This thread appears to be labeling coal, the "dirty fuel". In the past: yes it was.
Today's technology (used in the USA, granted due to environment regulations) allows coal to be burnt with only two products of combustion to be released into the atmosphere: H2O and CO2. (No carbon monoxide CO, today's power plant coal boilers are extremely efficient. CO has more BTU released when it is further oxidized to CO2)
The back end of the boilers capture the polutants before they reach the stack. The big concern is that recently CO2 has recently been added to the polutant list. Of the hydro-carbon fuels, coal has the greatest carbon to hydrogen ratio.
The only thing dirty about coal, today (in the USA) is its carbon footprint. When burnt it releases large amounts of CO2.
