Example 1 If the theoretical air required for complete comb actually supplied, what is the percentage of excess air? Solution 1 air are The amount of excess air is 6 kg (18 - 12 = 6). Expressed as a percentage of the theoretical air (12 kg), this would be: 612 100% = 50% (Ans.) Self-Test 1 The theoretical air to burn one kilogram of Powder River Basin coal is 7.5 kg. How much combustion air is needed to burn 25 kg of Powder River Basin coal if 20% excess air is required? 4: 7.5kg 100% - 20% 225 kg (Ans.)