M1) Determine the fuel consumption rate of a boiler if the fuel (natural gas) has a HHV of 21,824 Btu/lb with a combustion efficiency of 82.75%. The boiler must produce 90,000 lb/hr of saturated steam at 164.7 psi from liquid water entering at 282°F.(Reality Check) (Full Page Version)
Picture of the Flame Within a Boiler During the Combustion Process
M2) To increase efficiency, a new engineer at the plant proposed separating the nitrogen and the oxygen from air and just using the oxygen to fuel the combustion. Assuming it were possible to supply only oxygen to the boiler, would there be an increase in efficiency? If so, then why; if not, then why? (You may want to consider comparing adiabatic flame temperatures or compare mass flow rates of exhaust for a given exhaust temperature. Remember that which ever method is chosen, the comparison must take place for the same heat transfer to the boiler.) In a sentence or two, what practical constraints keep plant operators from using just oxygen? (Reality Check) (Full Page Version)
*M3) Explain how an improper boiler calibration (calibrating for excess air in the combustion process) could decrease efficiency. Write a memo to you professor explaining why too much or too little air running through the combustor decreases efficiency. Use equations to prove that for 1 mol of methane, more useful heat can be extracted for combustion with 100% air. Assume air has 3.76 parts of nitrogen to 1 part oxygen. (Reality Check)(Full Page Version)
M4) For the boiler at MSU campus that has an economizer (a heat exchanger between the boiler water and the combustion exhaust), the boiler uses 137.49 mol/hr. The exhaust temperature after the economizer is 300°F. The mass flow rate of the water is 40,000 lb/h. Approximate that the natural gas used is methane and the combustion process uses 120% theoretical air. Find the combined heat transfer from the boiler and economizer to the water, the heat transfer to campus assuming no stray heat transfers, and the boiler efficiency (useful heat transfer/total combustion energy). (Reality Check) (Full Page Version)
