Rory F. D. Monaghan

CFD Simulation of Entrained Flow Gasification With Improved Devolatilization and Char Consumption Submodels

In this work, we use a CFD package to model the operation of a coal gasifier with the objective of assessing the impact of devolatilization and char consumption models on the accuracy of the results. Devolatilization is modeled using the Chemical Percolation Devolitilization (CPD) model. The traditional CPD models predict the rate and the amount of volatiles released but not their species composition. We show that the knowledge of devolatilization rates is not sufficient for the accurate prediction of char consumption and a quantitative description of the devolatilization products, including the chemical composition of the tar, is needed. We incorporate experimental data on devolatilization products combined with modeling of the tar composition and reactions to improve the prediction of syngas compositions and carbon conversion. We also apply the shrinking core model and the random pore model to describe char consumption in the CFD simulations. Analysis of the results indicates distinct regimes of kinetic and diffusion control depending on the particle radius and injection conditions for both char oxidation and gasification reactions. The random pore model with Langmuir-Hinshelwood reaction kinetics are found to be better at predicting carbon conversion and exit syngas composition than the shrinking core model with Arrhenius kinetics. In addition, we gain qualitative and quantitative insights into the impact of the ash layer surrounding the char particle on the reaction rate.Copyright

Reduced Order Modeling of Entrained Flow Solid Fuel Gasification

Reduced order models that accurately predict the operation of entrained flow gasifiers as components within integrated gasification combined cycle (IGCC) or polygeneration plants are essential for greater commercialization of gasification-based energy systems. A reduced order model, implemented in Aspen Custom Modeler, for entrained flow gasifiers that incorporates mixing and recirculation, rigorously calculated char properties, drying and devolatilization, chemical kinetics, simplified fluid dynamics, heat transfer, slag behavior and syngas cooling is presented. The model structure and submodels are described. Results are presented for the steady-state simulation of a two-metric-tonne-per-day (2 tpd) laboratory-scale Mitsubishi Heavy Industries (MHI) gasifier, fed by two different types of coal. Improvements over the state-of-the-art for reduced order modeling include the ability to incorporate realistic flow conditions and hence predict the gasifier internal and external temperature profiles, the ability to easily interface the model with plant-wide flowsheet models, and the flexibility to apply the same model to a variety of entrained flow gasifier designs. Model validation shows satisfactory agreement with measured values and computational fluid dynamics (CFD) results for syngas temperature profiles, syngas composition, carbon conversion, char flow rate, syngas heating value and cold gas efficiency. Analysis of the results shows the accuracy of the reduced order model to be similar to that of more detailed models that incorporate CFD. Next steps include the activation of pollutant chemistry and slag submodels, application of the reduced order model to other gasifier designs, parameter studies and uncertainty analysis of unknown and/or assumed physical and modeling parameters, and activation of dynamic simulation capability. INTRODUCTION Carbon dioxide capture and storage (CCS) is recognized as one of a suite of technology options that can be used to reduce greenhouse gas (GHG) emissions from continued fossil fuel usage [1-3]. Several approaches to carbon dioxide (CO2) capture, the most expensive step in CCS, have been suggested, among them, pre-combustion CO2 capture systems, which employ gasification. Applications of gasification-based energy systems include IGCC plants for the production of power, and polygeneration plants for the production of industrial chemicals, fuels, hydrogen, and potentially power. There are three general families of commercial gasifier designs: fixed/moving bed, fluidized bed and entrained flow. According to the DOE/NETL 2007 Gasification Database, nearly all planned gasifiers will be of the entrained flow family [4]. The primary reasons for this are: high throughputs, high carbon conversions and very low concentrations of tars and hydrocarbons associated with entrained flow gasifiers (EFGs) [5]. Important characteristics of the main EFG designs are shown in Table 1. However, there are significant technical challenges associated with the operation of EFGs. Foremost among these are: 1. Lack of dynamic feedstock flexibility: changes in feedstock composition can lead to unacceptable syngas composition changes and unpredictable slag behavior. 2. Injector failure: high flame temperature and high particle velocities lead to short injector life. This is particularly true for slurry-fed designs. 3. Slag behavior: even under normal operating conditions, slag can freeze, causing corrosion and blockages inside the gasifier.

Energy recovery from thermal treatment of dewatered sludge in wastewater treatment plants

Sewage sludge is a by-product generated from municipal wastewater treatment (WWT) processes. This study examines the conversion of sludge via energy recovery from gasification/combustion for thermal treatment of dewatered sludge. The present analysis is based on a chemical equilibrium model of thermal conversion of previously dewatered sludge with moisture content of 60-80%. Prior to combustion/gasification, sludge is dried to a moisture content of 25-55% by two processes: (1) heat recovered from syngas/flue gas cooling and (2) heat recovered from syngas combustion. The electricity recovered from the combined heat and power process can be reused in syngas cleaning and in the WWT plant. Gas temperature, total heat and electricity recoverable are evaluated using the model. Results show that generation of electricity from dewatered sludge with low moisture content (≤ 70%) is feasible within a self-sufficient sludge treatment process. Optimal conditions for gasification correspond to an equivalence ratio of 2.3 and dried sludge moisture content of 25%. Net electricity generated from syngas combustion can account for 0.071 kWh/m(3) of wastewater treated, which is up to 25.4-28.4% of the WWT plants total energy consumption.

Effect of High Temperature Corrosion on the Service Life of P91 Piping in Biomass Co-Firing

The authors would like to acknowledge the funding from the Irish Research Council, Bord na Mona, and ESB under the Enterprise Partnership Scheme (No. EPSPG/2012/466). The authors would also like to express their gratitude to Dr. Fionn Griffin from the ESB and Mr. Barry Hooper from the Bord na Mona, industrial collaborators in this project. SEM and EDX work presented here was carried out at the NCBES Electron Microscopy Unit within the Center for Microscopy and Imaging at the NUI Galway, a facility funded by NUI Galway and the Irish Government’s program for Research in Third Level Institutions, Cycles 4 and 5, National Development Plan 2007–2013. The authors would also like to express their gratitude to collaborators of the METCAM project, which is supported by Science Foundation Ireland under Grant No. SFI/10/IN.1/I3015.

Integrated Oxygen-Free Gasification-Gas Turbine Power Concept: A Low-Emissions Alternative for Small-Scale Coal Plants

A power plant concept with the potential to replace the large fleet of ageing, small-scale (< 50 MWe ), inefficient, polluting, and soon-to-be obsolete coal-fired power plants is proposed. The proposed plant comprises a bituminous coal-fed oxygen-free gasifier, low-temperature syngas cleanup system and an open-cycle gas turbine with heat recovery. Heat is supplied to the gasifier through combustion of a portion of the cleaned syngas produced by it. Since the proposed plant employs only a gas turbine, with no steam bottoming cycle, heat recovery from the gas turbine and its integration with the rest of the plant is crucial. A thermodynamic model of the plant has been created to assess its feasibility based on overall efficiency and emissions of CO2 , SO2 , mercury and particulates. The model comprises submodels for feedstock composition and enthalpy, as well as first-order thermodynamic models for each of the plant components including the gasifier, feedstock preparation, heat exchangers and steam generators, contaminant removal, combustors and turbomachinery. The results of the analysis show base case plant thermal efficiency of 38.2% on a HHV basis, which is roughly 5% points higher than that for a similarly-sized pulverized coal combustion (PCC) plants. Emissions of CO2 , SO2 , mercury and particulates per unit electrical energy produced in the base case are: 0.774 kg/kWh, 47.2 g/MWh, 2.37 g/GWh and 28.2 g/MWh, respectively. These values are well below emissions from similarly-sized PCC plants, which have been assessed using a spreadsheet model. The model of the proposed plant has been used to assess overall performance when torrefied pine wood is co-gasified with coal. Results show a slight decrease in plant efficiency with increasing co-gasification, with large decreases in CO2 , SO2 and mercury emissions. Emissions of particulates increase slightly with co-gasification. Finally the model has been used to perform sensitivity analysis on the proposed system. Sensitivity analysis highlights the need for greater understanding of gasifier performance under a range of conditions.Copyright

Introducing a Novel Reactor Concept: Indirectly Fired Integrated Gasification and Steam Generation System

This paper introduces a novel gasification reactor that uses steam gasification of carbonaceous feedstock by indirectly heating the reacting flow through a high temperature heat exchanger without the need for partial combustion with oxygen. It demonstrates the importance of gasification as a method for increasing power plant efficiency and reducing emissions. This paper also describes the computational model created to model this novel gasifier and the results of the model that illustrates the efficiency and purity advantages of the new gasifier. The reactor was modeled as a 1D counter-reacting flows heat exchanger, using the effectiveness-number of transfer units (e-Ntu ) method. The heating flow was assumed to be fully combusted at the inlet. The gasification stream was modeled as a plug flow, where the reaction is kinetically controlled. A simplified version of the Random Pore Model (RPM) was used to predict the char consumption. The results indicate that the gasification of coal with steam without partial combustion with oxygen using this new concept is feasible. The gasification reaction rates are found to be slow at temperatures less than 1200°C, but most of the char conversion, which reached about almost 100% completion, occurred at higher than 1200°C.Copyright