in these installations are coke, anthracite, and certain types of lean coals.

Without dwelling on detailed theories of the processes of water-gas production, which may be found in the relevant literature ([1], [2], [3]), we shall note that questions concerning the mechanism of the reactions are far from completely resolved. This observation applies not only to the reactions of water-gas formation, but also to the reactions of producer-gas formation. The dynamics of the process of producer-gas formation under the operating conditions of water-gas generators has been especially poorly studied. Calculations of the quantity and composition of producer gas and water gas in these generators are usually made by the method of Prof. Grum-Grzhimailo [2], which reflects the actual course of the process only approximately, or on the basis of experimental data.

In the present work the question is considered of burning and utilizing the heat of air gas, which is essentially a by-product of the main process—the production of water gas by the intermittent method. The approximate calculations and experimental data presented show that air gas contains, in the form of chemical and physical heat content, 20–25% of the total heat of the fuel. Utilization of the heat of air gas will make it possible to raise the efficiency of the water-gas production process from 50–60% to 70–75%.

II. Possible Ways of Utilizing the Heat of Air Gas

The utilization of the physical and chemical heat of air gas may be carried out in various ways and for various purposes. The literature ([1], [2]) describes an installation in which a recuperator and a waste-heat boiler were provided. The products of the air blast were fed into the lower part of the recuperator. In view of the periodicity of the supply of air gas, ignition of the latter was carried out by means of an electric igniter. Tubular superheaters were placed in the upper part of the recuperator. One of them served to superheat high-pressure steam, produced in the waste-heat unit and supplied for the general needs of the enterprise; the other—to superheat low-pressure steam, produced in the water jacket of the generator and fed into the generator for gasification. After passing through the superheaters, the combustion products of the air gas entered a vertical water-tube waste-heat boiler, operating at a pressure of \(P = 14\) ata. The waste gases were removed by means of an exhauster into the exhaust pipe. Another installation described there, designed for utilizing the heat of air gas, included a superheater and a horizontal water-tube waste-heat boiler, operating at a pressure of 19 ata.

The literature [2] also describes an installation in which regenerators were provided for utilizing the heat of air gas. During the periods of the air blast, the air gas heated the checkerwork of the regenerators, and during the periods of the steam blast, the water vapor was superheated by the heat accumulated in the checkerwork.

Despite the expediency and possibility of utilizing the heat of air gas, at present there are very few installations in which such utilization is carried out. The reasons for this fact are various. At some installations with individual waste-heat boilers, especially of the water-tube type, frequent leaks occurred as a result of sharp temperature fluctuations associated with the periodicity of the supply of air gas. This was the reason for abandoning utilization of the heat of this gas. At some installations with a common waste-heat boiler, the hydraulic resistance of the system—the gas pipeline and the boiler—led to a limitation of the output of the installations in water gas. In some cases, apparently, the unsuccessful design of dampers also had an effect; these disconnected individual generators from the common air-gas pipeline during the periods of the steam blast. Losses, or, more precisely, leaks into the indicated gas pipeline of water gas, caused a reduction in the output of the latter for the main production purposes. At some installations the combustion of the gas is made difficult by its low calorific value.

In addition to the circumstances indicated, the reasons for abandoning waste-heat systems could have been difficulties in operating these systems in the form of “pops,” i.e., explosions of the combustible mixture. Pops often arose as a result of an incorrect hydraulic regime, untimely purging of the gas pipeline with inert gases, insufficiently careful observation by the personnel, etc.

Discharge into the atmosphere of air gas obtained at water-gas installations is in many cases impermissible not only because of the po—

heat losses, but also from the sanitary conditions of the districts in which the installations are located. In doing so, it is necessary to think through the utilization system carefully from the standpoint of preventing the formation of a combustible mixture in it, of partial or complete automation of switching operations, the use of tight shutoff slide valves, etc.

It seems to us that the basic principle of operation of utilization systems should be the continuous supply of air-blast gas into a common gas main under pressure. The pressure of this gas must be sufficient to eject the air required for combustion of the gas. Ejection burners possess a certain self-acting property, which is necessary in the event of unavoidable fluctuations in gas consumption. The presence of incandescent refractory walls will ensure continuous ignition and complete combustion of the combustible mixture with a small excess of air. If combustion in the burners is incomplete, afterburning will be possible in the furnace space of the boilers.

If an installation has a large number of generators, it is advisable to construct several independent utilization systems, each of which serves 4–6 generators.

III. Selection of the operating regime of water-gas generators when using air-blast gases

The requirements imposed at the individual stages of the process on the operating conditions of gas generators are contradictory. During the air-blast period it is desirable to develop high temperatures in the fuel bed, which in the subsequent period ensures a high degree of steam decomposition and a reduction in the content of \(CO_2\) in the water gas. On the other hand, from the standpoint of heat losses with the air-blast gas, strong heating of the bed is undesirable, since it leads to a considerable content of \(CO\) in this gas and to its high temperature. Similar contradictions arise in choosing the bed height. A great bed height leads, on the one hand, to an increase in the degree of steam decomposition and to higher productivity with respect to water gas, and, on the other hand, to an increase in losses with the chemical heat of the air-blast products and to the need to increase the pressure of the air blower. In this connection there are no fully established rules for choosing the height of the fuel bed. At various installations the height of the fuel bed ranges from 0.8 to 2.0 m.

The contradictions in the operating conditions of generators largely disappear when devices are available for utilizing the chemical and physical heat of the air-blast products. In these cases, an operating regime with a high fuel bed (on the order of 2 meters) becomes advisable. With a great fuel-bed height, a moderate temperature of the air-blast products is usually obtained, which is important for the operation of the slide valves and the air-gas pipeline. Intensification of the air blast is possible even with a high bed, but to ensure it, fans of increased pressure are required. If reserve generators are available, a reduced air-blast rate is permissible, which ultimately leads to a high heating value of the water gas.

Regulation of the temperatures in the fuel bed is associated with changing the ratio of the amounts of air and steam passed through the bed. With an increase in the amount of air at an unchanged amount of steam, the temperature of the fuel bed rises and the amount of heat accumulated in the bed increases.

The temperature in the bed and the composition of the gases are also affected by the duration of the air-blast and steam-blast periods and by the alternation of upper and lower steam blast. The longer the duration of the air blast,

the higher the temperature of the fuel bed, the greater the chemical and physical heat of the air gas. The duration of the steam blast is limited by the lowering of the bed temperature and by a decrease in the degree of steam decomposition. The sequence of the individual blast phases in one cycle is usually as follows: air blast, steam purging from below, upper steam blast, lower steam blast.

The duration of the complete cycle in different installations ranges from 4 to 15 minutes. The subdivision of cycles into phases varies greatly. The limits of the duration of the individual phases, as a percentage of the duration of the entire cycle, are given below.

The shorter the cycle, the more uniform the process is with respect to gas composition. However, when the cycle duration is too short, relatively much time is spent on switching valves and on purging, and a considerable amount of the products of the air blast becomes mixed with the water gas, and vice versa.

Analyzing the available data on the operation of existing installations, one may conclude that the expedient cycle duration may be taken as 6–9 minutes. The duration of the air-blast phase, when the air gas is utilized, should be selected in accordance with the condition of continuous delivery of the air gas into the common gas main. Thus, when three gas generators are operating, the duration of the air-blast phase should be 35% of the total cycle duration; when four gas generators are operating, the duration of the air-blast phase should be 28–30% of the total.

IV. Testing an Existing Water-Gas Station

The question of burning the gases of the air blast and utilizing their heat was developed as applied to the conditions of one of the existing water-gas stations. The work was carried out by a team from the Department of Theoretical and General Heat Engineering of the Tomsk Polytechnic Institute. Members of the department A. A. Gurchenok and Yu. A. Korolenko participated in the work.

The station has three gas generators. The internal diameter of the shafts is 1.8 m, and their height from the grate to the bottom of the gas-discharge branch pipe is 2.5 m. The general layout of the installation is shown in Fig. 1.

The air pressure in the common air duct is 360–400 mm water column. The steam pressure in the main is 3–4 ata. The pressure and temperature of the steam as it enters the fuel bed are, respectively, 500–800 mm water column and 170–180°C. The quantity of steam is not regulated. The average daily fuel consumption is 19–21 tons. The fuel is coal from the Aralichevsk deposit. Composition of the combustible mass: \(C^{r} - 89\%\), \(H^{r} - 4.1\%\), \(S^{r} - 0.7\%\), \(O^{r} - 4.2\%\). Volatile matter \(V^{r} - 10\%\). Coal moisture \(W^{p} = 5–6\%\), ash content \(A^{p} = 10–14\%\). Usually two gas generators are in operation; the third is kept in “hot” reserve and operates only during cleaning of the main units and at peak gas consumption. The main gas generators are cleaned twice per day; the reserve one, once per day.

The duration of the individual phases is as follows: 3 min—air blast; 15 sec—steam purging from below; 1.5 min—lower steam blast; 15 sec—air purging. The total cycle duration is 8.0 min.

The indicated cycle is only an example. Very often, in particular after fuel charging, steam is not supplied from above. Frequently the duration of the air-blast phase is increased.

Because of the absence of a gas holder of sufficient capacity, the gas generators are often forced to stand idle. Cleaning of the gas generators is carried out

Fig. 1.

1. Gas generator. 2. Sprayer. 3. Water seal of the sprayer. 4. Exhaust pipe. 5. Scrubber. 6. Water seal of the scrubber. 7. Gate valve for regulating and shutting off the air. 8. Gate valve for disconnecting the air duct from the gas duct during air blast. 9. Gate valve closed during upper steam blast. 10. Exhaust valve. 11. Steam gate valve. 12. Switching three-way cock.

manually, without the use of insert, so-called “false” grates. Before cleaning, the thickness of the fuel bed is reduced to a minimum, with the calculation that after removal of the slag a layer of incandescent fuel 25–30 cm thick remains on the grate.

For designing a device for burning air gas at the given station, it was necessary to obtain at least approximate data on their operation. Therefore tests of one of the generators were planned. During the tests the following measurements and determinations were made: 1) measurement of the air flow rate, 2) determination of the amount of charged fuel and the height of its bed, 3) measurement of the air pressure in the common air duct and under the grate, 4) measurement of the pressure and temperature of the air gas in the outlet branch pipe, 5) determination of the composition of the air gas. Visual observations of gas combustion were also made. The gas, at the outlet from an iron tube welded to the gas-discharge branch pipe, was periodically ignited. In the last tests, small diffusion burners were used.

Test No. 1. Before the start of the test (12 h 10 min), the height of the fuel bed was 0.67 m. This gas generator had been cleaned at 4 o’clock. The layer of slag accumulated in it before the start of the test was approximately 0.2 m (allowing for a content of 30% combustibles). At 12 h 15 min, fuel was charged and the bed height reached 1.02 m (the weight of fuel corresponding to 1 cm of bed height is ~23 kg). Over 2 h 10 min (the duration of the test), the bed height decreased to 0.7 m.

Figure 2 presents the measurement data from this test. As can be seen from the figure, the temperature of the air gas rose smoothly to \(1000^\circ\mathrm{C}\) as the fuel bed decreased, while its heating value correspondingly decreased from 1000 to \(400\ \text{kcal}/\text{m}^3\).

Fig. 2.

Composition of the air gas

The gas, when discharged from the iron pipe, burned during the first hour of the test at \(Q_p^н > 500\ \text{kcal}/\text{nm}^3\) and did not burn in the second hour of the test at \(Q_p^н < 500\ \text{kcal}/\text{nm}^3\).

The air pressure in the common air duct was \(360\ \text{mm water column}\), and under the grate it was \(210\ \text{mm}\). The pressure drop of \(150\ \text{mm water column}\) resulted from throttling of the air by the regulating gate valve. The resistance of the bed was \(\sim 80\ \text{mm water column}\).

An approximate calculation of the specific quantity of air gas can be made from the nitrogen balance according to the following relation:

\[ v = \frac{V_2 N_2^{\text{air}}}{N_2^{\text{gas}} G} \ \frac{\text{nm}^3}{\text{kg fuel}}, \]

where \(V\ \text{nm}^3/\text{h}\) is the calculated hourly air flow rate according to the readings of the pneumometric tube,

\(z\ \text{h}\) is the duration of the air-blast periods during the test,

\(N_2^{\text{air}}\) is the content of \(N_2\) in the air,

\(N_2^{\text{gas}}\) is the same in the producer gas,

\(G\) is the fuel consumption during the experiment, in kg, calculated from the change in bed height, without allowance for slag formation. In this test the following was obtained:

\[ v=\frac{1450\left(\frac{68}{60}\right)\cdot 79}{70\cdot 715}=2.6\ \text{nm}^3/\text{kg} \]

Test No. 2. This test was carried out after cleaning the gas generator. Cleaning was performed from 2:30 p.m. to 3:10 p.m. The height of the fuel bed after removal of the slag was \(0.27\ \text{m}\). Before the test, fuel was charged in an amount of \(1700\ \text{kg}\), and the bed height reached \(1.02\ \text{m}\). After 2 hours of operation the bed height decreased to \(0.6\ \text{m}\).

Figure labels: \(^{\circ}\mathrm{C}\); \(\text{kcal}/\text{nm}^3\); \(\text{nm}^3/\text{h}\); temperature of producer gas; \(Q_n^{h}\); air consumption; air blast; steam blast; cessation of blast.

Fig. 3.

As can be seen from Fig. 3, the heating value of the producer gas at the beginning of the test was \(800\ \text{kcal}/\text{nm}^3\), and subsequently it decreased to \(300\ \text{kcal}/\text{nm}^3\). The gas did not burn at \(Q_p^{h}<450\ \text{kcal}/\text{nm}^3\). The temperature of the producer gas changed from \(420\text{–}450^{\circ}\mathrm{C}\) at the beginning of the test to \(750\text{–}800^{\circ}\mathrm{C}\) at its end.

The air pressure in the common air duct was \(380\ \text{mm water column}\), and under the grate it was approximately \(150\ \text{mm water column}\). The approximate yield of producer gas in this test was \(2.2\ \text{nm}^3/\text{kg}\) of fuel.

Composition of air gas

Test No. 3. This test began 2 hours after partial cleaning of the gas generator. Twenty minutes after the start of the test, fuel was charged. During charging, the height of the fuel bed increased from 0.79 m to 1.57 m. By the end of the test the bed height had decreased to 1 m. Thus, the average height of the fuel bed was greater than the average bed height in the preceding tests. Increasing the fuel bed led to an increase in the heating value of the air gas. In this test the heating value was maintained at the level of 550 kcal/nm³; see Fig. 4.

Fig. 4.

Visible labels in the figure: °C; kcal/nm³; nm³/h; “air gas”; “air consumption”; “steam”; “air blast”; “steam blast”; “blast shutoff”; min.

The average yield of air gas was 2.1 nm³/kg of fuel.

Test No. 4 was carried out in order to check the operating conditions of the gas generator with an increased fuel bed. The test began with a bed height exceeding 1 m. Twenty minutes after the start of the test

Composition of the air gas

the height of the bed decreased to 1 m; fuel was then charged, and the bed height reached 1.35 m. The temperature of the air gas fluctuated within the range 400–600°C, and its calorific value within the range 500–400 kcal/nm³, Fig. 5.

Fig. 5.

Operational test of all gas generators.
It was noted above that, when a device for burning the air gas is present, it is necessary to ensure a continuous supply of air gas to the gas pipeline. To determine the possibility of continuously producing air gas under the existing conditions at the plant (the absence of a large-capacity gas holder, manual cleaning of the generators), it was planned to carry out operational tests of all the gas generators. The first test was carried out under a regime modified in comparison with the usual one, namely with a thickened fuel bed and with a single cleaning of the gas generators during the day.

During the 24-hour test, the total duration of interruptions in the production of air gas amounted to 1 h 12 min. With more careful control of the operating regime of the individual gas generators, interruptions in the production of air gas can be completely avoided.

The total time of supply of air and steam blast during the tests for the individual gas generators was somewhat different. Judging by fuel consumption, the air flow rates per unit time of air blast in generators 1 and 2 were approximately the same and were 1.7 times greater than the air consumption in generator No. 3. During the cleaning of one of the generators, the other two operated, and the durations of the air- and steam-blast phases were approximately the same and amounted to 4–5 min.

Conclusions from the preliminary tests

The preliminary tests showed that, under the existing operating conditions, producer gas is obtained with variable composition and temperature. To avoid significant fluctuations in the composition and temperature of the producer gas, it is necessary to arrange for periodic removal of slag (twice per day) while maintaining the overall height of the fuel bed. This is possible by using inserted (false) grates that support the bed during cleaning. It is also necessary to arrange more frequent charging of fuel in smaller portions (250–350 kg).

The tests showed that continuous production of producer gas is possible with an appropriate choice of the operating regime of the gas generators. In particular, this is possible by changing over to a sequential, uniform supply of air blast to all gas generators, with a slight overlap of the air-blast phases for the individual gas generators. According to the test data, the average calorific value and temperature of the producer gas may be taken as, respectively, 500 kcal/nm³ and 700°C. The approximate yield of producer gas is 2.3 nm³/kg of fuel.

V. Design of a device for burning producer gas

The low and variable quality of producer gas complicates the question of selecting the design of the combustion device. Turbulent or diffusion burners are not applicable in this case because of the poor stability of combustion. A furnace device with a refractory checkerwork appears most acceptable. The considerable accumulation of heat by the checkerwork will ensure ignition of the gas even in the event of brief interruptions, and combustion will proceed sufficiently completely with small excesses of air.

Probable interruptions in the gas supply make it advisable to eject air from the atmosphere by means of the gas, since, when air is supplied under pressure, cases are possible in which the gas pipeline is filled with a combustible mixture. Ejection of air by the gas is also desirable in view of a certain self-acting regulation of the air supply when the quantity of gas varies, as was already indicated above.

Figure 6 shows the design of a furnace with checkerwork made of standard brick, as applied to a station with an hourly coal consumption of about 1000 kg. It is assumed that the air will be ejected by the gas.

When the gas generators are operated with a high fuel bed and false grates are used (which is associated with a reduction in fluctuations in the composition of the gases), it is possible to use another version of a furnace for burning the gases. This version is shown in Fig. 7. It is a furnace with a small checkerwork located in the front wall of a Shukhov–Berlin boiler. This design belongs to the type of tunnel flameless burners developed by VNIIT for burning blast-furnace gas [4]. In this version it is also assumed that air will be ejected by the gas. In cramped boiler rooms, forechambers located inside the boilers may be used for burning producer gas.

Fig. 6

Fig. 7

Fig. 8

In-figure label: “Shukhov–Berlin boiler.”
Visible dimensions: 2700; 2645.

References

1. Ginzburg, D. B.—Gasification of Fuel and Gas-Generator Installations. 2nd ed., 1938.
2. Ginzburg, D. B.—Gas-Generator Installations, Part II, 1937.
3. Dolikh, P.—Water Gas, 1938.
4. Arseev, A. V.—Combustion of Gases, 1952.