UO 09 STEAM GENERATOR 1. INTRODUCTION In many engineering processes the first step is the production of heat energy by combustion. One way to distinguish between combustion processes is according to whether the energy produced by combustion is intended to be retained in the products of combustion or transferred to an external fluid (such as water). In boilers for electricity generation, space heating or raising of process stream, the usefulness of the combustion process depends on the proportion of the calorific value of the fuel transferred as heat energy to the external working fluid; any heat retained in the flue gases is largely wasted. The steam generator pilot plant uses water as the feed fluid and propane as the combustion fuel to produce steam. The steam vapour is subsequently condensed and recycled back around to the steam generator. The Air/Fuel ratio and flowrate of gas can be moderated. The water feed flowrate using the positive displacement pump can be varied. The output combustion gases can be analysed, allowing the calculation of an energy balance over the unit. Various key temperature, pressure, flowmeter indicators are fitted. The steam pressure can also be moderated using a control valve (adjustable PRV, V7, see Figures 1 & 2). 27 30 31 34 36 37 38 225 2333353 28 29 1 Cooling water inlet valve V10 26 Waste gas analysis device 2 Cooling water flow meter F2 27 Flow safeguard 3 Cooling water feed 28 Exhaust pipe 4 Condensate return 29 Exhaust gas measuring point O2, CO₂. CO 5 Condensate return valve V11 30 Temperature measuring point T7 6 Process schematic 31 Steam boiler A2 26 25 24 23 22 21 20 19 18 17 16 15 14 7 Switch for condensate pump 32 Live steam pressure sensor P1 8 Main switch 33 Condensate trap 9 Steam connection for consumer units 34 Ignition electrodes 10 Main steam shut-off valve V8 35 Air feed to the boiler 13 11 Steam temperature controller for 36 Feedwater pump P1 12 superheater 11 12 Temperature selector 37 Fine adjustment valve for gas V1 10 13 Measurement readouts 38 9 Gas connection and gas main valve with thermal cutout 8 7 14 Signal lamps 39 Lockable roller 6 15 Reset button 40 Valve for draining (pipe) 5 # 16 Pressure sensor for condenser P2 41 4 Differential pressure sensor Feedwater level L1 3 2 17 Condenser A1 42 Valve for draining (feedwater tank) 18 Condenser pressure controller 43 Concentrate pump P2 19 Non-return valve V6 44 Valve for draining (condenser) 20 Live steam pressure controller 45 Water jet pump P3 21 Pressure retention valve V7 46 Shut-off valve for water jet pump V9 22 Steam safety valve V5 47 Power supply connection 23 Ventilation 40 41 42 43 44 45 46 47 48 49 50 48 Cooling water discharge 24 Feedwater tank B1 49 USB connector 25 Superheater W1 with water trap A3 50 Table frame 2. AIMS Figure 1: Steam generator module (Boxhammer, 2012) 1. To understand the operating principles of a steam generator plant. 2. Experimental determination of the turndown ratio of the burner for propane. 3. Calculation of the stoichiometric air/fuel ratios and net combustion heat release. 4. To understand the importance of determining a heat balance for a chemical reaction (combustion). 5. To understand the principles and calculation of combustion efficiency. 1 3. OPERATING PROCEDURE KTC Superheater W1 Water trap (T5 A3 P1 V5 X1 P3 V8 Main steam Water jet pump CO₂ 0₁₂ Condensate Vent pipe Boiler trap ☑ ☑IV9 V6 (T3) Feed- T1 T6 Conden water A2 ser tank B1 P2 A1 Cool- T2F2 ing (L1) 山 L V10 (P3 5 2 V1 V3 (F3 V4 -- Gas F1 T4 V12 P1 Feedwater pump Condensate return L V11 P2 Condensate pump V13 on the gas cylinder Figure 2: P&ID of steam generator (Boxhammer, 2012) Al Condenser A2 Boiler A3 Water trap B1 Feedwater tank P1 Feedwater pump P2 Condensate pump P3 Water jet pump X1 Condensate trap V1 Fine adjustment valve for gas V2 Gas solenoid valve V3 Gas pressure regulator V4 Gas main valve with thermal cutout V5 Steam safety valve V6 Non-return valve V7 Pressure retention valve V8 Live steam shut-off valve V10 Cooling water inlet valve V11 Condensate return valve V12 Feedwater non-return valve F1 Feedwater flow F2 Cooling water flow F3 Gas flow L1 Feedwater level P1 Live steam pressure P2 Pressure in the condenser P3 Gas pressure T1 Temperature in the condenser T2 Cooling water inlet temperature T3 Cooling water outlet temperature T4 Feedwater temperature T5 Live steam temperature superheater T6 Saturated steam temperature T7 Exhaust gas temperature Table 1: Key to P&ID V9 Shut-off valve for the water jet pump 2 NOTE: The exhaust chamber surface of the burner is extremely hot, this can reach in excess of 250 °C. Start-Up (refer to Figure 2 and panel): 1. Ensure that all valves are closed (especially check that V8 and V11 are fully closed, this is essential for safety), except V7 which should be fully open. 2. Switch on the power supply. 3. Open V10 (cooling water) and adjust the flowrate to 80 l/hr at F2. 4. Check that the feed water level in vessel B exceeds 2200 ml. If lower than 2200 ml ask academic staff to top this up. 5. Check that the level in condenser (A1) does not exceed 300 ml. If more than 300 ml in A1, excess liquid can be transferred to vessel B using the condensate pump, P2 (the start switch for P2 is on the panel next to the main power supply). 6. Ask Academic Staff to Do: Open main valve on gas cylinder, and open and adjust the pressure regulator to 1 bar. 7. Open V3 (yellow handled valve on gas cylinder line under the desk). 8. Open V1, by turning anticlockwise 3 complete turns. 9. Start feed pump P1 and set flowrate to 8 l/hr (visual display on pump) and start condensate pump P2. 10. Ignition stage: Maintain a distance of at least 30 cm from the burner housing. Press reset button on panel and then press reset burner button on panel. If the burner does not ignite wait 3 minutes before attempting to re-start. 11. Check ignition (look for flame in burner), adjust V1 to set a gas flowrate of approximately 5.0 1/min at F3 (note adjust slowly as valve is sensitive). 12. Fully open V9 and adjust V10 to the keep flowrate at 80 l/hr at F2. 13. Adjust V7 to set the steam pressure to 8 bar at P1 (Note: there is a lag between adjusting V7 and the pressure change so do this slowly). 14. Allow system time to stabilise (this occurs when you have a consistent flame in boiler, and instrument measurements level out) before commencing experiments. Note: Ensure A1 level does not exceed 400ml, and feed tank does not empty. Operation: Experiment 1 (Varying Gas Feed Flow): Start the equipment as described in the start-up procedure. Vary the gas fuel flowrate at F3 at values of approximately 4, 5, and 6 l/min. Keep the water feed flow at 8 l/hr and steam pressure at 8 bar. Experiment 2 (Varying Steam Pressure): With a fuel flowrate of 5 l/min, water feed flow rate of 8 1/hr, vary the steam pressure at values of 7, 7.5 and 8 bar by adjusting V7. Experiment 3 (Varying Feed Water Flow): With a fuel flowrate of 5 l/min, steam pressure of 8 bar, vary the water feed flow at values of 7, 7.5 and 8 1/hr. Experiment 4 (Varying Air Flow): With a fuel flowrate of 5 l/min, steam pressure of 8 bar, and feed flow of 8 1/hr, vary the air flow (ask staff to help do this). Experiment 5 (Turndown Ratio): Determine the turndown ratio of the boiler. Starting with the conditions described in the start-up procedure, adjust V1 until the maximum gas flowrate is obtained at F3. Then adjust V1 slowly until the flame in the boiler goes out. Make a note of the high and low gas flowrates. For experiments 1-4 you will need to record the temperature of the feed water in, the steam temperature and pressure, the feed water flow rate, the gas flow rate, the ambient air temperature, the flue gas temperature, the % by vol of O2 in the flue gas, the % by volume of CO2 in the flue gas, the air/fuel ratio (2) value (1.0 indicates stoichiometric air, 1.2 indicates 20% excess air) and the combustion efficiency 3 (n). These final values are obtained using the DRAGER gas analyser, the use of which is explained in the video (but will be recapped on the day). Shut down: 1. Close V1. 2. Close V3. 3. Switch of gas bottle (Staff to do this). 4. Switch off P1 and P2. 5. Wait 5 minutes. 6. Close V9 and V10. 7. Switch off power. 8. Close all valves except V7 which should be opened fully. 4. THEORY 4.1 Turndown ratio Turndown ratio refers to the width of the operational range of a device and is defined as the ratio of the maximum capacity to minimum capacity. For the steam generator the turndown ratio can be defined as: TD=max gas flow / gas flow when flame goes out 4.2 Heat Balances (see Felder and Rousseau (2016); Smith et al., (1996)) The stoichiometric equation for the complete combustion of propane is given as: C3H8 (g) +502 (g) → 3CO2 (g) + 4H₂O (1) (4.1) (4.2) In order to calculate the heat given out by this reaction equation (4.3) is used: AHReact Σni Hfi products - Σni Hfi reactants (4.3) where AHReact is the heat of reaction, n; is the number of moles of component i and Hƒ¡ is the heat of formation of component i (Note: All of this data is usually obtained at 25 °C and can be found in Appendix 1). In this case the heat of reaction is also known as the heat of combustion. There are different heats of combustion available in the literature as some reactions have the water product as a gas whilst others have the water product as a liquid (as in this case). The gross calorific value or standard enthalpy of combustion at 298 K is when the water product of combustion is in the liquid state (as in equations 4.2 and 4.3), whilst the net calorific value is found by taking into account the heat of vaporization of water at 298 K e.g. AHReact NetΣni Hfi products - Σni Hfi reactants + NH2O H20@298K (4.4) so that, C3H8 (g) +502 (g) → 3CO2 (g) + 4H₂O (g) (4.5) We now need to consider the overall heat balance in the steam generator, In the generator, propane and air are taken in (remember air is 21% by mol (or vol) O2 and 79% by mol N2) at room temperature and are then combusted to give out heat. This heat is taken up by the feedwater so that the water boils to create steam. The rest of the heat is either taken up by the flue gases or is lost to the environment. In order to calculate the amount of heat available for uptake by the boiler water we need to construct a van't Hoff box on the combustion system as shown in Figure 3. The heat balance over the box gives: AHoverall = AH1 + ▲H2+ AH3 (4.6) 4 Ti AHoverall Reactants AH₁ 298 K ΔΗΣ Figure 3: The van't Hoff box where, AH₁ = Σni Cpi, Ti (298 – Ti) AH₂ = AHReact Net AH3 = Σni Cpi, Tf (Tƒ – 298) AH3 Products 298 K (4.7) (4.8) (4.9) and Cpi is the specific heat of component i, T; is the ambient temperature and Tƒ is the flue gas temperature. Note: Values for heats of formation of individual species and specific heat capacities of gases are given in Appendix 1. A positive value of AH; signifies that heat is added to the system, while a negative value indicates that heat is lost by the system. The heat taken up by the water, which becomes steam can be determined from the enthalpy of the water feed subtracted from the enthalpy of the steam out: Qsteam-m (hg-hf) (4.10) where m is the mass flow rate of water into the system, hg is the enthalpy of steam at the given temperature and pressure and hƒ is the enthalpy of the feed water in at the given temperature of the feed. 4.3 Characteristic Values of the Steam Generator 4.3.1 Evaporation rate The boiler's evaporation rate (mst) is calculated from the supplied quantity of feedwater (Vw) and its density at the feedwater temperature. mst Vfwp@Tfeed (4.11) 4.3.2 Combustion efficiency The combustion efficiency indicates how much of the energy contained in the fuel is used during combustion by the steam generator. The remaining energy is lost in the form of exhaust gas losses. It is defined as follows: η = 1 - (ΔΗ3 / ΔΗ2) (4.12) Note: (i) When using the efficiency equations remember to use the magnitude for AH; not the +/- value. (ii) This value can be compared to the value given by the DRAGER gas analyser(eta [n] value). 5