The main fossil fuels -- coal, natural gas, and crude oil -- are
close substitutes in many industrial, commercial, and residential
applications, such as for space heating, mechanical energy,
electric power generation, and as a petrochemical feedstock.
Despite their overlapping end uses and potential for substitution,
differing physical characteristics of these fuels affect regional
and global markets for them. Please write a few sentences
describing how differences in physical characteristics affect
regional and global trade for these fossil fuels.