a/ CS-ESP = cold-side electrostatic precipitator ; HS-ESP = hot-side electrostatic precipitator ; FF= fabric filter ; FGD = flue gas desulphurization b/ PAC=powdered activated carbon ; SC=spray cooling ; PFF=polishing fabric filter c/ NA = not available In comparison , the estimated annual costs of Hg controls , as a function of plant size , lie mostly between the costs for low-NOx burners ( LNBs ) and selective catalytic reduction ( SCR ) systems .