Commercial entrained-flow coal gasifiers operate at high temperatures and pressures for increased efficiency. For reduced operation costs, the start-up burner (SUB) of a gasifier requires fast heating and pressurization by intensive combustion, minimizing damage to the refractory lining and to itself. We evaluated different designs of the SUB adopting diesel-oxygen diffusion flame under the stoichiometric condition to improve the performance of the existing premixed flame burner that has been subjected to frequent failures. Using computational fluid dynamics, the combustion characteristics of the SUB with a thermal input up to 2.28MWth and pressure up to 8 bar were predicted for tests in a pilot-scale cylindrical gasifier. The results showed that the injection of diesel and oxygen through separate multiple inlets can intensify the combustion, leading to short and stable turbulent diffusion flames. Varying the design of the burner tip and inlet arrangement reduced the influence of pressure on the flame length while achieving high heat flux to the wall, which is essential for a fast and stable start-up. Further investigations are required to understand the influence of the reactor geometry on the circulation flow of high-temperature gas back to the burner tip in the actual gasifier.