several existing and new stack-gas scrubbing and clean coal technologies; switching to all, or blending high-sulfur coal with, low-sulfur coal; switching to all natural gas, or cofiring coal and natural gas; "trimming," or reducing annual hours of plant utilization; retiring old units; repowering existing units with new coal or non-coal boilers; purchasing or transferring emissions allowances from other units; increasing demand-side management and conservation; or bulk power purchases from other utilities or non-utility generators from units using coal or other fuels.