A multi-stage selective catalytic reduction (SCR) unit (32) provides efficient reduction of NOx and other pollutants from about 50-550° C. in a power plant (19). Hydrogen (24) and ammonia (29) are variably supplied to the SCR unit depending on temperature. An upstream portion (34) of the SCR unit catalyzes NOx+NH3 reactions above about 200° C. A downstream portion (36) catalyzes NOx+H2 reactions below about 260° C., and catalyzes oxidation of NH3, CO, and VOCs with oxygen in the exhaust above about 200° C., efficiently removing NOx and other pollutants over a range of conditions with low slippage of NH3. An ammonia synthesis unit (28) may be connected to the SCR unit to provide NH3 as needed, avoiding transport and storage of ammonia or urea at the site. A carbonaceous gasification plant (18) on site may supply hydrogen and nitrogen to the ammonia synthesis unit, and hydrogen to the SCR unit.