What do you consider the safest, efficient and cost effective fuel to be used on a steam-boiler used in your work area?

The first thing you have to look at the location of the host plant that owns the boilers to see what fuel sources they have available in that area. If natural gas is available I would consider it as the number one fuel source because of its BTU value and cleaner emission standard.

For long-term, concentrated solar powered (CSP) boiler with natural gas as a back up + heat recovering system.However, it is better to change its place to be safe.

for me in my experience i thick coal is the most effective and low in cost.

I think LPG or natural are better than liquid and solid fuels if we consider emission, but cost-wise it differ depending on the location of your plant.