F?or the past 60 years, Columbia Gas Transmission LLC, a NiSource Gas Transmission & Storage company, has been transporting most of its natural gas from the Gulf of Mexico and Texas. With gas conditioning and processing as the gas comes on shore, we have had a steady, clean, dry fuel source with an average Btu of about 1,020 to 1,040. It has an average gas composition of 97% methane, <0.5% ethane, 2% propane, <1% isobutane, 0.5% n-butane, .01% pentanes and small amounts of other hydrocarbons. With this stable source of fuel gas, we have been able to set our engine parameters to run steadily, safely and meet emission compliance.

Another source of our gas supply has been the Appalachian Basin. This gas has an average Btu of about 1,240 and typically has higher propane content than the gas that comes from the Gulf region. When running engines on this fuel, the ignition timing is retarded slightly to keep the power cylinders from detonation.

?A chromatograph plot shows the Btu range of Columbia Gas Transmission’s fuel gas from 2002 to 2010.

Now, with the recent discovery and production of the Marcellus shale-gas play in West Virginia, Pennsylvania and New York, the natural gas composition is presenting several new challenges.

We were having issues with a compressor station that used a wide range of fuel gas sources—everything from coalbed methane with a Btu level of around 930, to transmission and storage gas at 1,030, to Marcellus gas at nearly 1,300 Btu. Due to the mixing of gas with various Btu levels, we found it challenging to set the controls to keep the engine out of detonation and to stay in emission compliance.

Figure 1 is a chromatograph plot showing the Btu of our fuel gas from 2002 to present. As one can see, from 2002 until 2008, the Btu level has been very steady at an average of 1,045. Since the Marcellus shale gas started coming into the station, the Btu level has varied from 950 to as high as 1,300, with an average of about 1,155 Btu.

But the challenge is the constant changing in levels.

We wanted to monitor the effects of the rapid Btu and ethane change. Because ethane has a lower auto-ignition temperature than methane, as the ethane percentage increases in the fuel gas, the air/fuel mixture in the cylinder lights off sooner. Table 1 shows the lower explosion level of methane, ethane and propane.

Peak firing pressures of a cylinder change as the ethane level increases.

Figure 2 shows the peak firing pressures of a cylinder as the ethane level increases. The ignition timing is stationary at 15 degrees before top dead center (BTDC). The first trace in blue is at 4% ethane. Here, the peak-firing pressure (pfp) is normal at about 570 per square inch (psi) with a pfp angle at 18.2 degrees after top dead center (ATDC). The next trace is at 7% ethane, with a pfp of 650 psi and a peak angle at 15 degrees ATDC. The third trace is at 9% ethane, with the pfp at 744 with a peak angle at 11.8 degrees ATDC. The last trace is at 13% ethane. At this point, audible detonation can be heard.

The pfp now is at 731, but the peak angle is at 6.4 degrees ATDC. The left side of the trace shows the early expansion of the air/fuel mixture causing very high pressures at compression pressure. This also causes issues because it wants to push backwards on the crankshaft and causes the engine to slow down, and then the governor opens to give the unit more fuel. In most four-cycle reciprocating engines, we like to keep the pfp angle between 16 to 22 degrees ATDC.

The ethane level on a test was steady at 14.97%. We saw a slight change in the CO, O2 and exhaust temperatures, but a huge difference in the NOx levels. The ignition timing was at 15 degrees BTDC. The NOx level was at 1,500 parts per million (ppm). By retarding the ignition timing, we can control not only the pfp, but also the pfp angle.

This figures shows the peak firing pressure of a cylinder with 12.63% ethane.

NOx is created with temperature and time. As the ethane lights off earlier than methane, the peak firing pressures are higher and closer to top dead center. As we retard the ignition timing to 12 degrees BTDC, the NOx level drops to 900 ppm. At 9 degrees BTDC, the NOx level drops to 600 ppm. Finally, in order to get the NOx numbers in the permitted level, the ignition timing needed to be retarded to 7 degrees BTDC.

Figure 3 shows the peak firing pressure of a cylinder with 12.63% ethane. The blue trace shows the peak firing pressure at 15 degrees BTDC. The pressure is higher and closer to TDC. The red trace is at 12 degrees BTDC. Here we are lowering the firing pressure and it is moving away from TDC. Then, a normal trace with the ignition set at 9 degrees BTDC shows the firing pressure is normal and the NOx levels are at the desired limits.

Solutions

With the Btu levels changing so frequently, we had to come up with something that would be dynamic and change the engine parameters as the Btu levels changed. To keep the engine from detonating and to stay in emission compliance, we needed to keep the peak firing pressure angle between 16 to 20 degrees ATDC.

At first we thought we could just track the Btu levels, but we found that we have better results tracking just the ethane level in the gas stream. Figure 4 shows the chromatograph results for April 7, 2010, with a heating valve of 1,190.6. On this day the ethane level was at 12.64% with ignition timing at 15 degrees BTDC and the unit was showing slight detonation.

On the following day the chromatograph showed a lower heating valve of 1,138.8 Btu, but the ethane level increased to 14.97%. The ignition timing was still at 15 degrees BTDC. The engine was now in severe detonation. The engine load, speed, and other ambient conditions were the same.

With an ethane level at 12.64% and ignition timing at 15 degrees BTDC, the engine shows slight detonation.

As the ethane level increases, the ignition timing retards. This moves the peak firing pressure angle away from top dead center, keeps the unit from detonating and keeps the NOx levels low. With further testing we have found that, as the ethane increases, our first step is to close the turbocharger waste gate, which gives us better control. This leans out the air/fuel mixture as much as possible, so we do not have to retard the ignition timing as much.

We have noticed that a higher ethane level does not affect all engines in the same way. This example shows how a slight increase in ethane percentage has a significant effect on firing pressure. This study was a lean-burn, four-stroke engine with pre-chambers. When doing the same test on a four-cycle rich-burn unit with 15% ethane, we saw very little effect on the peak firing pressures. This is due to the ethane-burning velocity.

In addition to this study, we are using other technologies to protect our engines. First, we are in the process of testing two detonation-monitoring systems. We use accelerometers mounted on the power cylinders to detect detonation. When that happens, a signal is sent to either close the turbocharger waste gate and/or retard the ignition timing. If detonation continues, another signal shuts the engine down.

Second, we use membrane fuel gas filters to maintain engine fuel gas at a constant Btu level. By maintaining a steady Btu level in our fuel gas, extra engine controls would not have to be installed.

Third, by placing a NOX sensor in the engine exhaust as soon as the Btu or ethane levels started to rise, the sensor would put out a signal to close the turbocharger waste gate and retard the ignition timing.

Fourth, although extraction plants are being built in the Marcellus area to extract the propane from the gas, at this time they are not removing the ethane. But there are plans to do so, which would solve most of these problems up front.