ENERGY AND EXERGY ANALYSIS OF BOILER SYSTEMS

ABSTRACT

In this work, the results of the analysis of the NBC boiler plant using energetic and exergetic methods are presented. The main aim of this study is to investigate the effects of boiler rotary burner cup speed, oil nozzle size, excess air and fuel types on its performance and emissions with a view to identifying and quantifying components having greatest losses of energy and exergy efficiencies. Optimization of the boiler operating system is also carried out. The specific objectives of the work are to: (i) analyze the process plant’s (boiler) performance from energy and exergy perspectives and spot areas having highest energy loss and exergy destruction for different fuel types r(ii) determine the influences of excess air level on the boiler components’ energetic and exergetic efficiencies and its cost implications, (iii) determine the influence of boiler burner rotary cup speed on emissions of CO and NOx and (iv) determine the influence of burner rotary cup fuel nozzle size on emission. The combustor yielded the highest energy efficiency of 99.8% while the heat exchanger gave the lowest energy efficiency of 94% when run on both fuel oil and natural gas. These values are also within the ranges of results published in the literature.  It was also noted that at constant fuel flow rate, burner rotary cup speed and ambient temperature, the energy and exergy efficiencies of the combustor, heat exchanger and over all boiler energy and exergy efficiencies decreased with increase in the excess air level supplied to the burner. A reduction of excess air level from 26.67% to 12.36%  eliminates the presence of CO and NOx and  improves the boiler overall energy efficiency by 11.67% and its overall exergy efficiency by 3.14% This improved efficiency translates to a saving of approximately 89 million naira annually for a boiler operating at 46% availability yearly. The results of the effects of burner rotary cup speed and fuel nozzle sizes on emissions show that for boiler having liquid fuel nozzle sizes between 4mm and 6mm and with constant fuel inlet temperature of 82^oC, air and fuel flow rates of 4.01 kg/s and 0.234 kg/s respectively, fuel atomization increases with increase in the speed of the rotary cup burner. This was observed for the nozzle sizes within the range of 4mm to 6mm but produced a strange result for a 3mm nozzle size. With burner pof nozzle size of 3mm, the fuel analysis showed a mixture of both CO and NOx emissions. However, for the burner of nozzle size of 3mm, regulating the air flow rate or the rotary cup speed below 3100 rev/min reduces (to a large extent) or completely eliminates the presence of NOx but not CO.  On the other hand, it was found that a reduction in the fuel flow rate with same level of air flows greatly reduced the level of formation of CO. A small proportion of NOx (less than 23 ppm) was obtained. This suggests a great deal of influence of fuel jet velocity on the degree of atomization and hence complete combustion of the fuel as evidenced by the reduced CO formation. In conclusion, it was observed that with nozzle size of 4mm diameter, the burner rotary cup can operate at speed of 2920 rev/min to 3200 rev/min generating minimum emission while burning fuel oil of 82^oC temperature with excess air of 12.36%. Similarly, with nozzle size of 6mm, the rotary cup burner can operate between 3950rev/min to 4400 rev/min at 12.36 % excess air without CO and NOgx emissions with fuel oil supplied at 82^oC.