Currently, emission standards for air curtain incinerators (ACIs) that burn only wood wastes, yard wastes and clean lumber have been promulgated under four different source categories: Large Municipal Waste Combustors, Small Municipal Waste Combustors, Commercial and Industrial Solid Waste Incineration Units, and Other Solid Waste Incineration Units. The division of ACI standards across four source categories has created uncertainty within industries about which rules apply. Additionally, some ACI standards require title V permitting while others do not. This action proposes to consolidate the rules for ACIs that burn only wood wastes, yard wastes, clean lumber, or a combination of the three, into a single location and resolve inconsistencies in the rules, including a proposal to remove the title V permitting requirement for this type of ACI. The EPA is also soliciting comment on revising the definition of "municipal waste combustion unit" to remove the reference to pyrolysis/combustion units in the EPA's Other Solid Waste Incinerators: New Source Performance Standards (NSPS) and Emission Guidelines (EG) for Existing Sources.