Effective utilization of heat in waste and biomass processing

Abstract

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.