There could be many reasons but the person who has done steam air decoking knows better. Some of the reasons are given below.
1- Reverse flow of steam and air may not be manitained properly, time wise and ratio wise, may be arch temperature kept low during decoking at forward and reverse flow , may be heater effluent not anayse at reverse flow for complete burning.
2- If decoking has done propery and it has happned during or after start-up, than it could be due to shight foam carry over from coke drums.
3- If it is not due to foam carry over from coke drum, than it could be due to after burning i.e. less flow of combustion air or very minor tube leaks.
4- it also could be due to less velosity steam injection .
Abdul Hakeem ahabid@ yahoo.com