A system for waste-to-energy conversion of municipal solid waste and urban forest residue includes as a central element a vortex-cyclone suspended combustion zone furnace, supplied via a shredder and a rotary preheat kiln, and followed by a waste heat boiler. The combustion furnace takes the form of a horizontal tunnel-like structure into which solid waste material from the preheat kiln is introduced near an entry end, with separate exit ports for hot gas and ash at an exit end. The furnace has spaced horizontal waterwall tubes, between which forced draft air is injected in a circular pattern aided by vanes, producing a swirling overfire air curtain surrounding the vortex-cyclone suspended combustion zone along the length of the furnace. In the manner of a cyclone separator, a cylindrical structure surrounds the central exhaust gas opening, and extends from the exit end wall into the combustion chamber to a circular leading edge. This cylindrical structure minimizes non-combustible particulate content in the gas flow directed out through the central exhaust gas opening.