Natural Gas based Diesel Fuel Research

Energy Alternatives

Dr. Dennis Witmer and Dr. Chuen-Sen Lin carry out natural gas based diesel fuel research at the Arctic Energy Technology Development Lab, at the University of Alaska Fairbanks.

Hundreds of Alaskan villages have individual power plants, which range from few kW to tens of MW and most of these power plants run on diesel fuels. Fuel cost, transportation, storage, and contamination caused by fuel leakages have long been considered to be critical problems in the development of rural power systems. A variety of efforts have been made to alleviate these problems, for instance testing of clean fuels, installation of alternate energy power plants, construction of new fuel storage facilities etc. Mechanical Engineering Department at the University of Alaska Fairbanks has a long history of being involved in these efforts. Examples for these efforts include research on fuel cells, wind turbines, synthetic and bio fuels, heat recovery, system remote sensing and monitoring etc.

Clean diesel-testing project is one of the most recent rural power related research projects, which involves many ME students and faculty members. The clean diesel project is being carried out at the UAF Energy Center with support from ICRC; an Alaska based company and DOE. The research comprises of measurement and analyses of emissions of two Syntroleum diesel fuels, energy balance, endurance testing, exhaust heat recovery, engine vibration analysis and engine cylinder pressure reconstruction using inexpensive sensors. Syntroleum fuels are synthetic diesel fuels produced from natural gas through a Gas to Liquid (GTL) process or Fischer Tropsch (F-T) process. Both fuels are manufactured by Syntroleum Corporation and are marked as the S-1 and S-2 brands. The F-T process is a catalyzed chemical reaction which converts hydrogen and carbon monoxide into liquid hydrocarbons of different forms to substitute petroleum. The general process may be represented by the following reactions:

CH4 + Â½ O2 â†’ 2 H2 + CO (Syn Gas)
(2n +1) H2 + n CO â†’ CnH2n+2 + n H2O (Syn Crude)

The Syn Crude is then broken into lengths comparable with those of conventional diesel fuels. The Syntroleum fuels are produced by Syntroleum, using a proprietary process called Syntroleum Process. For comparison, properties of S-2 and conventional diesel are listed in the table below:

Fuel Property	Sulfur (ppm)	Aromatics (ppm)	Cetane Number	Heating Value (Btu/gal)	Specific Gravity
Conventional Fuel	350	31	47	130,000	0.85
S2	0	0	74	120,000	0.77

One of the major objectives of this research is to test the synthetic fuels for its compatability with an existing diesel generator and its compliance to the emissions code without requiring expensive modifications of the diesel generator. The second major objective is to determine the effect of recovering heat energy from exhaust of ultra clean diesel on the maintenance of the diesel generator. The experimental set-up for fuel performance test includes a 235 kW DD-50 heavy-duty diesel generator, a 2-section bulk fuel storage tank with 3000 gallons and 5000 gallons capacity sections, two 300-gallon day tanks, a load bank with load factor control, a set of National Instrument data acquisition system and data analysis software, ECOM AC Plus emission analyzer, a JUM FID analyzer, a Model 50 MC2-6 laminar flow element coupled with a HART differential pressure transducer for mass flow measurement of inlet air, a home made gravimetric device for fuel flow rate measurement, accelerometers for engine vibration measurement, thermocouples, and pressure gauges. In addition to these, engine performance data is also extracted from the CAN Bus of the diesel generator and the engine operating conditions are adjusted for performance optimization using fuel injection timing control. For heat recovery testing, the set-up includes a gas to liquid heat exchanger for exhaust heat extraction, a unit heater for load simulation, a 3-way temperature control valve for load temperature control, circuit setters for flow balance and control, corrosion coupons for corrosion monitoring, and other necessary flow operation and performance measurement sensors, such as pump, pressure gauges, etc.

Presently, test for emissions, energy balance, and endurance are completed and results are obtained. For the 2000-hour testing performed, the engine ran under a varying load, which mimics the load of diesel generators used in Alaskan villages. During this 2000-hour period, the engine performance parameters were measured. The measured results showed that the performance was consistent and also it was observed that no abnormal performance was caused by the synthetic fuels. After the 2000-hour testing, the engine head and injectors were disassembled for investigation and no noticeable damages, obstructions or abnormal residues were found. Emissions were measured according to the procedure of the EPA 5 Mode Test Cycle. Emissions were also measured occasionally during the 2000-hour testing while the engine was under varying load. Emission results of EPA 5 Mode Test Cycle for conventional diesel and the synthetic fuel obtained from the same engine showed that the reduction in emissions due to synthetic fuels were about 30% for CO, 12% for NOx, and 20% for CH. It was also found that the BSFC of the synthetic fuels was lower (more efficient) than that of the conventional fuel by a margin of 0.2% to 3.3%.
Current activities of this research project include the analysis of engine vibration data, continuation of the instrumentation and installation of the heat recovery system, and planning of cylinder pressure recovery using inexpensive sensors. The expected finishing time of this research is September, 2006 and the expected additional results would include the correlations of engine vibrations with fuel type, injection timing, and load; feasibility of heat energy recovery from exhaust of ultra clean fuels for Alaskan village diesel generators and feasibility of cylinder pressure measurement using inexpensive sensors.