There are a number of factors dictating which heaters you are able to use in any particular situation. The main options you have are:-
For the single phase electric heaters the first consideration is that they will only give a maximum of 3kW from a standard 13A domestic plug. With electric fan heaters, they rely heavily on being able to re-circulate air multiple times. What this means is if you put a 3 kW heater in a large warehouse - it will never throw heat out particularly hot. Each time air is pulled through a few degrees is added - meaning in a small, for example 10 feet square room it will give out hot air.
Another issue with fan heaters is that they can kick up dust, if you are trying to heat a process which is sensitive to dust then fan heaters might not be the solution.
With radiant or infra-red heaters it works differently. These are like a light, and the light heats the surface of that the light shines on. This means they can be used in a large open area to heat something small. The main concern with them is that if left unattended the object being heated can get extremely hot over time.
If an electric fan heater seems like the correct solution, but you need more capacity - then 3 phase might be an option. If you have 16A 3 phase sockets, the most powerful heater you can use is the 13.9 kW FF13 Heater if you have a 32A socket then the most powerful you can use is the 28.9 kW FF29 Heater if you have a 63A Socket you can go for a 42 kW FF42 Heater or even larger FF80 or FF125. When ordering large three phase heaters - consider asking the supplier to fit the correct plug for your sockets. These can be either 4 or 5 pin plugs. Of course any of these could be wired in directly to an isolator by a qualified electrician.
If you need a lot of heat, and have no 3 phase electrics, then you need to burn a fuel. It may be possible to build a single phase heater that is of higher capacity than 3kW, but it will require either a 32A or larger plug, or will require wiring in. As a special it will also be expensive.
If you are burning a fuel you need to operate the heater in an area where there are no flammable or explosive materials, even cardboard and paper could be construed as a fire risk. You also need to operate it in a well-ventilated area due to the carbon dioxide and carbon monoxide emissions. Of gas heaters and oil fired heaters you tend to find gas heaters burn a little more cleanly, but are more expensive to run per kW of heat.
If you need lots of heat but have no 3 phase and cannot have fumes or sparks and embers, then the only viable option is an indirect diesel heater, these will burn in a steel combustion chamber, and expel the fumes and embers through a chimney that can be flued out. The only issue here is that a hole in the wall or the roof has to be made to accomodate the flue. When adding a flue, you must use rigid stainless steel flue, and it must rise a metre vertically before you add an elbow, at that point 45 deg is the maximum bend allowed at any point. The problem is that too much back-pressure will cause the heater to burn improperly and soot up the chamber.
Diesel Heaters of both indirect and direct firing type, do tend to require more regular servicing than gas or electric. They can be made to run on Kerosene, but Kerosene doesn't have the lubricity of diesel and so the pump pressures should be set lower for heaters running Kerosene to preserve the life of the pump. Obviously this will mean re-tuning the air flow and possibly swapping to a nozzle. Danfoss S type nozzles are calibrated for diesel, as Kerosene is lighter, the flow will be higher. If you buy ES type nozzles, these are calibrated for Kerosene. It is reccomended not to run pumps at over 9 bar when using kerosene.
The main issues with oil fired heaters are down to dirty fuel, fuel with water in it, and trying to run them in an environment where there is a lot of particle in the air - for example dust or paint fumes.
