Natural gas of the following composition by volume: methane 89%, nitrogen 7%, ethane 1.5%, hydrogen sulphide 1%, propane 1.5% is burnt in a boiler with 40% excess air and used to generate steam. All the hydrogen sulphide oxidises to sulphur dioxide. (a) Write all relevant chemical equations. [2 marks] (b) Calculate the composition of the exhaust gas on a wet basis. [8 marks] Planning permission requires SO2 emission to be less than 65ppm. The proposal is to add a desulphurisation unit before the boiler. The manufacturer guarantees 92% efficiency of the desulphurisation unit and a capacity of 500kg sulphur removed. (i) Calculate the flue gas composition on a wet basis after desulphurisation and show that SO2 is within the limit required. [8 marks] (ii) Calculate the time the desulphurisation unit can be on-line before its capacity is used up if the flow rate of natural gas is 3000m3/h at 0C and 1 atm. [2 marks]