The correct answer is A) cold start a boiler with diesel oil. Steam assist fuel atomizers are converted to straight mechanical atomizers in order to facilitate cold starting a boiler with diesel oil. This is because diesel oil is more viscous than the fuel oil typically used in steam-assisted atomizers, and the mechanical atomizer provides better atomization of the diesel oil to ensure proper ignition and combustion when starting a cold boiler. The other options are incorrect because they do not accurately describe the reason for converting to a mechanical atomizer. Option B is incorrect as raising steam on an idle boiler would not necessitate the use of a mechanical atomizer. Option C is incorrect as meeting minimum boiler steam demands does not require the use of a mechanical atomizer. Option D is incorrect as fuel economy is not the primary reason for converting to a mechanical atomizer in this context.