Boiler and method of heating liquid

Claim 1

A boiler, comprising: a fluidized bed combustion chamber, said chamber including a bed containing portion and first and second inlet means for admitting fluidizing gas to said bed containing portion to fluidize a bed of particles contained therein; said first inlet means introducing fluidizing gas into said bed containing portion at a first upper level to define a shallower zone of said bed containing portion, and said second inlet means being laterally offset from said first inlet means, and introducing fluidizing gas into said bed containing portion at a second deeper level lower than said first level to define a separate deeper zone of said bed containing portion; said first and second inlet means including riser tubes extending upward through a lower boundary surface, said riser tubes extending upward to be higher at said first inlet means than at said second inlet means; said first inlet means including a first plenum communicative therewith, and said second inlet means includes a second plenum communicative therewith, said first and second plenum being independent of each other, and said second plenum including an air plenum and a fuel plenum, said air and fuel plenums being independent of each other; charging means for charging combustible material into a fluidized bed within said bed containing portion above said second inlet means, without depositing combustible material above said first inlet means; a plurality of tubes positioned within said chamber extending from a lower position to a higher position, each of said tubes having a portion located above said first inlet means in said shallower zone but not above said second inlet means within said deeper zone, each said tube including an upper and a lower limb joined by a bend, said limbs and bend being positioned to extend over said first inlet means within said bed containing portion but not over said second inlet means within said bed containing portion, whereby fluid can flow through said tubes by natural convection and when a fluidized bed of particles is contained within said chamber so as to have a level upper surface, said charging means charges said deeper zone of said bed while not charging said shallower zone.