Reference no: EM13216189

A boiler burns gas with the following composition (v/v) - 80% methane (CH4), 15% ethane (C2H6), 4% CO2, and 1% nitrogen. The burner is operated with 20% excess air (21% O2, 79% N2). The inlet gas and air are both at 25oC.

The boiler can be considered in three separate stages: (i) mixing of gas and air at atmospheric pressure, (ii) combustion of the gases, and (iii) heat exchange between the gases (cooling) and the water (heating) to produce steam and flue gas.

Reactions of combustion are:
CH4+2O2?CO2+2H2O C2H6+7/2O2?2CO2+3H2O
Heat of combustion at 25oC to water vapour and CO2 are:
Methane 803 kJ/mole ethane 1429 kJ/mole
Specific heat capacities of gases can be assumed constant as follows:
methane 2.80 kJ/kgoC ethane 2.60 kJ/kgoC
CO2 1.05 kJ/kgoC O2 0.96 kJ/kgoC
H2O 1.93 kJ/kgoC N2 1.05 kJ/kgoC

(a) Perform a mass balance for the combustion of 1.0 m3 of gas at 1 atmosphere pressure to determine the flue gas composition (use molecular weights rounded to the nearest whole number).

(b) Perform an energy balance and estimate the temperature of the combustion gases immediately after combustion has occurred but before the gases have lost heat to produce the steam (assume complete combustion of the methane and ethane and no side reactions).

(c) Estimate the amount of heat that the combustion gases can lose as they cool to the exhaust flue gas temperature of 400oC.

(d) How much steam will the boiler produce per m3 of gas if the feed water/condensate mixture returning to the boiler is at 50oC and the steam produced is 39 bar.g and 400oC?

(e) What is the thermal efficiency of the boiler (useful heat as steam divided by heat in fuel)? Assume the steam leaves any equipment as saturated condensate at 39 bar.g.

(f) What is the dew-point of the flue gas?

(g) An economiser is installed to preheat the boiler feed water mixture using the flue gases. If the flue gases are cooled to 150oC what will the new feed water temperature be and what will be the increase in boiler efficiency?

(h) Estimate the increase in efficiency of the boiler if the boiler control system can maintain the excess air at 10% rather than 20% (assuming that the flue gas exhaust temperature is unchanged). You should not need to repeat the full calculation to make this estimate.