There are also wood-gassers, which home-built units produce hydrogen and methane for combustion at about a 30% loss of power (maybe not that much at higher altitudes). They don’t produce much pollution, but there wouldn’t be enough wood and waste for everyone to use them. They might be a small part of a solution, though, along with several other technologies and human adjustments.
Much of that gas is carbon monoxide. Be wary if building such a system.