Kirby S. Chapman

Virtual Pipeline System Testbed to Optimize the U.S. Natural Gas Transmission Pipeline System

The goal of this project is to develop a Virtual Pipeline System Testbed (VPST) for natural gas transmission. This study uses a fully implicit finite difference method to analyze transient, nonisothermal compressible gas flow through a gas pipeline system. The inertia term of the momentum equation is included in the analysis. The testbed simulate compressor stations, the pipe that connects these compressor stations, the supply sources, and the end-user demand markets. The compressor station is described by identifying the make, model, and number of engines, gas turbines, and compressors. System operators and engineers can analyze the impact of system changes on the dynamic deliverability of gas and on the environment.

Increasing Turbocharged Engine Operating Ranges Through Use of a Booster System

In the natural gas industry a large portion of the engines used for compression are lean-burn engines. When these engines operate at low equivalence ratios, oxides of nitrogen (NOX ) can be minimized. The lean-burn engines are turbocharged to deliver high air flow to the engine. However, varying ambient temperatures alter the mass flow rate of air delivered to the engine, changing the equivalence ratio the engine fires at. This may cause an engine to be de-rated, or taken off line reducing the gas throughput. This problem can be partially offset by the installation of a turbocharger booster system to increase the available energy at the turbocharger turbine inlet. One method to boost the energy available to drive the turbocharger is to increase the temperature of the exhaust before it enters the turbine via a relatively small dry low NOX burner. A turbocharger booster system was designed, prototyped, installed, and tested at the National Gas Machinery Laboratory (NGML) turbocharger test and research facility (TTRF). The test data show that the addition of a turbocharger booster system increased the speed of the turbocharger without increasing the emission levels. The increase in speed translates to an increased pressure ratio and mass flow rate of air produced by the compressor. By controlling the booster system, constant air flow rate can be achieved regardless of ambient conditions. This paper provides test results that show how the system can be used to increase an engine’s operating range and mitigate ambient effects.Copyright

Dynamic Modeling of Non-Isothermal Gas Pipeline Systems

Natural gas systems are becoming more and more complex as the usage of this energy source increase. Mathematical models are used to design, optimize, and operate increasingly complex natural gas pipeline systems. Researchers continue to develop unsteady mathematical models that focus on the unsteady nature of these systems. Many related design problems, however, could be solved using steady-state modeling. Several investigators have studied the problem of compressible fluid flow through pipelines and have developed various numerical schemes, which include the method of characteristics, finite element methods, and explicit and implicit finite difference methods. The choice partly depends on the individual requirements of the system under investigation. In this work, the fully implicit finite difference method was used to solve the continuity, momentum, energy, and equations of state for flow within a gas pipeline system. The particular solution method described in this paper does not neglect the inertia term in the conservation of momentum equation. It also considered the compressibility factor as a function of temperature and pressure, and the friction factor as a function of the Reynolds number. the fully implicit method representation of the equations offer the advantage of guaranteed stability for a large time step, which is very useful for the gas industry. The results show that the effect of treating the gas in a non-isothermal manner is extremely necessary for pipeline flow calculation accuracies, especially for rapid transient processes. The results indicate that the inertia term plays an important role in the gas flow analysis and cannot be neglected from the calculation.Copyright

Variations in Long-Term Emissions Data From NSCR-Equipped Natural Gas-Fueled Engine

This paper describes work by Kansas State University’s National Gas Machinery Laboratory and Innovative Environmental Solutions, Inc. on a project to characterize pollutant emissions performance of non-selective catalytic reduction (NSCR) technology, including a catalyst and air-to-fuel ratio controller (AFRC), applied to four-stroke cycle rich-burn engines. Emissions and engine data were collected semi-continuously with a portable emissions analyzer on three engines in the Four Corners area. In addition, periodic emissions measurements that included ammonia were conducted several times. Data collected from October, 2007, through August, 2008, shows significant variation in emissions levels over hours, days, and longer periods of time, as well as seasonal variation. As a result of these variations, simultaneous control of NOx to below a few hundred parts per million (ppm) and CO to below 1,000 ppm volumetric concentration was not consistently achieved. Instead, the NSCR/AFRC systems were able to simultaneously control both species to these levels for only a fraction of the time the engines were monitored. Both semi-continuous emissions data and periodically collected emissions data support a NOx -CO trade-off and a NOx -ammonia tradeoff in NSCR-equipped engines.Copyright

Mapping Study to Characterize NSCR Performance on a Natural Gas-Fueled Engine

Kansas State University’s National Gas Machinery Laboratory and Innovative Environmental Solutions, Inc. are conducting a project to characterize pollutant emissions performance of field gas-fired four-stroke cycle rich burn (4SCRB) engines equipped with non-selective catalytic reduction (NSCR) technology. Engine emissions and operating parameters are being monitored on three engines over an extended period. In addition, a mapping study was conducted on one engine. The NSCR was operated at various controlled air-to-fuel ratios (AF) while emission measurements were conducted and engine operating parameters monitored. NOx , CO, and oxygen were measured using both EPA reference method technology and the portable analyzer used in the long-term study. In the mapping study, ammonia, formaldehyde, and speciated hydrocarbon emissions were recorded in real-time using an extractive FTIR system. This paper focuses on the engine mapping phase. The mapping tests demonstrated a trade-off between NOx emissions and CO, ammonia, and hydrocarbon emissions. Richer engine operation (lower AF) decreases NOx emissions at the expense of higher CO, ammonia, and hydrocarbons. Leaner operation has the opposite effect. The results to date of the semi-continuous monitoring are presented in a separate paper.Copyright

Turbocharger Component Matching System Development

This paper outlines the development of the Turbocharger Component Matching System (TuCMS) software package that can be used to inexpensively analyze turbocharger performance, and match turbocharger components to integrate and optimize turbocharger-engine performance. The software system is being developed with the intent to reduce the time taken to experimentally match a turbocharger with an engine, a task that is key to engine emission reductions. TuCMS uses one-dimensional thermo-fluid equations to analyze both the compressor and turbine side of a turbocharger. The program calculates the velocities, pressures, temperatures, pressure drops, and efficiencies for a specified set of turbocharger geometry and atmospheric conditions. Both the compressor and turbine side include established loss models found in the open literature. Work and rotational speed are the parameters used in the turbine and compressor analysis algorithms to integrate the turbocharger system. TuCMS utilizes a component-based architecture to simplify model enhancements. TuCMS can be used as a cost effective engineering tool for preliminary turbocharger testing during engine upgrades and modifications.Copyright

Fuel-Efficient Operation of Compressor Stations Using Simulation-Based Optimization

One of the primary concerns in the operation of a compressor station is minimization of fuel consumption while maintaining the desired throughput of natural gas. In practice, the station operator tries to achieve this by shutting down units or controlling individual unit speeds based on experience. This is generally a trial-and-error process without any guarantee of optimality. In this paper we present a robust structured solution process for tackling this problem using simulation-based optimization. The first step to develop this solution process is to devise an analysis scheme that provides the simulation support required by the optimization. This was achieved by developing a fully implicit finite difference formulation of the continuity, momentum and energy equations for flow under non-isothermal conditions. The performance of each compressor unit was modeled by fitting polynomials to the compressor map. These polynomial equations were appended to the flow equations to obtain a complete set of system governing equations. The nonlinear algebraic equations resulting from this formulation were then solved using a Newton-Raphson iteration to obtain system performance. The problem of optimizing the operation of a compressor station was then formulated as a nonlinear programming problem (NLP) in which the design variables are the compressor unit speeds and the objective function to be minimized is the fuel consumption. A constraint was also placed on the minimum mass flow rate through the station to ensure that adequate flow is maintained while minimizing fuel consumption. This NLP was then solved using a sequential unconstrained minimization technique (SUMT) with a derivative-free grid search for handling the unconstrained minimizations. The simulation algorithm mentioned earlier is invoked whenever the optimization needs to evaluate the system response at a candidate operating point. The results obtained show that the simulation works very well in terms of predicting system response, and the proposed simulation-based optimization approach is highly effective in minimizing fuel consumption in a systematic way. The approach is successfully applied to single stations as well as to a sequence of stations along a pipeline, thereby establishing its applicability to station-level and network-level optimization.© 2004 ASME

Experimentally Determined Port Discharge Coefficients From Large-Bore Reciprocating Engines

This paper describes the collection and analysis of discharge coefficients from the ports of large-bore two-stroke cycle engines. The literature includes some information on discharge coefficients from very small ports. The literature was found to not include data collected from very large ports, such is in Cooper, Clark, and Worthington two-stroke cycle engines. The methodology was to construct and then use a flow bench that was sized for large-bore engine cylinder liners. The flow bench is designed to experimentally determine the discharge coefficients of large bore engine ports. The discharge coefficients are an integral part of determining the air flow rate through an engine, and in modeling and predicting the airflow through an engine system. This information can be used by designers to better match turbochargers and aftercoolers to engines. Large bore engine cylinders are typically are 35–56 cm (14–22 in.) in diameter, and have power outputs ranging 745–3730 kW (1,000–6,000 hp). In general, the majority of these engines were built in the 1940–1950’s. The importance of predicting the airflow rate through these engines has become paramount due to increasingly stringent EPA emission regulations. The data shows that there is a vast difference between the discharge coefficients of the three primary engines used in the natural gas industry.Copyright

Modeling of Lambda Sensor Output With Exhaust Gas Mixtures From Natural Gas-Fueled Engines

The increasingly strict emission regulations may require implementing Non-Selective Catalytic Reduction (NSCR) system as a promising emission control technology for stationary rich burn spark ignition engines. Many recent investigations used NSCR systems for stationary natural gas fueled engines showed that NSCR systems were unable to consistently control the emissions level below the compliance limits. Modeling of NSCR components to better understand, and then exploit, the underlying physical processes that occur in the lambda sensor and the catalyst media is now considered an essential step toward the required NSCR system performance. This paper presents the work done to date on a modeling of lambda sensor that provides feedback to the air-to-fuel controller. Several recent experimental studies indicate that the voltage signal from the lambda sensor may not be interpreted correctly because of the physical nature in the way the sensor senses the exhaust gas concentration. Correct interpretation of the sensor output signal is necessary to achieve consistently low emissions level. The goal of this modeling study is to improve the understanding of the physical processes that occur within the sensor, investigate the cross-sensitivity of various exhaust gas species on the sensor performance, and finally this model serves as a tool to improve NSCR control strategies. This model simulates the output from a planar switch type lambda sensor. The model consists of three modules. The first module models the multi-component mass transport through the sensor protective layer. Diffusion fluxes are calculated using the Maxwell-Stefan equation. The second module includes all the surface catalytic reactions that take place on the sensor platinum electrodes. All kinetic reactions are modeled based on the Langmuir-Hinshelwood kinetic mechanism. The model incorporates for the first time methane catalytic reactions on the sensor platinum electrode. The third module is responsible for simulating the reactions that occur on the electrolyte material and determine the sensor output voltage. The model results are validated using field test data obtained from a mapping study of a natural gas-fueled engine equipped with NSCR system. The data showed that the lambda sensor output voltage is influenced by the reducing species concentration, such as carbon monoxide (CO) and hydrogen (H2 ). The results from the developed model and the experimental data showed strong correlations between CO and H2 with the sensor output voltage within the lambda operating range between 0.994 to 1.007 (catalytic converter operating window). This model also showed that methane does not significantly influence the lambda sensor performance compared to the effect of CO and H2 .Copyright

Interpreting the Lambda Sensor Output Signal to Control Emissions From Natural Gas Fueled Engines

This paper presents the work done to date on a modeling study of the Non-Selective Catalytic Reduction (NSCR) system. Several recent experimental studies indicate that the voltage signal from the heated exhaust gas oxygen sensor commonly used to control these emission reduction systems may not be interpreted correctly because of the physical nature in the way the sensor senses the exhaust gas concentration. While the current signal interpretation may be satisfactory for modest NOX and CO reduction, an improved understanding of the signal is necessary to achieve consistently low NOX and CO emission levels. The increasingly strict emission regulations may require implementing NSCR as a promising emission control technology for stationary spark ignition engines. Many recent experimental investigations that used NSCR systems for stationary natural gas fueled engines showed that NSCR systems were unable to consistently control the emissions level below the compliance limits. Modeling of NSCR components to better understand, and then exploit, the underlying physical processes that occur in the lambda sensor and the catalyst media is now considered an essential step toward improving NSCR system performance. This paper focuses only on the lambda sensor that provides feedback to the air-to-fuel ratio controller. The goals of this modeling study are: • Improve the understanding of the transport phenomena and electrochemical processes that occur within the sensor. • Investigate the cross-sensitivity of exhaust gases from natural gas fueled engines on the sensor performance. • Serve as a tool for improving NSCR control strategies. This model simulates the output from a planar switch type lambda sensor. The model consists of three modules. The first module models the multi-component mass transport through the sensor protective layer. A one dimensional mass conservation equation is used for each exhaust gas species. Diffusion fluxes are calculated using the Maxwell-Stefan equation. The second module includes all the surface catalytic reactions that take place on the sensor platinum electrodes. All kinetic reactions are modeled based on the Langmuir-Hinshelwood kinetic mechanism. The third module is responsible for simulating the reactions that occur on the electrolyte material and determining the sensor output voltage. The details of these three modules as well as a parametric study that investigates the sensitivity of the output voltage signal to various exhaust gas parameters is provided in the paper.Copyright