OCR Text |
Show of cross-beam burners situated at locations three meters abart. The cross-beam burner , consisted of concentric pipes with air and gaseous fuel fed from one end of the horizontal pipes. The gas nozzles were located at the lower edge of the pipe thus,the issuing jet/flame is directed downwards. At locations lower than the crossbeam burners, i.e. at 1.0 m, six perepheral burners are located to assist the combustion and these act as secondary burners. The portion of air and fuel injected through these burners is small; the air necessary for combustion is fed from the lower end of the furnace by a fan that supplies both the cooling and combustion air. The air mixes with the gaseous fuel and the products of combustion are allowed to flow vertically upward towards the suction fan and exit. INSTRUMENTATION AND PRECISION-Gas temperature measurements were obtained with the aid of industrial type shielded thermocouples made of Chromel-Alumel wires and the sheath outer diameter was 0.02m. The precision of the gas temperature measurements was ±30 K . A stainless steel sampling probe was used to measure CO and CO 2 concentrations;the probe had an inner and outer diameters of 0.003 and 0.005 m respectively. The probe was connected to a portable CO/CO~ infrared gas analyser of the dry type, calibrated to yield CO % concentration and CO mole fraction 2in ppm . The species concentration values were subject to systernrnatic error of around ±lO%,see ref.8. The furnace outer surface wall temperatures were measured with an industrial sensor thermocouple Copper-Constantan wired. The obtained measurements were used to calculate the local heat flux at the wall using the following formula,ref.8: h S ( t s t ) ........... ~(5) a where the value of h,the heat transfer coefficient is obtained from the relevant heat transfer data books, t being the ambient temperature and t ~he surface temperature. The surface grea S is calculated for each location from the furnace geometry. The estimated error in such procedure is less than 10%. MEASURED PROPERTIES- Figure 5 shows some measured furnace temperature along the vertical axis. The measured distribution was taken at 0.15m from the furnace inside wall. The distribution shows a zone of high temperature near the zone 17 to 21 m from the furnace lower end. The temperature decayed towards t~e furnace exit from 1000°C to nearly 300 C 103 at a distance of 30 m from furnace inlet. The observed distribution identifies a fairly large combustion/calcination zone. Figure 6 shows the wall temperature distribution in terms of (t -t ) ,the average wall temperature at e~chalocation is obtained around the furnace circumference. The calculated heat transfer rate is then obtained with the aid of eq.5. The product h(t -t ) is shown also in figure 6,and is norffial~zed by the maximum value to yield the non-dimensional heat flux distribution. The abovementioned experiments were carried out under normal running conditions and reflect the type of scatter that one can get in such flow configuration. The measured axial distribution of the wall temperature showed clearly the distinct zones in the furnace. More comprehensive measurements are required to adequately analyse the furnace behaviour. RESULTS AND DISCUSSION The present computational procedure was applied to the flow situation in the vertical lime kiln. The numerical grid comprises 20x20 grid nodes and the flow pattern, temperatures and heat flux distributions were obtained when the governing equations were satisfied to within 0.01%. The converged solution was plotted for the flow configuration where the void fraction € is 0.45, air to fuel ratio of 0.7 and t~e fuel calorific value of 7117 kj/m were used3 .The total air volume flow rate was 0.667m /s and the c~rresponding fuel flow rate was 0.972 m Is. Figure 7 displays the radial profiles of mean axial velocity component at various locations along the furnace. The obtained predictions showed a peak velocity towards the wall which is attributed to the combustion generated accelation. It is worthnoting that the predicted velocities can be readily obtained although the experimental values were not available due to the complicated nature of the flow regimes and difficulty of measurements. In figure 8 the centreline temperature variation is shown for the same flow configuration and it clearly demonstrated the peak temperature zone where the calcination takes place . The temperature gradually increased from room temperature upstream until peak temperature persists for a distance of about one furnace diameter,D f • The temperature then decreased towaras the top of the furnace where the fresh charge is being loaded. Figure 9 shows measured and predicted wall heat flux distributions for the furnace under normal conditions. It can be seen that the peak heat flux occurs at about 4 D f and the heat flux then decays |