K. Clark Midkiff

Modeling and Experiments of Dual-Fuel Engine Combustion and Emissions

The combustion and emissions of a diesel/natural gas dual-fuel engine are studied. Available engine experimental data demonstrates that the dual-fuel configuration provides a potential alternative to diesel engine operation for reducing emissions. The experiments are compared to multi-dimensional model results. The computer code used is based on the KIVA-3V code and consists of updated sub-models to simulate more accurately the fuel spray atomization, auto-ignition, combustion and emissions processes. The model results show that dual-fuel engine combustion and emissions are well predicted by the present multi-dimensional model. Significant reduction in NO x emissions is observed in both the experiments and simulations when natural gas is substituted for diesel fuel. The HC emissions are under predicted by numerical model as the natural gas substitution is increased. The capabilities and limitations of the combustion model to simulate premixed combustion of air and natural gas were identified. It was found that the combustion model previously developed for diesel combustion provides adequately accuracy when extended to model the present dual-fuel cases. However, the accuracy of the predictions deteriorates for small pilot quantities. A brief discussion is given of a new combustion modeling approach that is applicable to very low pilot diesel fuel cases.

An expert system for lighting energy management in public school facilities

: Lighting accounts for a large part of the energy expenses of a facility, ranging from 40% to 60% of the total energy cost. Therefore, it represents a critical factor to consider in any energy management program. Lighting audits involve complex energy calculations, such as the estimation of light intensity per square unit area, quality of illumination, and required number of light sources to obtain an adequate illumination level in the facility. In this paper we discuss the development and implementation of an easy-to-use expert system for lighting energy management in public schools. The prototype computer-based system can evaluate 11 different areas of a school facility, and it can identify suitable lighting solutions from 17 distinct bulb types and 38 ballast types. The system determines a facilitys required number of bulbs, and it performs the cost and saving analysis of its final recommendation. The constructed expert system was successfully validated by two human experts through two case studies, involving the evaluation of a conventional classroom and a medium-size office.

Including heterogeneous combustion in first-order and distributed activation energies models of coal nitrogen release

Abstract Models of fuel-nitrogen release in pulverized-coal flames are developed to account for 1. (1) the occurrence of parallel heterogeneous and homogeneous coal-N removal and 2. (2) the release of coal-N by numerous different first-order reactions characterized by a normal distribution of activation energies. These models are fitted to previously reported flat-flame, coal-dust burner data. Including heterogeneous combustion effects in a model of coal-N release during the early, largely devolatilization, stage of the flames results in a first-order coal-N devolatilization rate constant with an activation energy that is significantly, but not substantially, higher than the activation energy that is determined neglecting the effects of heterogeneous combustion. Allowance is made for coal devolatilization in excess of that predicted by the ASTM proximate analysis test. A distributed activation energies model is fitted to the combined coal-N release data for particular coals burnt at two different stoichiometries. The mean and standard deviation of the activation temperature for nitrogen devolatilization including the effects of heterogeneous combustion are 8200 ± 600 K for the bituminous coal studied and 5100 ± 2300 K for a subbituminous coal.

Sensitivity Analyses of NOx Formation in Micro-Pilot Ignited Natural Gas Engines

A sensitivity analysis of NOx formation in micro-pilot ignited natural gas dual fuel engines is performed based on a phenomenological combustion model. The model’s NOx formation mechanism incorporates a super-extended Zel’dovich mechanism (up to 43 reactions). The sensitivity analysis compares the contribution of each major reaction to NOx formation, and identifies the rate controlling NOx formation reactions. The formation rates for reactions involving NOx are also investigated to reveal the primary NOx formation paths. Results show that there are two main NOx formation paths both in the packets zone and the burned zone. The rate limiting reactions for the packets zone are identified as: O + N2 = NO + NN2 + HO2 = NO + HNO Rate limiting reactions for the burned zone are: N2O + M = N2 + O + MN2 + HO2 = NO + HNO Since the aforementioned reaction significantly influence the net NOx prediction, it is important that the corresponding reaction rates be determined fairly accurately. Finally, because the quasi-steady-state assumption is commonly used for certain species in NOx modeling, a transient relative error is estimated to evaluate its use. The relative error in NOx prediction with and without this assumption is of the order of 2 percent. Clearly, sensitivity analysis can provide valuable insight into understanding the possible NOx formation pathways in engines and improve the status of current prediction tools to obtain better estimates.Copyright

The Advanced Low Pilot Ignited Natural Gas Engine: A Low NOx Alternative to the Diesel Engine

High nitrogen oxides (NOx ) and particulate matter (PM) emissions restrict future use of conventional diesel engines for efficient, low-cost power generation. The advanced low pilot ignited natural gas (ALPING) engine described here has potential to meet stringent NOx and PM emissions regulations. It uses natural gas as the primary fuel (95 to 98 percent of the fuel energy input here) and a diesel fuel pilot to achieve compression ignition. Experimental measurements are reported from a single cylinder, compression-ignition engine employing highly advanced injection timing (45°–60°BTDC). The ALPING engine is a promising strategy to reduce NOx emissions, with measured full-load NOx emissions of less than 0.25 g/kWh and identical fuel economy to baseline straight diesel operation. However, unburned hydrocarbons were significantly higher for ALPING operation. Engine stability, as measured by COV, was 4–6 percent for ALPING operation compared to 0.6–0.9 percent for straight diesel.Copyright

The Advanced Injection Low Pilot Ignited Natural Gas Engine: A Combustion Analysis

The Advanced Low Pilot Ignited Natural Gas (ALPING) engine is proposed as an alternative to diesel and conventional dual fuel engines. Experimental results from full load operation at a constant speed of 1700 rev/min are presented in this paper. The potential of the ALPING engine is realized in reduced NOx emissions (less than 0.2 g/kWh) at all loads accompanied by fuel conversion efficiencies comparable to straight diesel operation. Some problems at advanced injection timings are recognized in high unburned hydrocarbon (HC) emissions (25 g/kWh), poor engine stability reflected by high COVimep (about 6 percent), and tendency to knock. This paper focuses on the combustion aspects of low pilot ignited natural gas engines with particular emphasis on advanced injection timings (45°–60°BTDC).Copyright

Modeling of NOx Emissions Using a Super-Extended Zel’dovich Mechanism

A kinetic model for NOx production has been developed to predict NOx emissions. The reaction scheme is a modified super-extended Zel’dovich mechanism (SEZM), which includes 43 reactions and 20 species instead of just the three reactions typically used in the extended Zel’dovich mechanism. The NOx emissions predicted by both mechanisms are compared using two separate models. First, a theoretical investigation of the two mechanisms is made for an SI engine using prescribed temperature and pressure histories. Then each of the two mechanisms is combined with a phenomenological combustion model for a single-cylinder Caterpillar 3400 series diesel engine to calculate the NOx emissions. The predictions from both mechanisms are compared with experimental results. It is shown that the SEZM can predict NOx emissions more accurately than the extended Zel’dovich mechanism. Results show that the SEZM increases the predicted NOx by about 25 percent. The difference between the two models is more pronounced for lean combustion, in which NO2 and NH play an important role in the NOx formation. In addition, the effects of several parameters on diesel engine NOx production are investigated. The super-extended Zel’dovich mechanism for NOx formation is expected to be more appropriate for lean combustion, such as in diesel or natural gas engines and other engines that typically operate at lean conditions.Copyright

Strategies for Reduced NO

The performance and emissions of a single-cylinder, natural gas fueled engine using a pilot ignition strategy have been investigated. Small diesel pilots (2–3 percent on an energy basis), when used to ignite homogeneous natural gas-air mixtures, are shown to possess the potential for reduced NOx emissions while maintaining good engine performance. The effect of pilot injection timing, intake charge pressure, and charge temperature on engine performance and emissions with natural gas fueling was studied. With appropriate control of the above variables, engine-out brake specific NOx emissions could be reduced to the range of 0.07–0.10 g/kWh from the baseline diesel (with mechanical fuel injection) value of 10.5 g/kWh. For this NOx reduction, the decrease in fuel conversion efficiency from the baseline diesel value was approximately 1–2 percent. Total unburned hydrocarbon (HC) emissions and carbon monoxide (CO) emissions were higher with natural gas operation. Heat release schedules obtained from measured cylinder pressure data are also presented. The importance of pilot injection timing and inlet conditions on the stability of engine operation and knock are also discussed.Copyright