Solid Waste Incineration Modelling for Advanced Moving Grate Incinerators
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sustainability Article Solid Waste Incineration Modelling for Advanced Moving Grate Incinerators 1, 1, 1,2, Mingtao Jiang y, Adrian C. H. Lai y and Adrian Wing-Keung Law * 1 Environmental Process Modelling Centre, Nanyang Environment and Water Research Institute, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore; [email protected] (M.J.); [email protected] (A.C.H.L.) 2 School of Civil and Environmental Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore * Correspondence: [email protected]; Tel./Fax: +65-6790-5296 Mingtao Jiang and Adrian C. H. Lai contributed equally to this work as first authors. y Received: 27 August 2020; Accepted: 22 September 2020; Published: 28 September 2020 Abstract: Currently, the design of advanced moving grate (AMG) incinerators for solid waste is aided by computational simulations. The simulation approach couples a waste bed model to characterize the incineration processes of the waste material on top of the moving grate, with a computational fluid dynamics (CFD) model to reproduce the heated air movement and reactions in the incinerator space above. However, the simulation results of AMG incinerators are rarely compared with actual field measurements for validation in the literature so far. In this study, we first examine the sensitivity of pyrolysis kinetics in the waste bed model using three existing alternatives. The predictions of combustion characteristics, including the bed height, flow and temperature distributions, composition of stack gases and gas emissions are obtained for the three alternatives and compared with measurements from a simple laboratory furnace. The results show that the pyrolysis kinetics mechanism can significantly affect the outputs from the waste bed model for incineration modelling. Subsequently, we propose a new coupling approach based on a recent AMG waste bed model (which includes the complex pyrolysis kinetics inside the waste bed on top of the moving grate) and the freeboard CFD simulations. The new approach is then used to predict the field performance of a large scale waste-to-energy (WTE) plant and the predictions are compared directly with the real measurements in various operational scenarios. The comparison shows an overall satisfactory agreement in terms of temperature and exit gases composition given the complexity of the real life operations, although the CO emission is slightly underpredicted. Keywords: waste bed model; freeboard model; computational fluid dynamics (CFD); waste-to-energy (WTE); gas emission; combustion 1. Introduction Sustainable waste management is of critical importance worldwide and particularly for the developing countries in Asia [1–5]. An effective tool for sustainable waste management is the incineration of solid waste through waste-to-energy (WTE) plants. A schematic diagram for a WTE plant is shown in Figure1 (from [ 6]). The WTE plant can largely reduce the waste volume and alleviate the requirement for landfills and the steam turbines can also recover a major portion of the energy contained in the solid waste. At the same time, a comprehensive understanding of the physical and chemical processes for the waste incineration is essential for the engineers to properly design the incinerator to meet the strict requirements and regulations set by the government agencies. Sustainability 2020, 12, 8007; doi:10.3390/su12198007 www.mdpi.com/journal/sustainability Sustainability 2020, 12, 8007 2 of 15 Sustainability 2020, 12, x FOR PEER REVIEW 2 of 15 Figure 1. SchematicSchematic diagram diagram of of a a waste-to-energy waste-to-energy (WTE) plant (from [6]). [6]). Among the various types of solid waste incineratorincinerator nowadays, the advanced moving grate (AMG) incinerator is is the most common choice. Its Its main main components components are are the the waste waste feeding feeding system, system, moving grate and grate bars. When When the the operation operation begins, begins, the the solid solid waste waste is transported into the incinerator through through the the moving grate at the bottom.bottom. The The complex complex processes processes of of drying, drying, pyrolysis, pyrolysis, gasificationgasification andand charchar combustioncombustion (on (on top top of of the the grate) grate) then then take take place place in thein the waste waste bed bed due todue the to high the hightemperature temperature inside inside the incinerator, the incinerator, while while the gaseous the gaseous products products and volatiles and volatiles released released from from the solid the solidwaste waste are further are further mixed andmixed combusted and combusted in the freeboard in the regionfreeboard above region the waste above bed. the Experimental waste bed. Experimentalinvestigations investigations on the combustion on the characteristics combustion characteristics inside laboratory inside incinerators laboratory have incinerators been reported have beenin several reported previous in several studies [previous7–9]. These studies experimental [7–9]. These measurements experimental provided measurements important insightsprovided to importantimprove the insights understanding to improve of thethe mechanismsunderstanding involved. of the mechanisms However, to theinvolved. best of However, our knowledge, to the fieldbest ofmeasurements our knowledge, inside field AMG measurements incinerators inside under AMG various incinerators operational under scenarios various haveoperational been very scenarios rarely havereported been so very far. rarely reported so far. Computational simulations have been used to pr predictedict the performance of AMG incinerators for design improvement and verification. verification. Early Early attempts attempts to to model model the incineration process date back to at least the 1970s [10] [10] and were based on the very limited computational resources available at that time, which werewere insuinsufficientfficient totorepresent represent the the complexities complexities involved. involved. With With continuous continuous improvement improvement in incomputational computational power power and and improved improved understanding understanding of thermo-fluid of thermo-fluid dynamics dynamics and chemical and chemical reaction reactionkinetics, recentkinetics, simulations recent simulations have advanced have significantly advanced [11significantly–13]. However, [11–13]. accurate However, modelling accurate of the modellingAMG incinerator, of the AMG which incinerator, undergoes thewhich complex undergoes processes the ofcomplex drying, processes pyrolysis, of gasification drying, pyrolysis, and char gasificationcombustion, and is still char a majorcombustion, challenge is still at present. a major challenge at present. For industrial industrial applications, applications, a acommon common approach approach to tomodel model the thewaste waste bed bedis through is through the one- the dimensionalone-dimensional Lagrangian Lagrangian approach approach whereby whereby the the waste waste be bedd is isrepresented represented as as a aseries series of of non-interfering non-interfering Lagrangian elements.elements. TheThe changeschanges in in its its mass, mass, composition composition and and temperature temperature as wellas well as theas the resulting resulting gas gasproducts products from from these these elements elements are tracked are tracked over time.over Atime. mathematical A mathematical model model was developed was developed by [14, 15by] [14,15]using the using above the approach above approach to account to account for the bed for volumethe bed changevolume and change various and thermochemical various thermochemical processes. processes.An improved An radiationimproved model radiation was model adopted was by adopted [16] that by simulated [16] that thesimulated chemical the reaction chemical kinetics reaction in kineticsdetail and in theirdetail model and wastheir validated model was by theirvalidated own experimental by their own measurements experimental in measurements a laboratory reactor. in a laboratoryMathematical reactor. modeling Mathematical was carried modeling out by [17 was] of grate-firedcarried out boilers by [17] by incorporatingof grate-fired a standaloneboilers by incorporatingbed model coupled a standalone with the computationalbed model coupled fluid dynamics with the (CFD) computational simulations forfluid the dynamics freeboard, (CFD) which simulationsincluded the for energy the freeboard, equations which for both included the fuel the and energy gases andequations accounted for both for the the heat fuel transferand gases among and accounted for the heat transfer among the multi-phases. The Lagrangian approach was extended by [18] to two-dimensional, which is beneficial potentially considering that the waste composition might Sustainability 2020, 12, 8007 3 of 15 the multi-phases. The Lagrangian approach was extended by [18] to two-dimensional, which is beneficial potentially considering that the waste composition might be spatially non-uniform on top of the moving grate in practice. Other studies involved further improvement to incorporate additional effects such as the effects of the devolatilization rate and moisture content [9,19]. A recent research approach for the waste bed model is the particle-based approach of the discrete element method. The waste bed is represented by a large group of discrete particles undergoing the various heat and mass transfer processes with different chemical reactions.