Methods and apparatus for high efficiency generation of electricity and low oxides of nitrogen (NO.sub.x) emissions are provided. The electricity is generated from combustion of hydrogen-rich gases produced in waste conversion units using ultra lean fuel to air ratios in the range of 0.4-0.7 relative to stoichiometric operation in internal combustion engine-generators or ultra lean operation in gas turbines to ensure minimal production of pollutants such as NO.sub.x. The ultra lean operation also increases the efficiency of the internal combustion engine. High compression ratios (r=12 to 15) can also be employed to further increase the efficiency of the internal combustion engine. Supplemental fuel, such as natural gas or diesel oil, may be added directly to the internal combustion engine-generator or gas turbine for combustion with the hydrogen-rich gases produced in waste conversion unit. In addition, supplemental fuel may be reformed into a hydrogen-rich gas in a plasma fuel converter and then introduced into the internal combustion engine-generator or a gas turbine for combustion along with supplemental fuel and the hydrogen-rich gases produced in waste conversion unit. The preferred embodiment of the waste conversion unit is a fully integrated tunable arc plasma-joule heated melter with a common molten pool and power supply circuits which can be operated simultaneously without detrimental interaction with one another. In this embodiment, the joule heated melter is capable of maintaining the material in a molten state with sufficient electrical conductivity to allow rapid restart of a transferred arc plasma.