WAYS TO IMPROVE THE ECONOMY OF FUEL USE BASED ON THE OPERATING EXPERIENCE OF ONE STATE DISTRICT POWER PLANT

Engineer N. S. VASILIEV

The economy of fuel use achieved at one of the state district power plants as a result of many years of persistent and painstaking work by its staff is of general interest for a number of reasons, the main ones being: 1. At this power plant, new reserves have been identified and are being realized for a further substantial increase in economy. 2. This state district power plant operates on coal from the Moscow region and has steam drying in an open-cycle scheme. Its long operating experience should be of considerable interest in connection with the development of the most rational scheme for using high-moisture coals, especially in high- and superhigh-pressure installations.

The state district power plant is divided into two sections that are to a considerable extent separate: Stage I, with turbine pressure of 15 ata, accounting for about 20% of the station’s total capacity, and Stage II, with turbine pressure of 29 ata, accounting for over 80% of the station’s capacity.

Stage II is of the greatest interest.

At present, the level of economy is characterized by the following data:

Note. At this state district power plant, the efficiency of the central pulverized-fuel preparation with an open-cycle steam-drying scheme and the efficiency of the boiler house proper, without pulverized-fuel preparation, are determined separately, which more correctly and clearly reflects the economy of each of these links in the process.

Centralized Pulverized-Fuel Preparation System with an Open-Circuit Steam-Drying Scheme

Drying high-moisture coals in an open circuit, and moreover with low-pressure exhaust steam, is one of the important factors in increasing the economy of fuel utilization and, according to the latest data from the drying laboratory of the All-Union Thermal Engineering Institute, is the most economical in thermal terms. Thus, according to data from Stalin Prize laureate Candidate of Technical Sciences N. I. Mikhailov, the use of steam drying with exhaust steam at pressures within the range of 1.2–4 ata gives a fuel saving, compared with gas drying, of respectively 3.7–1.8% of the total fuel consumption at the power plant.

At this state district power plant, the steam tubular dryers are supplied with steam at 2.5 ata from two back-pressure turbines. The coal is dried from 31–33% to 15–16%, for which about 8% of the boiler steam produced is consumed, and 700–750 kcal are expended per 1 kg of evaporated moisture. The electricity generated in such a heat-supply consumption, with a specific consumption of standard fuel of 165 kg/kWh, amounts to 3% of the station’s total output. This reduces the station-wide specific fuel consumption by 2%, or by 10 g/kWh. At the same time, however, the question arises of the magnitude of the additional losses specific to an open drying scheme.

The advantages of such a scheme for high-moisture coals, from the standpoint of operation of the furnace-boiler unit—consisting in the independence of the furnace (in particular, air) regime from the drying system, in the more efficient use of furnace radiation, in better burnout of combustibles, in the possibility of having a high temperature of the primary air, in the more complete use of the air-heater surface, etc.—have already been discussed in the press and appear to us sufficiently obvious.

Here we shall consider only the specific losses of the open drying scheme, namely: mechanical fuel losses (carryover) and losses for heating the drying agent (air) discharged to the atmosphere. The drying scheme under consideration is opened before pulverization immediately after the dryers, i.e., before coal grinding, when the content in it of fractions finer than 200 μ does not exceed 10–15%, whereas after the mill system it amounts to 85–90%. With such a small content of fine fractions in the coal stream, and also because of the low velocity of the steam-air mixture in the shaft downstream of the dryer, the carryover of coal into the shaft from the stream before the dust collector amounts to about 2.5–3% of the total quantity of coal.

This is the great advantage of opening the circuit before pulverization, rather than after it, as has hitherto been proposed in most cases. At present, cyclone dust collectors of one of the latest modifications of NIIOGAZ are being installed at these dryers, with a collection efficiency of 90–94% according to tests at this state district power plant (gas-cleaning laboratory of VTI). For each drying unit a group of 6 such cyclones is installed. Thus, the carryover loss due to the open circuit in the drying installation under consideration is about 0.3% of all coal consumed by the station. The heat loss with the drying agent (air) discharged to the atmosphere amounts to 55 kcal/kg of evaporated moisture, which corresponds to about 0.4% of fuel consumption.

Comparing the total losses of the open circuit, amounting to 0.7%, with the saving obtained from the use of exhaust steam for drying and amounting, as indicated above, to about 2%, we find that the drying system under consideration is, in thermal terms, by itself more economical than closed gas systems, providing a saving of more than 1% of fuel, in addition to the saving obtained in the furnace-boiler unit as a result of the above-mentioned advantages of such a scheme.

To summarize the foregoing, the following may be said: a) an open-cycle drying scheme makes it possible to achieve the highest economy of the boiler unit in thermal terms; b) separation of the drying and grinding processes and opening the circuit up to the mill ensure that the lowest fuel losses with carryover are achieved, not exceeding 0.3%; c) the use of exhaust steam for drying not only compensates for, but also substantially exceeds, the specific losses of the open drying scheme and gives an additional saving of more than 1% in fuel consumption.

Boiler Unit

Principal Features of the Furnace Regime

The dust-preparation system considered created favorable conditions for attaining high efficiency of the boiler unit, and these were used to the maximum by the station personnel in adjusting the most economical operating regimes.

The chief of these conditions is the independence of furnace operation from the operation of dust preparation, as a result of which the following are achieved: a) constancy of the optimum moisture content (15–16%) and fineness of grinding of the coal dust ($R_{30} = 12\%$; $R_{70} = 40\%$); b) constancy of the optimum excess air over time and under changes in load; c) a high level of temperatures in the furnace, necessary for intensifying heat transfer to the radiant heating surfaces, for the most complete burnout of combustibles, and for maximum deposition of ash (slag) in the furnace, with the aim of reducing ash wear of the working surfaces of the boiler and reducing their fouling; d) a high temperature of the primary air, owing to which additional drying of the dust occurs before the burners.

One of the decisive factors in the high efficiency of the boiler unit under consideration is the minimum excess air both in the furnace and downstream of the unit.

The optimum excess-air coefficient at the furnace outlet, established by tests, is 1.1, and downstream of the unit (i.e., downstream of the air preheater) it is 1.35. With this excess air and a gas temperature downstream of the unit of about 190°C, the loss with flue gases is 6.8%. Losses from chemical and mechanical incomplete combustion do not exceed 1% in total, while the loss from mechanical incomplete combustion does not exceed 0.2–0.3%. The loss from chemical incomplete combustion (0.7–0.8%) lies, in our opinion, at the lower limit of the accuracy of its determination under operating conditions with a normal Norze apparatus.

Taking also into account the heat loss from the unit to the surroundings, which according to detailed measurements is about 1.2%, we obtain a gross efficiency of the boiler unit of 91%, which is also confirmed by annual reporting data when calculating the efficiency directly from steam and fuel.

Such a low excess air proved possible because the furnace is not ballasted by the discharge from the drying systems, which includes, in addition to the drying agent and the evaporated fuel moisture, also the considerable air inleakage always present in closed gas-air drying systems.

At the same time, this excess air proved to be the most advantageous from the economic standpoint and acceptable from the standpoint of normal furnace operation, because special attention is paid to the uniform distribution of fuel and air among all burners, and thereby in the furnace space, as well as over time.

Such proportioning is ensured by the continuous remote monitoring, developed at the station, of the temperature of the aeromixture before each...

by the burner, depending on the ratio of fuel and primary air, since intensive drying of the dust takes place in the stream of the latter. An increase or decrease in the temperature of the aerodust by \(1^\circ\)C corresponds to a decrease or increase in the delivery of the pulverized-coal feeder by 3%. In terms of air, proportioning is ensured by the appropriate adjustment of the burners on the air side by means of special air dampers at the “Calumet” burners.

Under such furnace operating conditions, the temperature in its most active zone is 1500–1600°C, which considerably exceeds the level of maximum temperatures in furnaces with closed drying circuits operating on Moscow-region coal. The indicated proportioning of fuel and air makes it possible, as a rule, to operate without slagging of the boiler tube banks and without the formation of local slagging spots. At the same time, with respect to slag separation, the furnace operates under severe conditions, since 45–50% of all the fuel ash is released in it—partly in the form of softened and partly in the form of dry granulated slag, which is quite unusual for pulverized-coal furnaces with closed circuits, especially on Moscow-region coal, whose ash is mainly difficult to melt.

The indicated amount of slag dropout in the furnace has been determined by measurements both from the ash balance, when measuring the concentrations of fly ash in the gases, and by direct measurements of slag carryover to the ash hoppers, and is reliable. Thus, the furnace under consideration is, as it were, “semi-liquid-slag,” with a high degree of ash capture, which is of substantial importance for installations operating on high-ash fuel and suffering greatly from ash wear of the heating surfaces, smoke exhausters, and other elements of the gas path.

The applied proportioning of fuel and air, in combination with the high temperature in the furnace, makes it possible to operate, despite the low excess air, with the minimal incomplete combustion indicated above.

Maintaining Optimal Technological Parameters

In addition to the basic factors of high economy considered above, the degree to which the optimal parameters of the working process are maintained is of great importance; deviations from these parameters in the boiler house in question are, as a rule, smaller than the operating tolerances established, namely:

for 1950:

Note. The actual data are the result of daily processing of the records of the recording instruments.

The significance of this factor is characterized by the corresponding saving obtained in comparison with the tolerances established by the MES, amounting, for the indicated parameters, to more than 3000 t of reference fuel per year. This is achieved by a combination of the skill of the operators on duty at the central heat-control board, who conduct the working process of the boilers, with the automation of combustion-process control for all boilers in this boiler house, automation of steam-temperature control, and developed remote control. Under such conditions, the basic control of all boilers, including supervision of the operation of the automatic devices, with the exception of water supply, is carried out by one operator on duty at the central heat-control board with an assistant. At present-

centralization is also being carried out on this same panel for control of the feedwater supply, simultaneously with improvement of its automatic controls.

Air Inleakage

Air inleakage within the heating surfaces is characterized by an increase in the excess-air coefficient from the furnace to the outlet from the air heater by an amount of 0.20–0.25 and, as a rule, falls within the standards of the Rules for Technical Operation of Power Stations.

This is achieved by systematic and thorough work to identify and eliminate points of inleakage, chiefly during major and current repairs of the boilers.

Prospects for Further Improvement of Economy

The achieved average operating level of the gross efficiency of the plant’s own boiler house—91%—is not yet the limit, and work has been undertaken at the station to increase it further. From the data given above it is evident that the temperature of the exit gases, despite the reconstruction carried out on the tail heating surfaces, still remains elevated, averaging about 190°C, and for individual boilers—from 180 to 200°C.

Further development of economizer surfaces could be achieved only as a result of fundamental reconstruction by replacing the standard air heaters with small-size tubular ones, developed and proposed by VTI, made of 37/40 mm tubes and with two air passes. In this case it might be possible to reduce the exit-gas temperature by about 40°C and to increase the boiler efficiency by 2%. However, in this case the water temperature downstream of the economizer already appreciably exceeds the limit desirable for ensuring reliable circulation in these boilers.

The station has developed another way of reducing the temperature of the exit gases, consisting in installing low-pressure waste-heat boilers downstream of the boiler air heaters, with the steam from them used in the regenerative system of the turbine hall. The essence of this solution is that the heat of the exit gases, which is presently being discharged, should replace the heat of steam from the second extraction of the turbines at a pressure of 5 ata, used to feed atmospheric-type deaerators and throttled in this process to 1.2 ata.

For this purpose, on each of the boilers there should be installed four waste-heat boilers of the tubular—smoke-tube type, each with a heating surface of 365 m² and a steam capacity of 4.5–5.0 t/h.

To supply the deaerators, 12 waste-heat boilers with a pressure of 1.75 ata from 3 main boilers should operate. (Diagram 1).

The heat of the exit gases from the remaining 2 main boilers should be used in the feedwater heater at a pressure of about 3 ata, likewise instead of steam from the second extraction. For this purpose, 8 waste-heat boilers from 2 other main boilers should operate at a pressure of 3 ata. (Diagram 2).

The reduction in the temperature of the exit gases should amount to about 50°C and, correspondingly, the increase in the efficiency of the main boilers to more than 2.5%.

Taking into account the displacement of regenerative extraction, the fuel saving relative to its consumption for Stage II of the station will be about 2%, and relative to the fuel consumption for the entire station—about 1.6%, or about 8 g/kWh.

The costs for such a utilization installation for all main boilers should amount to 1.1 million rubles. The value of the fuel saved will be 1.7 million rubles. Thus, the costs will be repaid in less than 1 year.

Steam and Condensate Losses

Steam and condensate losses for the station as a whole average 1.5% per year, as against the MPE limit of 3%. Blowdowns of boiler water, in addition, amount to about 0.5%.

Variants for using the heat of exhaust gases in a regenerative system

This level of steam and condensate losses has been achieved by the painstaking work of the station personnel in three principal directions:
a) systematic maintenance of the tightness of steam-and-water lines in all elements;
b) maximum use of the various blowdowns, drains, etc.;
c) reduction of unavoidable operating losses to the necessary minimum.

At present, a number of additional measures are being developed with the aim of reducing steam and condensate losses to 1% instead of 1.5%. This work is being carried out under the banner of the complete elimination of open blowdowns and drains to the atmosphere that still exist at the present time.

In the present article, the achieved level of economy is set forth in its main features only for the boiler installation with pulverized-fuel preparation. Meanwhile, the level of economy of the turbine installations of the station’s second stage is also considerable as a result of the work carried out by the personnel; there, as in the boiler installation, a number of new measures have been developed and will be implemented in the near future, substantially increasing economy compared with the level achieved to date.

The Minister of Power Stations of the USSR has set before this state district power plant a responsible and difficult task: to reduce the specific fuel consumption by a further 4%, i.e., to make the station to an even greater degree exemplary in terms of economy—

[[unclear: beginning of word]]ty, as well as in other respects. At present, the station’s staff is working intensively and confidently to solve this task.

With the implementation, during 1951–1952, of the measures already developed for further improving the efficiency of the pulverized-fuel preparation, boiler, and turbine installations, the specific consumption of standard fuel for the second stage will decrease by another 15 g/kWh, corresponding to a gross station efficiency of 27%. However, there are still possibilities for further improving efficiency. Thus, for example, work has now begun on studying the use of waste heat from the coal dryers. A positive solution to these questions, as well as to certain others, may raise the station’s operational average annual efficiency for the second stage to the level of 27.5%. The attainment of such a high level of efficiency by a medium-pressure condensing power station (30 atm) proves to be entirely realistic, provided that, through proper organization of operation, the considerable advantages of a rationally designed open-cycle scheme for steam drying of coal are properly utilized.