Elsevier

Energy

Volume 149, 15 April 2018, Pages 230-249
Energy

Dynamic simulation of a municipal solid waste incinerator

https://doi.org/10.1016/j.energy.2018.01.170Get rights and content

Highlights

  • Detailed dynamic process simulation model of 60 MWth municipal solid waste incinerator is developed.

  • All process, automation and electrical components of the real incinerator are modelled.

  • Good agreement between model predictions and design data.

  • The relative errors of water/steam and flue gas parameters are between 1% and 5% at nominal load.

  • Hot shut-down/start-up simulation of the municipal solid waste incinerator is presented.

Abstract

For first time in literature, a dynamic process simulation model of a municipal solid waste incinerator is generated. The developed model of the 60 MWth incinerator describes in detail the flue gas path with its vertical and horizontal passes including grate, primary and secondary combustion zones as well as auxiliary burners, in addition to the water/steam side with its economisers, superheaters and natural circulation evaporators. All control structures required for plant operation are implemented, e.g. feedwater tank, boiler drum, steam turbine bypass system, condensers, air supply systems and attemperators. Through careful development, the only boundary conditions of the incinerator model are the inlet temperature and the mass flow rate of cooling water into condenser as well as the composition of the municipal solid waste. The model is verified towards design data, showing good agreement. The relative deviations of water/steam and flue gas parameters are all within 5%. The incinerator behaviour during shut-down and hot start-up procedures is then evaluated with the validated model.

Introduction

It is obvious that the suitable solid waste management is one of the major challenges that nowadays society should deal with [[1], [2], [3], [4], [5]]. In last few decades, due to rapid development of national economies, urbanisation of rural areas and continued improvement of living standard, the solid waste output is increasing constantly. In order to effectively minimise the municipal solid waste from an economic and ecological point of view, different solutions have been suggested such as reduction of waste generation, recycling, thermal treatment and as a last option landfilling [6]. A proven approach to dispose large quantities of municipal solid waste is the thermal treatment of waste in grate systems (incinerator). The incineration scenario becomes a preferred option because of the advantages it offers such as cost reduction of remaining landfill disposal due to lower volume of end products (one tenth of the original volume) and such as the lower total organic carbon (TOC) of waste, resulting in more inert residues unable to produce landfill gas. The released heat during the combustion of waste (lower heating value (LHV): approximately 10 MJ/kg) can be transferred from the flue gas path to the water/steam circuit and used to generate steam for a Rankine cycle in order to supply electricity and district heating [7]. Accordingly, the residual waste can be used as substitute fuel for conventional fossil fuels.

A municipal solid waste incinerator consists of a water/steam side, a flue gas path and its cleaning equipment. In a waste bunker, the delivered raw waste is classified and treated. Here, the bulky components are crushed and incombustible materials are discharged. The combustion system comprises of a primary combustion zone on the grate, a post-combustion zone (also known as a secondary combustion zone), in which the secondary air is injected and a zone with auxiliary burners. On the grate, the incineration of solid waste takes place at different stages by means of primary air (air/fuel ratio between 0.4 and 1.3). At these stages, the thermal processes are drying, pyrolysis, combustion of volatile matters and burn-out of char. In the post-combustion zone, the remaining combustible substances are burned with excess secondary air (approximately 1.5 over stoichiometric level). The exhaust gas flows into the flue gas path, including vertical passes (also known as radiation passes) and a horizontal pass (also known as a convection pass), in which shell and tube heat exchangers are installed.

Municipal solid waste incinerators, although most pollutants are destroyed by the combustion, can emit high quantities of pollutants to the atmosphere such as solid residues (particulate matter), heavy metals, acid gases and nitrogen oxides [8]. Therefore, complex air pollution control devices are required. In order to remove the solid residues from the flue gas, separators are attached at transition regions between vertical passes and over the entire length of horizontal pass. Furthermore, a commercial dust filter such as electrostatic precipitators or fabric filter is installed at the outlet of the flue gas path. The collected solid residues (slag and fly ash) are fed to the slag bunker and ash silos, stored temporarily and then discharged to be used as a filler material in road construction or in the cement industry. The acid gas concentrations in the flue gas, e.g. hydrogen chloride and sulphur dioxide are directly related to the chlorine and sulphur content of the solid waste. For separation of these pollutants, particularly acid gases, sulphur oxides and heavy metals, wet gas scrubbing or dry absorption with the addition of calcium compounds and/or activated carbon are used generally. The formation of nitrogen oxides is dependent on the nitrogen quantity of solid waste, combustion temperature and air/fuel ratio. Nitrogen oxides can be removed from the flue gas by employing a selective non-catalytic reduction (SNCR) in addition to conventional primary measures (e.g. staged combustion, especially air staged supply). Finally, the cleaned flue gas passes through the stack into the environment.

In addition to its main task, the volume reduction of municipal solid waste, the waste incinerator can be used to treat sewage sludge. The term sewage sludge refers to the residual material, resulting as a by-product during the treatment of industrial or municipal wastewater. Nowadays, approximately 10.2 million tonnes of dry sewage sludge per year are accumulated in Europe according to Eurostat [9]. The dry sewage sludge contains up to 70% organic components and the remaining 30% are composed of, among others, silicates, phosphates, phosphor and heavy metals. Because of its valuable components, the sewage sludge can be recycled in agriculture and landscaping to improve plant nutrition and to enhance the physical and chemical properties of soil. Because of its higher heating value (9–12 MJ/kgdried), however, more than half of the sewage sludge was incinerated in Germany in 2010.

The availability and utilization of municipal waste incinerators can be improved via experimental works, providing fundamental insights into the thermal and hydrodynamic behaviour of the process. However, detailed operation data from real incinerators is difficult because of the lack of accessibility, the harsh environment and the costs of measuring devices. Mathematical models, including steady-state simulation and dynamic simulation, offer significant contributions to the direct measurements. In steady-state simulation, the time derivations disappear from the conservation equations. In case of zero-dimensional modelling (thermodynamics calculation models), the local discretisation is not considered. The modelling of thermal power plant components such as heat exchanger, pump, condenser, turbine, etc., results in an algebraic system of equations with inputs and output parameters of components (pressure, enthalpy, mass flow and concentration). In case of one-dimensional modelling, the power plant components are discretised between the inlet and the outlet along the flow in finite objects, so-called numerical grid. Here, an algebraic system of equations at each discrete location is obtained. In case of two or three-dimensional modelling (is generally known as computational fluid dynamics (CFD)), additional local discretisation along the coordinates is required, resulting in more detailed calculation of the thermal power plant components. In dynamic simulation, the time derivations must be taken into consideration. In case of zero-dimensional modelling, the components are modelled without local discretisation. For one-dimensional modelling, the transient behaviour and local discretisation along the flow are computed. In case of two or three-dimensional modelling, a further discretisation along the local coordinates is required. While the two or three-dimensional simulations are often used for individual components to visualise flow patterns, the one-dimensional simulations are applied to entire systems, which are of high relevance in order to understand the interaction between different components.

Compared to the three-dimensional simulations, the one-dimensional dynamic process simulations have the advantage of lower computational time. They can predict the design and control of the entire power plant, providing rapid assessments of:

  • new power plant design

  • power plant optimisation and efficiency improvement

  • process modifications and retrofitting of existing power plants

  • power plant security and safety

  • operating behaviour at base loads, off-design, start-up and shutdown

  • operating behaviour during malfunctions

The main disadvantage is that the one-dimensional dynamic process models cannot resolve the flow and heat transfer processes in three-dimensional space (e.g. the concentration field, the temperature field and the velocity field). The CFD models, by contrast, offer a detailed insight into the flow pattern and heat transfer process, providing qualitative and in many cases quantitative predictions. Generally, the numerical methods of CFD are applied to individual components of thermal power plants, e.g. combustion chambers or heat exchangers, due to the high computational time.

Nowadays, dynamic process simulation becomes an integral part of design, operation and optimisation of thermal power plants. The basic task of dynamic simulation is to calculate the plant response and the behaviour of its control circuits to malfunction or to a change in load demand. This implies strong requirements on both model accuracy and efficiency of the numerical solver. Generally, the process simulation codes are based on the governing conservation equations of mass, momentum, species and energy. The specific mathematical formulation of the conservation equations depends on the used flow model, i.e. mixture flow model and two-fluid model. Due to its relative simplicity and suitability for a wide range of practical applications, the mixture flow model is of considerable relevance, since the calculation of average mixture properties is often sufficiently accurate for system-level analysis. The two-fluid model offers the possibility to consider thermodynamic non-equilibrium phenomena. Thus, it is more suitable for detailed analysis of specific components and application cases characterised by intense mass and heat transfer between the phases. The resulting partial differential equation system is discretised and typically closed with empirical correlations, which are selected according to the prevailing flow regime [10].

When looking into the scientific literature, simulation studies of municipal waste incinerators using 3D CFD software were frequently reported, considering plant simulation and optimisation, among other recently published works [[11], [12], [13], [14]], but there are no studies on the dynamic process simulation of municipal waste incinerators. Several dynamic process simulation models of the most important thermal power plant technologies were available such as combined-cycle power plant [[15], [16], [17], [18], [19], [20], [21], [22]], pulverised-coal fired power plant [[23], [24], [25], [26], [27], [28]], nuclear power plant [[29], [30], [31], [32]] and concentrated solar power plant [[33], [34], [35], [36], [37], [38]], summarized in a recent review paper on dynamic simulation of thermal power plants [10]. In this work and for the first time in the process simulation literature, investigations into the dynamic behaviour of a municipal waste incinerator are presented, showing advancement over the actual state-of-the-art in the simulation field of municipal waste incinerators. The dynamic process simulation model of the 60 MWth municipal solid waste incinerator, erected in Finland is generated using the advanced process simulation software (APROS). The incinerator burns 5.7 kg/s of solid waste with an ideal low heating value of 10.5 MJ/kg and discharges 36.3 kg/s of flue gas at 160 °C as well as 0.7 kg/s of slag at 450 °C. The water/steam circuit consists of 4 economisers, 7 superheaters and 5 evaporators with natural circulation. At full load, the gross electrical output of the steam turbine is approximately 16 MWel, which yields an electrical gross efficiency of 26.6%. The comparison between model predictions and the design data at base load shows a high degree of agreement. The developed model is then applied to evaluate the dynamic plant behaviour during shut-down and hot start-up procedures.

Section snippets

Dynamic process simulation software

There are several in-house developed codes and commercial software programmes available for process simulation of thermal power plants, e.g. ASPEN Plus DYNAMICS, DYMOLA and MATLAB/SIMULINK. In this study, the advanced process simulation software (APROS), developed by VTT Technical Research Centre of Finland is used [39]. APROS provides specialised component libraries and solution techniques for the dynamic simulation of different thermal power plant technologies. Base modelling is carried out

Characterisation of the incinerator

The investigated 60 MWth incinerator has been built in Finland, designed by Steinmüller Babcock Environment GmbH [51]; a company specialised in engineering and construction of waste treatment and flue gas cleaning systems. Some of the most relevant projects of the company are: the first hazardous waste incineration plant using rotary kiln technology in Bürrig (Germany) and the world’s first flue gas desulphurisation plant, erected in a dry cooling tower in Matra (Hungary).

The 60 MWth

Incinerator model

The municipal solid waste incinerator model is built by selecting and connecting the process, automation and electrical components, based on real design specifications, construction drawings and control schemes of the real plant, obtained from Steinmüller Babcock Environment GmbH. Only the solid waste composition, the air ambient temperature and the inlet temperature and mass flow rate of the cooling water into the condenser are set as boundary conditions. Even though these boundary conditions

Results

In the following sections, the analysis of the developed model to nominal load, shut-down and hot start-up is presented.

Conclusion

In this work, a dynamic process simulation model of the 60 MWth municipal solid waste incinerator was generated using the advanced process simulation software APROS. The developed model includes the flue gas path with its combustion system and the water/steam side with its heat exchangers. A set of control concepts was implemented for components of the feedwater tank, boiler drum, attemperators, steam turbine bypass system, condenser, auxiliary burners and air supply systems. Through careful

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