Circulating fluidized bed boiler technology is a clean coal-burning technology with high efficiency and low pollution, which has developed rapidly in recent decades. According to the theoretical design and operation of circulating fluidized bed boiler published in China in recent years, the principle, characteristics, start-up and operation of circulating fluidized bed boiler are introduced as follows:

First, the working principle of circulating fluidized bed boiler:

(1) fluidization process:

When the fluid flows upward through the granular bed, its operating state changes. When the flow rate is low, the particles are stationary and the fluid only passes through the gaps between the particles. When the flow rate increases to a certain speed, the particles are no longer supported by the distribution plate, but are all supported by the friction of the fluid. At this time, for a single particle, it no longer depends on the contact surface with other adjacent particles to maintain its spatial position. On the contrary, each particle can move freely in the bed without the mechanical support in front; As far as the whole bed is concerned, it has many fluid-like properties. This state is called fluidization. The lowest velocity when the granular bed changes from static state to fluidized state is called critical fluidization velocity.

The fluid-like characteristics of fluidized bed mainly include the following points.

(1) The static state at any height is similar to the solid in the unit bed section above this height.

The weight of the particles.

(2) No matter how inclined the bed is, the surface of the bed is always horizontal, and the shape of the bed keeps the shape of the container;

(3) solid particles in the bed can be discharged from the bottom or side holes like fluid;

(4) The object bed with higher density than the bed surface will sink, and the object with lower density will float on the bed surface;

(5) The particles in the bed are well mixed and uniformly dispersed in the bed, which is called "dispersed" fluidization. So when the bed is heated, the temperature of the whole bed is basically uniform. Generally speaking, gas and solid, gas will not flow evenly through the particle bed. Some bubbles escape through the short circuit of the bed, and the particles are grouped for turbulent motion. The porosity in the bed changes with location and time, so this fluidization is called "aggregation" fluidization.

The combustion process of coal is a gas-solid fluidization process.

Second, the principle and characteristics of circulating fluidized bed:

Under different gas velocities, the flow state of solid particle bed in circulating fluidized bed is also different. With the increase of gas velocity, solid particles present solid bed, bubbling fluidized bed, turbulent fluidized bed and pneumatic conveying respectively. The ascending section of circulating fluidized bed usually runs in the state of fast fluidized bed, and the formation of hydrodynamic characteristics of fast fluidized bed is very important for circulating fluidized bed. At this time, the solid fuel is fluidized by the air flow with the velocity greater than the final velocity of a single fuel, and moves up and down in the form of particle clusters, resulting in high backmixing. Particle clusters move in all directions, and they constantly form and disintegrate. In this fluid state, the gas flow can also carry a certain number of large particles, although its terminal velocity is much higher than the average gas velocity in the cross section. In this gas-solid operation mode, the gas-solid two-phase velocity is very different, that is, the relative velocity. Circulating fluidized bed is composed of gas-solid fuel separation device and solid fuel return device in fast fluidized bed (ascending section).

The characteristics of circulating fluidized bed are as follows:

(1) has no interface like bubbling fluidized bed, and solid particles fill the whole rising space.

(2) There is strong fuel backmixing, and particle clusters are constantly formed and disintegrated, and run in all directions.

(3) The relative velocity between particles and gas is relatively large, which is related to bed porosity and particle circulation flow.

(4) The operating fluidization speed is 2-3 times that of the bubbling fluidized bed.

(5) The bed pressure drop varies with fluidization velocity and particle mass flow.

(6) The particles are well mixed laterally.

(7) Strong particle backmixing, external circulation of particles and good lateral mixing make the temperature distribution in the whole ascending section uniform.

(8) By changing the storage capacity of the ascending section, the residence time of fuel in the bed can be adjusted within the range of several minutes to several hours.

(9) The overall property of fluidization gas is plug flow.

(10) Fluidized gas can be added at different heights of the reactor as required.

Three, the main types of fluidized bed fuel equipment:

At first, fluidized bed operation was mainly used in chemical industry. Since 1960s, fluidized bed has been used as coal fuel, and it has quickly become one of the three main fuel modes, namely, fixed bed fuel (stratified combustion). The theory and practice of fluidized bed fuel and suspension combustion (pulverized coal combustion) have also greatly promoted the development of fluidization discipline. At present, fluidized bed combustion has become one of the main application fields of fluidization, and people pay more and more attention to it.

Fluidized bed combustion equipment can be divided into bubbling fluidized bed boiler and circulating fluidized bed boiler according to hydrodynamic characteristics, and can also be divided into atmospheric pressure and pressurized fluidized bed boiler according to working conditions. Therefore, fluidized bed combustion boilers can be divided into atmospheric pressure bubbling fluidized bed boiler, atmospheric pressure circulating fluidized bed boiler, pressurized bubbling fluidized bed boiler and pressurized circulating fluidized bed boiler, which are in the industrial demonstration stage.

(4) Characteristics of circulating fluidized bed boiler:

(1) Working conditions of circulating fluidized bed boiler:

Project value

The temperature (℃) is 850-950, and the pressure drop of the bed is kpa 1 1- 12.

Fluidization speed (m/s) 4-6 Particle concentration in furnace kg/m3 150-600 bottom

Particle size of bed material (micron) 100-700 10-40 upper part of furnace.

Bed material density (kg/m3) 1800-2600 Ca/s molar ratio 1.5-4.

Fuel particle size (mm) < 12 wall transmission 2 10-250

The particle size (mm) of the desulfurizer is about 1.

(2) The characteristics of circulating fluidized bed boiler:

Circulating fluidized bed boiler can be divided into two parts. The first part is composed of furnace gas (block velocity fluidized bed), solid material separation equipment, solid material recovery equipment, (cyclone separator) and so on. The above parts form a solid substance circulation loop. The second part is the convection flue, which is equipped with superheater, reheater, economizer and air preheater.

In a typical circulating fluidized bed boiler combustion system, the primary air and secondary air required for combustion are sent from the bottom of the furnace and the side wall of the furnace respectively, and the fuel combustion is mainly completed in the furnace. Water walls are arranged around the furnace to absorb part of the heat generated by the fuel, and the solid materials carried out of the furnace by the airflow are collected in the gas-solid separation device and returned to the furnace for re-combustion.

The basic characteristics of circulating fluidized bed combustion boilers can be summarized as follows:

1, low temperature power controllable combustion;

Circulating fluidized bed combustion (CFB) is a fluidized combustion reaction process in which high-speed flue gas is in close contact with strong turbulent solid particles, and a large number of particles are back-mixed. At the same time, most high-temperature solid particles are captured outside the furnace, and these particles are sent back to the furnace to participate in the combustion process again and organize combustion repeatedly. Obviously, the burning time of fuel in the furnace is prolonged. Under this combustion mode, due to the limitation of the optimal desulfurization temperature, the temperature level in the furnace is generally around 850℃, which is much lower than that of ordinary pulverized coal furnace (generally 1300- 1400℃) and also lower than the ash baking point of ordinary coal (1200-65400℃).

This low-temperature combustion method has many advantages, such as slagging and alkali metal precipitation in the furnace are much better than those in the pulverized coal furnace, the sensitivity to ash characteristics is reduced, and there is no need to use a lot of space to cool high-temperature ash, so the amount of nitrogen oxides is low. And can be combined with a low-cost and efficient desulfurization process organized in a furnace. From the point of view of combustion reaction kinetics, the combustion reaction in circulating fluidized bed boiler is controlled in the dynamic combustion zone (or transition zone). Because the combustion temperature in circulating fluidized bed boiler is relatively low, and there is a lot of strong mixing of solid particles, the combustion rate in this case mainly depends on the chemical reaction rate, which in turn depends on the combustion temperature level. The physical factor of surface combustion is no longer the dominant factor to control the combustion rate, and the fuel burn-out rate of circulating fluidized bed boiler is very high. Generally, the combustion rate of a circulating fluidized bed boiler with good performance can reach above 98-99%.

2. The fluidized circulation process of solid materials with high speed, high concentration and Qualcomm;

Solid materials in circulating fluidized bed boiler (including fuel carbon residue, desulfurizer and inert bed material, etc.). ) through the furnace, separator, material return device for external circulation. At the same time, the materials in the circulating fluidized bed boiler participate in both in-furnace and out-of-furnace circulation operations. The whole combustion process and desulfurization process are gradually completed in the dynamic process of these two forms of cyclic operation.

3. High-intensity heat, quality and operation transfer process:

In a circulating fluidized bed boiler, a large amount of solid materials pass through the furnace under strong turbulence. Manual operation can change the circulating amount of materials and the distribution law of materials in the furnace to adapt to different combustion conditions. Under this organization mode, the heat, mass and momentum transfer in the furnace is very strong, which makes the temperature distribution in the whole furnace height uniform, which has been fully proved by practice.

4, circulating fluidized bed boiler compared with other furnace type:

Generally, the combustion of solid fuel can be divided into stratified combustion, fluidized bed combustion and close floating combustion, and fluidized bed combustion can be divided into bubbling fluidized bed combustion and circulating fluidized bed combustion. In order to understand the advantages of circulating fluidized bed boiler and the problems that need to be further studied and solved, it is necessary to compare circulating fluidized bed boiler with other boiler types.

(1) Comparison of combustion processes;

Characteristic layer combustion furnace circulating fluidized bed suspension combustion furnace

Average diameter of fuel particles (mm) < 300 0.05-0.10.02-0.08.

Wind speed in fuel chamber area (m/s)1-3 3-1215-30.

The operating state of solids is static, mostly upward and partly downward.

Heat transfer coefficient between bed and heating surface w.m2.k50-150100-250 50-100.

Wear is small, medium and small.

(2) Comparison of desulfurization processes:

Calcium-based desulfurizer (such as limestone, white stone or hydrated lime) is directly injected into the pulverized coal furnace, and the desulfurizer burns in a large area at high temperature to carry out melon reaction;

500℃-900℃

Calcium carbonate, calcium oxide and carbon dioxide

500℃-900℃

Magnesium carbonate? Calcium hydroxide+magnesium oxide+carbon dioxide

500℃-900℃

Calcium (hydroxyl) calcium 0 (sulfur) +H2O 1

At normal combustion temperature, the combustion process is basically completed within 200ms (the particle size of desulfurizer is about 10μm), and porous calcium oxide particles are formed after desulfurization. Once the desulfurizer is burned to generate CaCO, it will react with calcium sulfate.

2 calcium oxide+sulfur dioxide+oxygen and calcium sulfate

According to the calcium injection test of pulverized coal boiler, the optimum calcium injection temperature is about 1 100℃, and the desulfurization efficiency is better when the limestone particle size is between 8- 10μm, and the desulfurization efficiency is generally 20% and 50%. The combustion desulfurization process of circulating fluidized bed boiler is to send desulfurizer (lime or white stone) into the furnace, and then react with sulfur dioxide gas produced by combustion to achieve the purpose of desulfurization. Like the pulverized coal boiler, the desulfurizer enters the circulating fluidized bed boiler, burns in a large area to form calcium oxide, and then reacts with sulfur dioxide gas.

In the circulating fluidized bed boiler, due to the unique design and operation conditions, the main circulation loop of the whole circulating fluidized bed boiler runs in the optimal temperature range of desulfurization (850-900℃). At the same time, due to the circulation of solid materials inside and outside the furnace (through the separation device and the return device), the residence time of desulfurizer in the furnace is greatly prolonged, usually the average residence time can reach tens of minutes. In addition, the strong turbulent mixing in the furnace is also very beneficial to the desulfurization process of circulating fluidized bed boiler. When Ca/S is 1.5-2.5, the desulfurization efficiency can reach 90% and the utilization rate of desulfurizer can reach 50%, which will double the desulfurization effect of pulverized coal.

(3) Comparison of main technical and economic indexes of various boilers:

Boiler model

Main technical and economic indicators of YG-35/39-m3

Circulating fluidized bed boiler BG-35/39-M pulverized coal boiler L-35/39-W/I chain boiler

Actual thermal efficiency of boiler (%) 87.8 87.96 50

Fuel type lean coal lean coal lean coal

Low calorific value (kj/kg) 21736 22003 21736

Coal consumption of boiler (kg/hr kg/hr) 4959 4883 8707

Standard coal consumption of boiler (kg/hr kg/hr) 3684 3677 6468

Total power consumption of auxiliary machines (kW) 470 587. 1 362.3

The total power consumption capacity of auxiliary engine is converted into standard coal (kg) 100 235 145.

Total standard coal consumption (kg/h) 3872 42 18 66 13

Ton of standard coal consumption (kg)110.69109.25188.94

Combustion efficiency (%) 98-99 98-99 88. 1

The load adjustment range is large, small and large.

The adaptability to coal is wider than that of single coal.

The operation and maintenance level is generally high and simple.

Boiler equipment cost (body) (ten thousand yuan) 82.68 97 86.59

System investment cost (ten thousand yuan) 245,400,200.7

Boiler steel consumption (ton) 157 165 186

Sulfur dioxide emissions plus limestone can be desulfurization, all emissions, all emissions.

Nitrogen dioxide emissions produce less, produce more and produce more.

Fly ash emissions are large and small.

Note: Boiler investment is estimated in the early 1990s.

Comparison between circulating fluidized bed boiler and other types of boilers

boiler

Characteristic chain furnace pulverized coal furnace circulating fluidized bed furnace

Bed height or fuel combustion zone height M0.2 15-40 27-45

Section wind speed m/s 1.24-8 4-6

Excess air coefficient1.2-1.31.2-1.251.3.

Section heat load MW/m2 0.5- 1.53-54-6

The particle size of coal is below mm 6-326 and below 0. 1.

The load adjustment rate is 4. 1.3: 4. 1.

Combustion efficiency%

Nitrogen dioxide emission ppm 400-600 50-200 400-600

The desulfurization efficiency in the furnace is 80-90%

As can be seen from the above table, the circulating fluidized bed boiler is obviously superior to other types of boilers.

Advantages of verb (abbreviation of verb) circulating fluidized bed boiler:

Advantages: Due to the unique hydrodynamic characteristics and structure of circulating fluidized bed boiler, it has many unique advantages, which are briefly introduced below.

1, fuel adaptability:

This is one of the main characteristics and advantages of circulating fluidized bed boiler. In a circulating fluidized bed boiler, the fuel is only 1%-3% of the bed material, and the rest are nonflammable solid particles, such as desulfurizer, ash or sand. The special hydrodynamic characteristics of circulating fluidized bed boiler make gas, solid and solid mixed with solid fuel very well, so the fuel is mixed with a lot of bed material concrete soon after entering the furnace, and the fuel is heated to a temperature higher than the ignition temperature at this speed, but the bed temperature is not obviously reduced. As long as the calorific value of the fuel is greater than the heat required to heat the fuel itself and the air required for the fuel to reach the ignition temperature, the circulating fluidized bed boiler does not need auxiliary fuel, but uses any raw materials. Circulating fluidized bed boilers can use not only high-quality coal, but also all kinds of inferior coal, such as high-ash coal, high-sulfur coal, high-ash high-sulfur coal, coal gangue, peat, oil shale, petroleum coke, slag skin, waste wood and garbage.

2, high combustion efficiency:

The combustion efficiency of circulating fluidized bed boiler is 97.5-99.5% higher than that of chain boiler, which is equivalent to that of pulverized coal boiler. The high combustion efficiency of circulating fluidized bed boiler is due to its following characteristics: good gas-solid mixing and high combustion rate. Especially for coarse powder fuel, most unburned fuel is recycled to the furnace for re-combustion. At the same time, the circulating fluidized bed boiler can maintain high combustion efficiency in a wide operating range. Even when burning fuel with high fines content.

3, efficient desulfurization:

The desulfurization of circulating fluidized bed boiler is more effective than other boiler types, and the desulfurization rate of typical circulating fluidized bed boiler can reach 90%. Unlike the combustion process, the desorption reaction is slow. In order to completely convert calcium oxide (burning limestone) into calcium sulfate, the sulfur dioxide gas in the flue gas must have a long enough contact time with the desulfurizer and a large enough reaction area. Of course, it is impossible to completely cover the inside of desulfurizer particles. The average residence time of gas in the combustion zone is 3-4 seconds, and the particle size of limestone in circulating fluidized bed boiler is usually 0. 1-0.3 mm, which is superior to other boilers in both the utilization rate of desulfurizer and the removal rate of sulfur dioxide.

4. Low emission of nitrogen oxides (NO2):

The low emission of nitrogen oxides is a very attractive feature of circulating fluidized bed boilers. The operating experience shows that the emission range of nitrogen dioxide from circulating fluidized bed boiler is 50- 150PPM or 40- 120mg/mJ. The reasons for low NO2 emission are: firstly, low temperature combustion, when nitrogen in the air generally does not produce NO2; Second, staged combustion, which inhibits the conversion of nitrogen in fuel into NO2 and reduces some NO2 produced.

5. Low emissions of other pollutants:

The emissions of other pollutants such as CO, HC 1, HF and other circulating fluidized bed boilers are also very low.

6. High combustion intensity and small cross-sectional area of the furnace.

The high heat load per unit cross-sectional area of the furnace is one of the main advantages of circulating fluidized bed boilers. The cross-sectional heat load of circulating fluidized bed boiler is about 3.5-4.5MW/m2, which is close to or higher than that of pulverized coal boiler.

7. Give less coal:

Due to the large cross-sectional area of the circulating fluidized bed boiler and the good mixing and expansion of the combustion area, the number of coal feeding points required is greatly reduced, and only one coal feeding point is needed, which also simplifies the coal feeding system.

8. The fuel pretreatment system is simple:

The coal particle size of circulating fluidized bed boiler is generally less than 12mm, so the fuel pulverizing system is greatly simplified compared with pulverized coal boiler. In addition, the circulating fluidized bed boiler can directly burn high-moisture coal (moisture can reach more than 30%). When burning high moisture coal, no special treatment system is needed.

9. It is easy to realize the comprehensive utilization of ash;

Because the combustion process of circulating fluidized bed boiler belongs to low temperature combustion, and the excellent burn-out conditions in the boiler make the carbon content of boiler ash low, it is easy to realize the comprehensive utilization of ash. For example, ash can be used as cement admixture or building material, and warm firing Toya is beneficial to the extraction of rare metals.

10, wide load adjustment range and fast load adjustment;

When the load changes and it is necessary to adjust the coal feed, air volume and material circulation, the load adjustment ratio can reach (3-4): 1. In addition, due to the large cross-sectional wind speed, high heat absorption and easy heat absorption control, the load adjustment speed of circulating fluidized bed boiler is also very fast, generally reaching 4% per minute.

1 1. There is no heating surface buried pipe in the circulating bed;

There is no buried heating surface in the circulating fluidized bed boiler bed, so there is no wear problem. In addition, it can be directly started after starting, stopping and coking treatment is short-term and long-term pressure fire.

12, medium investment and operating costs:

The investment and operation cost of circulating fluidized bed boiler is slightly higher than that of conventional pulverized coal boiler, but lower than that of pulverized coal boiler with desulfurization device 15-20%.

Six, circulating fluidized bed boiler problems to be further studied:

In order to optimize the design and operation of circulating fluidized bed boiler and give full play to the advantages of circulating fluidized bed, the following aspects need further study.

1. Separation of circulating materials

The working temperature of the separation device of circulating fluidized bed boiler can be divided into high temperature, medium temperature and low temperature separation, and the function forms of separation can be divided into cyclone separation and inertial separation. Judging from the current operation of circulating fluidized bed, the high-temperature cyclone separator is relatively mature. However, the problem of wear when using high ash fuel has not been solved. And the separated volume is also very large, which is basically the same as the diameter of the furnace. Limited by the maximum size of cyclone separator, large-capacity circulating fluidized bed boiler must be equipped with multiple separators. Because the cyclone separator is lined with thick wear-resistant refractory and has large thermal inertia, the start-up time of the boiler is prolonged. The dynamic characteristics of load change become worse, so it is worth discussing to adopt inertial separator because of its simple equipment, small volume and convenient structural arrangement. The flow resistance is also relatively small. In addition, the medium and low temperature separator should not be operated. According to the development requirements of circulating fluidized bed boiler, a material separation device with high efficiency, small volume, low resistance, little wear and tear and convenient manufacture and operation will be designed.

2. Selection of solid particle concentration in circulating fluidized bed;

The concentration of solid particles in circulating fluidized bed has great influence on combustion process, desulfurization process and heat transfer process. However, it is difficult to determine the proper concentration of solid particles in circulating fluidized bed. At present, an important parameter of particle concentration in circulating fluidized bed (CFB) manufacturers is circulation ratio. The circulation ratio of some circulating fluidized bed boilers in China is usually below 10, while the circulation ratio abroad often reaches 50 or even higher. When analyzing the working process of circulating fluidized bed boiler, we should not only consider the internal circulation of materials, but also consider the external circulation of materials, especially when operating at high wind speed. Therefore, the reasonable choice of solid particle concentration in circulating bed has an impact on a series of factors such as combustion desulfurization, heat transfer, wear and energy consumption.

3, heating surface layout and temperature control in the furnace

In order to ensure that the furnace temperature of circulating fluidized bed boiler is controlled within a certain range, a part of heat must be absorbed in the solid particle circulation loop. At present, there are two main ways to absorb heat in the furnace: one is to arrange water walls or partitions in the furnace; The other is to arrange part of the heating surface (such as superheater) in the furnace, and then arrange the fluidized bed heat exchanger on the solid material circulation loop. Both forms are feasible. However, the two methods control the bed temperature in different ways. The former mainly changes the heat transfer coefficient of the water wall by adjusting the return amount, thus adjusting the solid particle concentration in the bed. So as to change the heat absorption in the furnace to control the bed temperature; Otherwise, the bed temperature can be controlled only by adjusting the ratio of solid materials entering the fluidized bed heat exchanger to the hot connection furnace, which is relatively flexible and especially suitable for large-capacity circulating fluidized bed boilers.

4. Determination of working wind speed (or cross-sectional heat load)

The operating wind speed of circulating fluidized bed boiler is an important parameter. General operating wind speed is 4- 10m/s/. Increasing the running wind speed will make the furnace more compact. The cross-sectional heat load increases accordingly. At this time, in order to ensure that the fuel and limestone particles have enough residence time and arrange enough heating surfaces, the height of the furnace must be increased. This not only increases wear and tear, but also increases the cost of the boiler. The fan power will increase, and the auxiliary power will increase accordingly. But if the wind speed is too low, the advantages of circulating fluidized bed will not be brought into play, so it should have the best operating wind speed for all kinds of fuels.

5. Return mechanism:

In the circulating fluidized bed, the separated solid materials must be sent back to the furnace through the feeding mechanism. The material return mechanism should also flexibly adjust the material return amount. However, due to the high temperature and great wear of the material return mechanism, if adjusting devices such as general mechanical valves are used, it is easy to cause jamming and ineffective rotation. At present, non-mechanical valves are widely used in circulating fluidized beds. (L valve) and fluidized bed feeding mechanism, on the one hand, adjust the material flow, on the other hand, prevent the fuel from being connected in series with the separator in the combustion chamber, resulting in short circuit. At present, many manufacturers keep the return mechanism confidential.

6, circulating fluidized bed boiler parts wear:

Due to the high particle concentration and high wind speed in circulating fluidized bed boiler, the wear of boiler parts is more serious. Wear is mainly related to wind speed, particle size and inhomogeneity of flow field, and wear is directly proportional to wind speed and concentration. In the design, the form of sudden contraction and sudden expansion of flue gas corridor should generally be prevented. At present, research is relatively weak.

7, low pollution fuel:

Circulating fluidized bed boilers have developed rapidly. One of the important reasons is the low-pollution fuel characteristics of circulating fluidized bed. At present, the research on desulfurization is relatively consistent, but there are still many contents to be studied in the optimal desulfurization temperature and efficient utilization of desulfurizer. Such as reducing NO2, bed temperature, flue gas recirculation, ammonia injection and the influence of desulfurizer on NO2 need further study.

9. Design of tail heating surface;

At present, in circulating fluidized bed boilers, the design of the heating surface of the tail flue is generally ignored, and how to arrange the heating surface of the tail flue more reasonably needs further study.

10, dust removal:

At present, most tail flue in China adopts electrostatic dust removal.

Seven, the development of circulating fluidized bed boiler:

Abroad: The research started in 1960s, and it was Austrond Company in Finland. The first commercial circulating fluidized bed boiler was transformed from an oil-fired boiler with a thermal power of 15mw. Later, the United States and Ba Telie multi-solid circulating fluidized bed boilers in Germany, Sweden, Canada, Italy and other countries made various types of circulating fluidized bed boilers, the largest of which was the circulating fluidized bed boiler, with the power generation capacity of 165MW and that of Canada of 65438+.

In the 1990s, circulating fluidized bed boilers should meet the following technical standards:

(1) combustion efficiency100%;

(2) The power plant efficiency is more than 40%;

(3)SO2 emission is less than 65438±00 ppm；;

(4)NO2 emission is less than 30PPM.

)。