Natural gas, assuming 100% methane is combusted with air in a 10 wt% excess (exc

Question

Natural gas, assuming 100% methane is combusted with air in a 10 wt% excess (excess air). There is 100 m³ (STP) of natural gas burned per hour, with the temperature and pressure of the combustiongas are respectively 300 ° C and 1 atm. How efficient is the combustion(energy into output energy of the combustion gas in front of)? say that this hot air is sent over a countercurrent- heat exchanger (200 W / m²K), with an exchange surface of 4.5 m², until what temperature can 1000kg/h water of 10°C be heated without making the water in the gas to condensate

Explanation / Answer

CH4 + 1.1 x 2(O2 + 3.76 N2) -> CO2 + 2 H2O + 0.2 O2 + 8.272 N2 H = -890 kj/mol

mass of methane = 100*101325*16/8.314*573 = 34030 g

amount of energy released in this reaction = 34030 g CH4 *{1 mol CH4 /16 g CH4}*{890 kj/ 1 mol CH4 } = 1892962 kJ

power given per sec = 1892962/3600 kj = 525.82 KW

power of the heat exchanger per sec = 200*4.5*573 = 515.7 KW

efficiency =output energy/ input energy = 515.7/525.82 = 98%

Calculate energy required to change temperature of water

energy = specific heat capacity of water x mass of water x change in water temperature

energy = 4.184 J K-1g-1 x 1000000 g x {t-10}/3600 = 515700

4.184*{t-10} = 515700*3600/1000000 = 1856.52

t = 453.7 oC

actually there is lack of clarity in the last part of the question without making the water in the gas to condensate makes no sense.

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