A system and method relate to a reductant dosing for use in the reduction of NOx in an exhaust stream is disclosed. The system and method incorporates a separate first or start-up cartridge and an insulated mantel or housing containing at least one to a plurality of main cartridges. The first and main cartridges store an ammonia adsorbing/desorbing material, which releases ammonia gas upon application of sufficient heat. The start-up cartridge permits the initial release of ammonia gas into the exhaust stream even during start-up of an engine, and because it is separate from the main cartridge, the first cartridge cools faster than the main cartridge and can be replenished with ammonia sooner than the main cartridge. The start-up cartridge is housed in or surrounded by a non-insulating debris shield.