Paper presented at the 6th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 30 June - 2 July, 2008.
Effective utilization of heat in thermal processing of waste
and biomass plays an important role since it contributes to
environmental and economic optimization of the processes and
equipment. In case of utilizing energy released during thermal
oxidization (incineration) of municipal solid waste (MSW) or
of hazardous waste for generation of process steam or for cogeneration
(combined heat and power systems - CHP) we can
consider the thermal processing as a certain kind of recycling.
Since waste has sufficient heating value, it belongs to
renewable energy sources which enable to save fossil fuel as a
primary energy source. Then we speak about waste to energy
systems (WTE). In addition to environmental benefit, effective
utilization of released energy has a positive impact on
economics of the process including reduced operating costs.
WTE can partially or completely compensate costs of waste
treatment (costs of auxiliary fuel for incineration of low
calorific industrial and/or hazardous waste) and it can even
bring profit to the operator in case of waste with high calorific
value.
Typical examples of units for the thermal processing of both
MSW and hazardous waste are shown with the objective to
evaluate main factors influencing energy balance of the
processes, while taking into account various regimes of
operation. Basic rules of selection of the systems for efficient
heat utilization including CHP are summarized and illustrated
on concrete industrial examples. Conventional methods of
energy availability are discussed and analyzed. Heat flows in
the incineration plant are evaluated as well as factors like plant
efficiency and/or energy utilization rate.
A novel and original technology for combustion of various
types of biomass and fytomass consisting of a feeding system,
boiler, heat recovery system and flue gas cleaning system (in
case of contaminated biomass) is presented.
Moreover, it is necessary to take into account specific
features of flue gas (and/or off-gas) as a process fluid. For an
optimum design of heat exchangers as equipment and
integrated items it is necessary to follow a top-down approach
“process – heat recovery system – heat exchanger” while
respecting specific features of the concerned process. A
combination of intuitive design, know how and sophisticated
approach based on up-to-date computational tools with
emphasis on computational fluid dynamics (CFD) is shown in
the paper.
After selecting a convenient process for the given type of
waste and/or biomass, the available energy for heat recovery is
evaluated and a heat recovery system is designed. Novel design
of air pre-heaters, heat recovery steam generators and special
heat exchangers (e.g. those for sludge pre-heating) is shown.
This approach always respects the primary role of the process,
while stressing also the importance of analysis aimed at
selection of heat exchangers and their design including specific
features and fouling problems.
