The development of boilers can be divided into two aspects: boiler and furnace. In the first half of the 18th century, steam engines used in British coal mines, including J Watt's early steam engines, used steam pressure equal to atmospheric pressure. In the second half of the 18th century, it switched to steam at higher than atmospheric pressure. In the 19th century, the commonly used steam pressure increased to about .8 MPa. Accordingly, the earliest steam boiler was a large-diameter cylindrical vertical boiler shell containing water, and later it was changed to a horizontal boiler shell (Figure 1a), and the fire was burned in a brick furnace body below the boiler shell. With the boiler getting bigger and bigger, in order to increase the heating area, a fire tube is installed in the boiler shell, and the fire is ignited at the front end of the fire tube. The flue gas comes out from the back of the fire tube and is discharged to the chimney through the brick flue to heat the outside of the boiler shell, which is called a fire tube boiler. At first, only one fire tube is installed, which is called a single fire tube boiler or a Cornish boiler (Figure 1b); Later, it was added to two fire drums, called double fire drum boiler or Lancashire boiler (Figure 1c). Around 183, after mastering the production and tube expanding technology of high-quality steel tubes, the fire tube boiler appeared (Figure 1d). Some fire tubes are installed in the boiler shell, which constitute the main heating surface of the boiler, and the fire (flue gas) flows through the tubes. Install as many fire tubes as possible below the water storage line of the boiler shell, which is called horizontal external combustion tempering tube boiler. Its metal consumption is low, but it needs a lot of masonry. Fig. 1e shows a fire tube boiler with a fire tube. The flue gas flows out of the fire tube and then flows through the fire tube, which is called a Scottish marine boiler. Its shape and size can be well matched with the engine room of a ship, and the boiler itself is light, so it has been used on ships. The locomotive boiler in Figure 1f is equipped with a fire box with a water jacket in front of the boiler shell with only fire tubes, and the lower part of the fire box is equipped with a grate for fire, which is compact in arrangement. All steam locomotives use this kind of boiler. Fig. 1g shows a small vertical fire tube boiler. Fire tube boiler and fire tube boiler are collectively called shell boiler. The fire tube boiler has been eliminated, but the fire tube boiler is still in use.

in the middle of 19th century, water-tube boilers appeared. The heating surface of the boiler is the water pipe outside the boiler shell, which replaces the boiler shell itself and the fire tube and fire tube inside the boiler shell. The increase of boiler heating area and steam pressure is no longer limited by the diameter of boiler shell, which is beneficial to improve boiler evaporation and steam pressure. The cylindrical shell in this kind of boiler was renamed as drum, or steam drum. The initial water-tube boiler only used straight water pipes, and its development is shown in Figure 2. The pressure and capacity of straight water tube boilers are limited. At the beginning of the 2th century, steam turbines began to develop, which required boilers with high capacity and steam parameters. Straight-tube boilers can no longer meet the requirements. With the development of manufacturing technology and water treatment technology, curved water tube boilers have appeared (Figure 3). At first, it was a multi-pot cylinder. With the application of water wall, superheater and economizer and the improvement of steam-water separation elements in drum, the number of drum is gradually reduced, which not only saves metal, but also helps to improve the pressure, temperature, capacity and efficiency of boiler. By 193s, 2 ~ 4 MPa, 385 ~ 4℃ bent water tube boilers with water walls were widely used with 6 ~ 12 MW thermal power units. After World War II, the boiler industry developed rapidly. In the 194s, boilers with 5 MW generator sets at 1 MPa and 51℃ were adopted. In 195s, boilers with 1-2 MW generator sets at 54-57℃ and around 14 MPa were adopted. In 196s, subcritical pressure (17 ~ 18.5 MPa) boilers with 3 ~ 6 MW generator sets were adopted. The single unit capacity of the largest natural circulation boiler in the 197s has reached 85 MW.

The former fire tube boilers, fire tube boilers and water tube boilers all belong to natural circulation boilers (Figure 4), and the water vapor flows naturally due to the density difference caused by different heating conditions in the ascending and descending pipelines. While developing natural circulation boilers, once-through boilers have been applied since 193s. In the 194s, auxiliary circulation boilers were used.

auxiliary circulation boiler, also known as forced circulation boiler (fig. 5), is developed on the basis of natural circulation boiler. A circulating pump is installed in the downcomer system to strengthen the water circulation of the evaporation heating surface. There is no drum in the once-through boiler (Figure 6), and the feed water is sent to the economizer by the feed water pump, and turned into superheated steam through the evaporation heating surfaces such as water wall and superheater, and sent to the steam turbine. The flow resistance of each part is completely overcome by the feed water pump. After World War II, these two types of boilers developed rapidly, because the generator set required high temperature, high pressure and large capacity at that time. The purpose of developing these two kinds of boilers is to reduce or not use the drum, to use small-diameter tubes as heating surfaces, and to arrange the heating surfaces more freely. With the development of automatic control and water treatment technology, they are becoming mature. In the 197s, the largest single auxiliary circulating boiler was a 17 MPa pressure with a 1 MW generator set. At supercritical pressure, once-through boiler is the only one that can be used. In 197s, the largest single unit capacity was 27 MPa pressure with 13 MW generator set. Later, a compound cycle boiler composed of an auxiliary cycle boiler and a once-through boiler was developed. In the process of boiler development, fuel types have great influence on the furnace and combustion equipment. Therefore, it is not only required to develop various furnace types to adapt to the combustion characteristics of different fuels, but also to improve the combustion efficiency to save energy. In addition, the technical improvement of furnace and combustion equipment also requires minimizing pollutants (sulfur oxides and nitrogen oxides) in boiler flue gas.

in the early years, the boiler shell used fixed grate, burning high-quality coal and firewood, and adding coal and removing slag were manually operated. Mechanized grates have been used since the appearance of straight water tube boilers, among which chain grates have been widely used. The air supply under the grate has developed from the "unified warehouse air" without segmentation to the segmented air supply. The early hearth was low and the combustion efficiency was low. Later, people realized the role of furnace volume and structure in combustion, made the furnace higher, and adopted furnace grate and secondary air, thus improving the combustion efficiency. Chain grate can adapt to most kinds of coal, but it can't burn strongly bonded bituminous coal. The lower feeding grate also appeared very early, which is only suitable for burning high-quality bituminous coal. The coal thrower appeared in the 194s. The coal thrower can be arranged on a fixed hearth or on a chain grate to form a coal thrower chain grate. The power of the generator set exceeds 6 MW-h, and the grates of the above-mentioned layer-fired furnaces are too large in size and complicated in structure, which makes it difficult to arrange. Therefore, chamber-fired furnaces were used in the 192s, and they burned pulverized coal and oil. Coal is ground into pulverized coal by the coal mill and then injected into the furnace by the burner for combustion, so the capacity of the generator set is no longer limited by the combustion equipment. Since the beginning of World War II, almost all boilers in power stations have adopted chamber-fired furnaces. U-shaped flame was used in the pulverized coal boiler manufactured in the early years. The pulverized coal gas flow from the burner first drops in the furnace, and then turns and rises. Later, a swirl burner arranged on the front wall appeared, and the flame formed an L-shaped torch in the furnace. With the increase of boiler capacity, the number of swirl burners has also increased, which can be arranged on both side walls or on the front and rear walls. Around 193, DC burners arranged at the four corners of the furnace appeared, and most of them were tangential combustion. In 196s, some countries used tangential combustion mode of DC burner in polygonal furnace to burn lignite. After World War II, oil was cheap, and oil-fired boilers were widely used in many countries. The automation degree of oil-fired boiler is easy to improve. After the oil price increase in the 197s, many countries turned to coal resources. At this time, the capacity of power plant boilers is also increasing, which requires that the combustion equipment not only can burn completely, ignite stably, run reliably and have good low-load performance, but also must reduce the pollutants in the exhaust gas. In the 194s and 196s, in order to intensify combustion and reduce fly ash, liquid slag-discharging pulverized coal furnaces and cyclone furnaces were once used, but they have been rarely used since the 197s because of too many nitrogen oxides generated by this combustion method.

staged combustion or low-temperature combustion technology is adopted in power plant boilers burning coal (especially lignite), that is, delaying the mixing of pulverized coal and air, mixing flue gas in the air to slow down the combustion, or dispersing burners to suppress the furnace temperature, which can not only inhibit the generation of nitrogen oxides, but also reduce slagging. Boiling combustion is a kind of low-temperature combustion. In addition to combustible solid fuel with very high ash content, limestone can also be mixed into the fluidized bed for desulfurization.

The main development trends of boilers are:

① Further improving the thermal efficiency of boilers and power stations;

② reduce the equipment cost per unit power of boilers and power stations;

③ improve the operation flexibility and automation level of boiler units;

④ develop more boiler varieties to adapt to different fuels;

⑤ improve the operation reliability of boiler unit and its auxiliary equipment;

⑥ reduce environmental pollution.