Ronald W. Breault

SYSTEM ISSUES AND TRADEOFFS ASSOCIATED WITH SYNGAS PRODUCTION AND COMBUSTION

The purpose of this article is to provide an overview of the basic technology of coal gasification for the production of syngas and the utilization of that syngas in power generation. The common gasifier types, fixed/moving bed, fluidized bed, entrained flow, and transport, are described, and accompanying typical product syngas compositions are shown for different coal ranks. Substantial variation in product gas composition is observed with changes in gasifier and coal feed type. Fuel contaminants such as sulfur, nitrogen, ash, as well as heavy metals such as mercury, arsenic, and selenium, can be removed to protect the environment and downstream processes. A variety of methods for syngas utilization for power production are discussed, including both present (gas turbine and internal combustion engines) and future technologies, including oxy-fuel, chemical looping, fuel cells, and hybrids. Goals to improve system efficiencies, further reduce NOx emissions, and provide options for CO2 sequestration require advancements in many aspects of IGCC plants, including the combustion system. Areas for improvements in combustion technology that could minimize these tradeoffs between cost, complexity, and performance are discussed.

A reduced-order model for heat transfer in multiphase flow and practical aspects of the proper orthogonal decomposition

Abstract This paper discusses two practical aspects of reduced-order models (ROMs) based on proper orthogonal decomposition (POD) and presents the derivation and implementation of a ROM for non-isothermal multiphase flow. The POD method calculates basis functions for a reduced-order representation of two-phase flow by calculating the eigenvectors of an autocorrelation matrix composed of snapshots of the flow. The flow is divided into transient and quasi-steady regions and two methods are shown for clustering snapshots in the transient region. Both methods reduce error as compared to the constant sampling case. The ROM for non-isothermal flow was developed using numerical results from a full-order computational fluid dynamics model for a two-dimensional non-isothermal fluidized bed. Excellent agreement is shown between the reduced- and full-order models. The composition of the autocorrelation matrix is also considered for an isothermal case. An approach treating field variables separately is shown to produce less error than a coupled approach.

Acceleration techniques for reduced-order models based on proper orthogonal decomposition

This paper presents several acceleration techniques for reduced-order models based on the proper orthogonal decomposition (POD) method. The techniques proposed herein are: (i) an algorithm for splitting the database of snapshots generated by the full-order model; (ii) a method for solving quasi-symmetrical matrices; (iii) a strategy for reducing the frequency of the projection. The acceleration techniques were applied to a POD-based reduced-order model of the two-phase flows in fluidized beds. This reduced-order model was developed using numerical results from a full-order computational fluid dynamics model of a two-dimensional fluidized bed. Using these acceleration techniques the computational time of the POD model was two orders of magnitude shorter than the full-order model.

Practical Aspects of the Implementation of Proper Orthogonal Decomposition

This paper discusses two practical aspects of the implementation of reduced-order models based on proper orthogonal decomposition (POD). The POD method calculates basis functions used in a reduced-order representation of two-phase flow in fluidized beds by calculating the eigenvectors of an autocorrelation matrix composed of snapshots of the flow. The aspects discussed are: (i) the time sampling of snapshots that minimize error in the POD reconstruction of the flowfield, and (ii) the form of the autocorrelation matrix that minimizes error in the POD reconstruction of the flowfield. Two regions in the flow are identified, a transient region and a quasi-steady region. Two methods are then proposed for time sampling the flow to retain additional snapshots in the transient region. Both methods are shown to produce less error than the case where snapshots are sampled a constant frequency. A time sampling rate based on a logarithmic distribution with 200 snapshots is shown to produce error on the same order as an evenly spaced snapshot database with 800 snapshots. The composition of the autocorrelation matrix is also considered. An approach treating field variables entirely separately is shown to produce less error than a coupled approach when the field variables are reconstructed.

Reaction Kinetics for Flue Gas Treatment of NOx

During the past several years, control of NOx emissions has become a national issue. NOx emissions are a leading contributor to acid rain as well as contributing strongly to photo-chemical smog. In this regard, NOx emissions have the most widely spread detrimental impact on air quality, vegetation and human health of any regulated emission. Current state-of-the-art technologies either modify the combustion zone in an effort to control the temperature, residence time and stoichiometry thereby lowering NOx emissions or provide a post combustion reducing agent that consumes the oxygen in the NOx molecule producing nitrogen and water. These two approaches have essentially reached their maximum potential. Combustion modifications can no longer achieve the emission levels required in specific regions throughout the country, namely southern California and the North East. Therefore, processes are required to add post combustion control as well. These technologies function either with or without the aid of a catalyst. A major problem with these techniques is that secondary pollution by the reducing agent (preferably NH3 due to its reactivity and selectivity) is becoming an issue and monitoring its emissions is required (Medros et al. 1989). Furthermore, oils and other compounds can reduce the effectiveness of an expensive catalyst, eventually requiring the replacement of it.

The Effect of Thermal Treatment of Hematite Ore for Chemical Looping Combustion of Methane

For chemical looping processes to become an economically viable technology, an inexpensive carrier that can endure repeated reduction and oxidation cycles needs to be identified or developed. Unfortunately, the reduction of hematite ore with methane in both batch and fluidized beds has revealed that the performance (methane conversion) decreases with time. Previous analysis had shown that the grains within the particle grew with the net effect of reducing the surface area of the particles and thereby reducing the rate and net conversion for a fixed reduction time. To improve the lifespan of hematite ore, it is hypothesized that if the grain size could be stabilized, then the conversion could be stabilized. In this work, series of tests were conducted in an electrically heated fluidized bed. The hematite ore was first pretreated at a temperature higher than the subsequent reduction temperatures. After pretreatment, the hematite ore was subjected to a series of cyclic reduction/oxidation experiments. The results show that the ore can be stabilized for cycles at different conditions up to the pretreatment temperature without any degradation. Details of the pretreatment process and the test results will be presented.

CO2 Adsorption: Experimental Investigation and CFD Reactor Model Validation

The National Energy Technology Laboratory is investigating a new process for CO2 capture from large sources such as utility power generation facilities as an alternative to liquid amine based adsorption processes. Many of these advanced dry processes are based upon sorbents composed of supported polyamines. In this analysis, experiments have been conducted in a laboratory-scale fluidized bed reactor and compared to CFD reactor predictions using kinetics obtained from TGA tests. Batch experiments were conducted by flowing a mixture of CO2, H2O, and N2 (simulated flue gas) through a fluidized bed of sorbent material. The exit gas composition time series data is compared to CFD simulations using a 3-dimensional nonisothermal reacting multiphase flow model. The effects of the gas flow rate, distributor design, and particle size are explored through the CFD simulations. It is shown that the time duration for CO2 adsorption decreased for an increase in the gas flow. Fluid bed hydrodynamics indicated that there were regions in the reactor where the inert FCC particles segregated and defluidized; without adversely affecting the capacity of the sorbent to adsorb CO2. The details of the experimental facility and the model as well as the comparative analysis between the data and the simulation results are discussed.

Experimental Investigation of Gas-Solid Disengagement in Bubbling Fluidized Bed Cold Model

The aim of this experimental study is to investigate the separation performances of a new gas-solid disengagment (GSD) device in a 10 cm bubbling fluidized bed cold model. An impactor separator is designed to be fitted internally onto the loop seal and install in the bubbling fluidized bed based on the impingement separation concept. The effects of operating parameters, such as static bed height, the length of the GSD device’s dip leg, and effect of dip leg porting were examined. The results indicated that a higher static bed height increases the mass of elutriated particles from the bed column. The length of the GSD device’s dip leg has a small effect on its separation ability. A longer dip leg can reduce the amount of particles elutriated due to the reduced pressure it experiences on the solids exit as compared to a shorter dip leg. The addition of fluidization ports to the dip leg has a negative effect on preventing particles from escaping through the gas outlet. The fluidizing gas will bypass the gas inlet of the GSD device through the dip leg and no particles will impact the baffle to be separated from the gas flow. Increasing the diameter of the solid exit for the dip leg increases the efficiency of the GSD device. The higher rate of particles passing through the dip leg reduces the chance of gas flow blockage by particles accumulating in the GSD device. The research concludes that with an effective design for a gas-solid disengagement device will reduce up to 75% of the particles that enter the filtration system that elutriate from the bubbling fluidized bed.

Impact of Hydrogen Assisted Lean Operation (HALO) on Natural Gas Engine Combustion

Two key challenges facing natural gas engines used for cogeneration are spark plug life and high NOx emissions. Using Hydrogen Assisted Lean Operation (HALO), these two key issues are simultaneously addressed. HALO operation, as demonstrated in this project, allows stable engine operation to be achieved at ultra-lean conditions and significantly reduces NOx production. For example, at 8% hydrogen supplementation by energy based upon lower heating values, NOx values of 10 ppm (0.07 g/bhp-hr NOx ) at an exhaust O2 level of 10% were demonstrated, which is a 98% NOx emissions reduction compared to the leanest operating condition achievable without hydrogen. Spark ignition energy reduction (which will increase ignition system life) was successfully achieved using hydrogen at an exhaust O2 level of 9%, leading to a NOx emission level of 28 ppm (0.13 g/bhp-hr NOx ). At this operating condition, it was found that spark energy could be reduced 22% (from 151 mJ supplied to the coil) with 13% hydrogen supplementation based on lower heating values, and even further reduced 27% with 17% hydrogen supplementation, with no reportable effect on NOx emissions for these conditions and with stable engine torque output. Another important result is that the combustion duration was shown to be primarily dependant on hydrogen supplementation, not a function of ignition energy (until the ignitability limit was reached). The next logical step leading from these results is to see how much the spark energy reduction translates into increase in spark plug life by performing durability testing.Copyright

Hydrogen Storage Using Slurries of Chemical Hydrides

The usual storage technologies considered for hydrogen are compressed hydrogen, liquid hydrogen, metal hydrides, and carbon-based storage systems. Over the past couple of years another method of hydrogen storage and transmission has been under investigation that offers some significant advantages over the usual hydrogen storage technologies. Thermo Power Corporation has been developing a chemical hydride slurry approach. In this approach, a light metal hydride is used as the hydrogen carrier and storage media. Light metal hydrides such as lithium hydride, magnesium hydride, sodium hydride, and calcium hydride produce hydrogen when they react with water. These materials are typically dry solids at ambient conditions. The oil in the slurry protects the hydride from unintentional contact with moisture in the air and makes the hydride pumpable. At the point of storage and use, a chemical hydride/water reaction is used to produce high-purity hydrogen. An essential feature of this approach is the recovery and recycle of the spent hydride at centralized processing plants to produce new hydride slurry, resulting in an overall low cost for hydrogen. This chemical hydride slurry system has several benefits: it greatly improves the energy transmission and storage characteristics of hydrogen as a fuel, it provides a hydrogen storage medium that is stable at normal environmental temperatures and pressures, it is pumpable and easily transported, it has a high gravimetric and volumetric energy density, with the use of a properly designed reactor it can provide hydrogen at elevated pressures without the use of a compressor, it produces the hydrogen carrier efficiently and economically from a low cost carbon source, and since the production of the hydride is a carbo-thermal process performed at a centralized plant, CO2 resulting from the carbo-thermal process for refining lithium is concentrated and amenable to sequestration.