Laboratory Notes. Heat transfer measurements in fluidized bed combustion reactor (approx. 2-3 hours laboratory exercise)

Transcription

1 Laboratory Notes Heat transfer measurements in fluidized bed combustion reactor (approx. 2-3 hours laboratory exercise) By Jeevan Jayasuriya /Arturo Manrique Division of Heat and Power Technology STOCKHOLM 3-feb-05 The original text was written in Swedish. Andres Norstrand translated the text and Jeevan Jayasuriya has reviewed it and prepared the laboratory note. Laboratory Notes Avdelningen för Kraft- och Värmeteknologi Kungliga Tekniska Högskolan STOCKHOLM

2 Introduction Fluidised Bed Combustion has became a matured technology today. This has been introduced and developed in last 20 years. The main working principle is that the fuel is combusted in a turbulent bed of sand and ash which implies good heat transfer and mixing. Since 1990 a coal fired Pressurised Bed (PFBC) is used as a base load unit in Stockholm. Similar units are reported in operation in Spain, Germany, Japan and United States. The main advantage with PFBC is the low combustion temperature which implies very low emissions of sulphur and NOx. PFBC units are compact in size. The PFBC unit, which is operating in Stockholm has been built as a part of combined cycle power generation and it produces heat and power. Disadvantages are the price and the complexity of the system. Mainly for these reasons no more PFBCs were built in Stockholm. In Sweden, there are a lot of Circulating Fluidized Beds (CFB) in operation using biomass as fuel. In CFB applications, the air entering from the bottom has a higher velocity and it makes whole bed blowing away from the furnace. At the top the bed, material is separated from the flue gases and it returns to the bottom. One disadvantage using biomass could be high temperature corrosion on steam tube packages. Characteristics A way of describing the process of fluidisation is to imagine a bed of solid particles kept in position on a plate inside a vertical tube. The plate is perforated so that air can enter from the bottom through the plate and pass through the bed. Different air flows give different properties to the bed. Static Bed Starting Fluidization Full Fluidization Figure 1: Illustration of different bed conditions

3 At very small flows the air will pass between the particles without disturbing their position. The bed is then called static bed and acts like a filter. By increasing the air flow to a higher level, some particles in the bed will start to move depending on the uneven air distribution in the bed. At higher air flows particles will be separated from each other and the bed volume increases. This is incipient fluidisation. A higher contact area between the air and the particles will be the result, which is an advantage during combustion. Even at higher air flows the particles will be mixed with each other and the bed is more like a boiling fluid. Now we are talking about full fluidisation. This is the domain where most advantages with fluidisation can be found. Continuous increase of the air flow means bigger size of the air bubbles and at the end these will occupy the whole width of the bed. The phenomenon is called slugging and it is very common when using gas as a fluidisation medium. When the flow is increased further more, the particles will be blown away together with the air stream. This is called pneumatic transportation of particles. CFB boilers work in this domain. Combustion in a fluidised bed During combustion in a fluidised bed the air is supplied through a distribution plate placed at the bottom of the bed. The air fluidises the bed. The best way of using the bed is to use the right amount of air that makes the full fluidisation in the bed. The bed itself consists of fuel, ash, sand and a sorbent (could be dolomite). The mass fraction of fuel is very small, only around 1 %. Sand is only used if there is not enough amount of ash or sorbent which could be the case if oil is used as a fuel. The sorbent is used for capturing the sulphur through reactions in the bed. There are 7 important parameters which determine the characteristics of the bed: 1. Bed Temperature. Normal temperatures used are around 750ºC-950ºC depending on the use and the fuel characteristics. To control the temperature of the bed, bed must be continuously cooled. This is done circulating water through the cooling tubes immersed in the bed. The tubes normally contain water for steam- or hot water production. 2. Particle Size. This is strongly connected to the fluidisation velocity. Lower the velocity the lower particle size is necessary to keep the same fluidisation. 3. Fluidisation Velocity. Depending on the particle size the fluidisation velocity is kept in the interval 0.4 m/s- 4 m/s. The oxygen in the air is necessary for combustion which means higher velocities in deep beds. Low beds with big cross sectional area use lower fluidisation velocity.

4 4. Bed height. Common values are 1 metre 4 metre. (4 m in PFBC). Increased fluidisation velocity gives higher bed level. 5. Tube location. I the case with immersed cooling tubes, the tube size and the tube placement strongly affect the effectiveness of the bed. Wrong dimensioning of tube placement will lead to pressure drops and worse heat transfer. 6. Heat Transfer Coefficient. Heat transfer from the bed to the cooling tubes is due to convection and radiation. The radiation part is affected by the temperature only. The convection part is affected by particle size, temperature, distance between tubes and the fluidisation velocity. Convective heat transfer increases when the tube temperature increases, smaller particles and higher distance between the tubes. The fluidisation affects the convective heat transfer in such a way that the particles destroy the thin air layer surrounding the tubes. Increased convective heat transfer will be the result. 7. Pressure drop. The pressure drop in a fluidised bed is dependent on the fluidisation velocity at low air flows. The pressure drop increases with increasing velocity up to incipient fluidisation. Increasing the velocity from this point does not affect the pressure drop anymore. The pressure drop remains constant until pneumatic transportation starts. In a power plant there are also pressure drops through eventual dust separators and distribution plates. These drops increase continuously with increasing velocity. Advantages with combustion in a fluidised bed In the fluidised bed combustion the combustion temperature is kept low compared to conventional combustion. This gives several advantages. Since the temperature is kept in the interval of 750ºC-950ºC (which is around 1000ºC-1600ºC for conventional combustion furnaces), the following advantages can be achieved. Low NOx emissions. There are no thermal NOx at all (from air nitrogen). This production needs a temperature from 1500ºC, which means that the NOx produced in a fluidised bed comes from fuel nitrogen only. The sulphur in the fuel reacts with the sorbent at the chosen temperature and the product leaves with the ash. The cleaning of sulphur is close to 100 %. A minimum of corrosive products are created. The low temperature prevents ash fusion and keeps alkali levels low enough to satisfy corrosion considerations. No need for similar high temperature materials in superheaters.

5 Good heat transfer due to particle convection. This means that the heat transfer surface can be constructed smaller for the same power. Even better if the unit is pressurised. Fuel flexibility Disadvantages with fluidised bed combustion Erosion problems on immersed tubes Ash fusion because of wrong air distribution Fuel feeding is complex (PFBC) Ash removal (PFBC) Production of N 2 O which is a very strong greenhouse gas. Description of Measurement equipment In the fluidised beds, which are used for steam production the bed is cooled by steam tube packages. In this laboratory exercise, the operation of the bed is set at opposite. The bed is heated with electrical elements. The characteristics of the bed will anyhow be the same. The central part of the equipment is the fluidised bed, which consists of a glass tube with metal plates at the top and at the bottom. Air enters the bed through the lower plate and the distribution filter. The purpose of the distribution plate is to distribute the air for bed operation and also to make sure that the bed material will not fall down when the unit is switched off. To prevent the bed material to blow away at high air flows, there is a filter at the top plate. The air from the pressurized system enters the unit through a reducing valve, from which it is possible to regulate the air flow. Afterwards the air flows through two rotameters connected in series. The meter has the measurement range from 0.4 to 4 m3/h and the second meter has the measurement range from 1.6 to 16 m3/h. The first rotameter gives the best accuracy and it is advisable to choose the first meter when it is possible. The air then goes through the bed and afterwards out to the surrounding air. An electrical heating element is mounted at the top plate of the bed. It is easy to move the element up and down in the bed. The power from the element can be regulated from a thyristor aggregate. The regulation can sometimes, when the power is turned to zero, suddenly jump to higher values. If this happens, turn up the voltage dividing resistor a little, and the indicating pointers again will change to low values.

6 There are four thermocouples in the fluidised bed but there is only one temperature display panel available. By using the selection keys, it is possible to choose the corresponding thermocouple to display the required temperature. The keys corresponding to display temperature at different thermocouples: Button 1 Button 2 Button 3 Button 4 Thermocouple under the filter Thermocouple in the bed Thermocouple at the top of the heating element Thermocouple on the side of the heating element When the buttons 1-3 are switched on, thermocouple 4 is connected to a temperature safety device which cuts off the power when the temperature reaches 200ºC. When button 4 is switched on, thermocouple 3 is connected to the safety device. The pressure drop through the bed can be achieved with a movable probe, which measures the static pressure. The pressure can be read on a U-pipe which contains water. Figure 2: A picture of the fluidised bed laboratory equipment 1 Rotameters 13 Ammeter 2 Jack panel 14 Voltage recorder 3 Inlets 15 Temperature indicator 4 Glass tube 16 Temperature safety device 5 Pressure indicator 17 Air inlet control 6 Thermocouple 2 18 Power control 7 Heating element 19 Key set for thermocouples 8 Thermocouples 3 and 4 20 Safety device 9 Fluidised bed 21 Circuit breaker 10 Distribution filter 22 Tube connection 11 Thermocouple 1 23 Cord 12 U-pipe

7 Rotameters 2 Fluidized Bed Voltimeter Temperature Guard Amperimeter Temp Display 3 U pipe Switch box 4 1 Power Regulator Fuse Valve Presurised Air Figure 3: Layout of the equipment V: Voltage indicator A: Ammeter T: Temperature security device C: Temperature indicator R: Power regulation Button Display shows Security connected to 1 Thermocouple 1 Thermocouple 4 2 Thermocouple 2 Thermocouple 4 3 Thermocouple 3 Thermocouple 4 4 Thermocouple 4 Thermocouple 3 Table 1: Key settings for thermocouple selection

8 PERFORMANCE 1.0 Measurement of pressure drop and bed height Adjust the flow according to the values given in table 2 and perform following at each setting: - Measure the pressure drop through the bed. Please note that two values are necessary because of the drop through the top filter - Measure the bed height and note special observations Air flow Rota meter m 3 /h Meter 1 Meter Pressure drop mm vp Sand bed Top sandbed +top filter filter Bed height (mm) Special observations Table 2: Test conditions and data table for pressure drop and bed height measurement When the measurements are completed, mark those points on diagram 1.

9 2.0 Determining the heat transfer coefficient One of the main advantages of fluidised bed technology is the high heat transfer coefficient between the bed and the immersed tubes. The students are supposed to calculate (1) the heat transfer coefficient between the heating element and the bed and (2) the heat transfer coefficient between the element and the free air stream above the bed. For heat transfer is given: P = h *A * t where P = power [W] h = heat transfer coefficient [W/m2,K] t = temperature difference during heat transfer [K] A = heating element area = m2 2.1 The element in the bed, constant surface temperature Because of the long heating time for the sand at low air flow, it is advantageous to start with the highest air flow. Before starting the measurements, the element is submerged 4 cm into the bed. It is easier if the bed is fluidised a little bit. Perform the following measurements for the air flows due to table 3: - Adjust the air flow - Turn the power and try to find an equilibrium state when the surface temperature (thermocouple 3) shows 190ºC. At the first measurement point you have to wait around 10 minutes to reach stability because of the heating of the sand. At a flow rate of around 6 m 3 /h the stable sand temperature would be around 55ºC. - Read voltage, current and bed temperature to calculate the heat transfer coefficient. The temperature in the bed should be measured as close to the element as possible. At small power it is difficult to read the voltage on the recorder, so in this domain it is better to calculate the power by setting the resistance to 120 ohm.

10 When all the values are calculated, plot the heat transfer coefficient as a function of the flow in diagram 1. Air flow Rotameter 1st 2nd m 3 /h Current A Voltage V Bed temp C Element temp C t K Heat transfer coefficient W/m 2 K Table 3: Test conditions and data table for measurement part The element in the air, constant surface temperature Perform same measurements as in 2.1, but keep the element above the bed. You only need two measurements (two points), one in higher flow and other is smaller flow. Calculate and plot the heat transfer coefficient in diagram 1. Draw a straight line between the points. Air flow Rotameter 1st m 3 /h Current A Voltage V Bed temp C Element temp C t K Heat transfer coefficient W/m 2 K Table 4: Test conditions and data table for measurement part The element in the bed, variable surface temperature The heat transfer coefficient, in addition to the flow rate of air, also depends on the surface temperature of the heating element. To investigate this parameter, if there is enough time, students can make measurements by varying the surface temperature (heating element) at constant air flow. The element should be submerged into the bed 4 cm. In order to save time, take one value from table 2 where the surface temperature is kept at 190ºC. Calculate, for the different temperatures, the heat transfer coefficient in the same way as earlier.

11 Air flow Rotameter 1st m 3 /h Element temp C Bed temp C t K Current A Voltage V Heat transfer coefficient W/m 2 K Table 5: Test conditions and data table for measurement part 4 REFERENCES Fransson, T. H. et al.; CompEduHPT: Computerized Educational Program in Heat and Power Technology Division of Heat and Power Technology, KTH, SE Stockholm, Sweden. Kunii, D., Levenspiel, O.; Fulidization Engineering 2 nd Edition, Butterworth-Heinemann Cooperation

12 Diagram Air flow (m3/h) Heat transfer coefficient (W/m2K) Pressure drop (mm H2O)