fuels (chemistry) unconventional energy sources 18 hydrogen 18 power engineering 17 combustion 15 coal 13 biomass 12 more ... energy sources 12 cars 10 biofuels 9 consumption 9 gas fuels 8 internal combustion motors 7 heat-generating plants 7 petrochemistry 6 fuels (transport) 6 natural gas 6 liquid fuels 5 boilers 5 lubricants 5 heating 5 air pollution 5 issues 4 energy 4 solid fuel boilers 4 waste disposal services 4 personal cars 4 steam boilers 4 pellets 4 permanently sustainable development 4 less ...