ENERGY RECOVERY

Removing Heat Energy from the hot exhaust gas of a thermal process, has to date been the most common method of capturing a source of energy that would otherwise have been lost to atmosphere

There are other methods of energy recovery available, one of which is Direct Electrical Generation by thermopile and another is the capturing of a Volatile Gas containing the energy.  Generating a Volatile Gas from waste is described in the section on GASIFICATION, below

In practice, where the amount of Heat Energy removed from the exhaust stream is less than 3,000 Kw (10MM Btu/Hr), and/or there is no on-site use for the recovered heat, that energy is just re-radiated to atmosphere

Heat recovery systems that do not use the energy are found downstream of Solid Waste incinerators handling between 150 Kg/Hr and 500 Kg/Hr plus any Thermal Reduction process that must have its exhaust stream 'scrubbed' (cleaned of particulate and chemicals).  Most gas scrubbing technologies cannot handle the 1,000oC Incinerator exhaust gas temperature so heat must be removed, even if there is no application for it

Heat Recuperation is the act of capturing Heat Energy from the thermal reduction process exhaust stream and then re-directing that energy back, into the thermal reduction process

High-volume Waste Gas and Vapour streams can be pre-heated before entering the reduction process using the recovered energy in order to reduce the amount of heat energy released by auxiliary fuel.  Combustion Air streams can be pre-heated to achive the same end, while many liquid fuel/waste can be pre-heated in this fashion to improve atomization at the nozzle

Similarly, as shown in the Waste Gas Thermal Oxidizer description, above, heat energy removed from the reduction process exhaust can be used remotely to reduce fuel consumption by the process which generated the waste gas to begin with.  Uses for the recovered energy can be found in many industrial, clinical and commercial applications

SOLID WASTE INCINERATION WITH HEAT RECOVERY

CONTROLLED-AIR, FIXED HEARTH

Shown below is the Equipment Layout plus the Process/Energy Balance for a continuous, 24-Hr Incineration Process at 31 Metric Tonne per day Municipal, Solid Waste throughput

1,300 Kg/Hr is the maximum practical design throughput for a Stepped, Fixed Hearth incinerator in a modular configuration; the minimum stepped hearth throughput being approximatley 250 Kg/Hr, operating continuously but for no more than 12-hr per day unless it has automatic ash removal

For Incinerators between 25 Kg/Hr and 250 Kg/Hr reduction rate, please see Solid Waste Incineration in THERMAL REDUCTION

The information presented here is intended to be an application guide and should not be used for design purposes

CONTROLLED-AIR, ROTARY KILN

Rotary Kilns have been in use for over a century to mix, process, heat and even incinerate a very wide variety of granular solids and sludge.  Where as a fixed-hearth incinerator will find it difficult to process granular and sludge waste, I have applied rotary kiln incineration to 'oily riverbed mud' with very good, but very fuel dependent results 

Rotary Kilns are generally more expensive to install than fixed hearth incinerators and so it is rare to find a kiln in use to reduce Municipal Solid Waste (MSW) where the waste throughput is less than 45 Metric Tonne per day

Below are the Layout Drawings and Process/Energy Balance for a Rotary Kiln reducing 90 Metric Tonne per day of MSW under sub-stoichiometric conditions, with Secondary, Cyclonic Oxidizing Chamber, Heat Recovery Boiler and Exhaust Gas Scrubber

90 MTPD MSW throughput suits the needs of an Alberta county for which a design study has been completed, parts of which are shown below

WASTE GASIFICATION & SYNTHETIC GAS RECOVERY

UP-DRAFT, FLUIDIZED BED

Gasification of organic compounds is just as it sounds; organic solids are turned to gas and ash in the presence of high heat and very little oxygen

There exist a number of commerical, gasification technologies reducing waste wood, coal, sewage sludge, municipal waste and industrial waste including plastics.  The technology has long been in use, where the gasification of coal illuminated street lights with 'coal gas' before the widespread adoption of electricity

Waste entering the bed must be sorted with more vigor than needed for incineration, especially the removal of metals and other inert material.  Composition of the MSW stream used for this Gasifier design is that shown above for Incineration - The difference in sorting is apparent in the followng table;

More to come...