Air Toxics Emissions From Gas-Fired Engines
Authors: Meeks N.N. 
Published: December 2007

In 1990, 14 natural-gas-fired internal combustion engines (ICE's) inoilfield service were tested in Santa Barbara County, CA, to satisfy Californiaair toxics legislation. The combustion exhaust was tested for formaldehyde, acetaidehyde, acrolein, benzene, toluene, xylenes, naphthalene, and polycyclicaromatic hydrocarbons. The fuel was test ed for aromatics to enable calculationof destruction efficiencies. Two-stroke and four-stroke engines were tested Four-stroke engines ranging from 39 to 208 hp were used in pumping unit andconstant load service. Emissions from four-stroke engines were unrelated tosize and service. The two stroke engines produced considerably higher emissionsthan the four stroke engines. Test results indicate natural-gas-fired ICE'sproduce toxic substances in small amounts. Formaldehyde and benzene dominatedthe toxic emission profile.

Introduction

Because the regulatory framework already exists for criteria pollutants such as NO, SO., and reactive organic compounds, pollutants such as NO, SO., andreactive organic compounds, legislative focus is being directed toward toxicair emissions. Title M of the 1990 U.S. Clean Air Act Amendment is a recentexample of federal legislation that will affect the oil industry. Variousstates, including California, New Jersey, and Illinois, have preceded the Clean Air Act Amendment by enacting stringent air toxics regulations. California's Air Toxics "Hot Spots" Information and Assessment Act has the mostrigorous testing requirements. This right-to- know law requires biennial testdata on chemical releases into the air. Test methods were developed andexecuted to comply with regulatory requirements. Testing was directed towardmeasurement of 22 potential compounds in ICE exhaust. Of these, only six werefound at levels exceeding 0.5 lbm/MMcf of fuel.

Selection of Compounds Tested

Application of thermodynamic principles indicates combustion products morecomplex than CO2, in measurable quantities, must result from interruption ofthe combustion process. Quenching of the combustion reaction through transferof heat and work was postulated to be the principal means for formation oftoxic substances. Thus, plausible principal means for formation of toxicsubstances. Thus, plausible reaction intermediates were considered within thecontext of fuel molecular structures to predict the relative abundance ofcombustion products. These tests support the proposition that a fuel composedof compounds with simple molecular structures favors products with equallysimple structures. products with equally simple structures. Natural gasconsists primarily of three simple molecules: methane (one carbon), ethane (twocarbons), and propane (three carbons). In the presence of heat and sufficientoxygen, combustion takes place, causing bonds to rearrange to lower energystates while releasing heat and work. Theoretically, if a sufficientoxygen/fuel mixture is allowed to reach thermodynamic equilibrium, only CO2 andwater will be measurable. Measurable amounts of products of incompletecombustion (PIC) indicate poor mixing, quenching, insufficient oxygen, orinsufficient reaction time. Tests were performed to determine the abundance oftoxic PIC. Aldehydes are a class of plausible stable-reaction intermediates(Fig. 1). A reaction between a single methane and oxygen molecule can directlyproduce formaldehyde as a stable intermediate combustion product. Quenching theformaldehyde molecule before it can be acted upon by oxygen can leaveformaldehyde in the exhaust. Similarly, acetaldehyde and acrolein are stablecompounds that can be produced from partial combustion of ethane. Becausenatural gas largely consists of methane and ethane, aldehyde testing waslimited to their direct analogs: formaldehyde, acetaldehyde, and acrolein. Froma thermodynamic perspective, creating more complicated molecules such asmonocyclic aromatic compounds (Fig. 1) from methane, ethane, and propane duringcombustion should not occur frequently. Benzene cannot be formed through asingle bimolecular collision between oxygen and small organic molecules, although formaldehyde can be formed this way. In contrast, benzene can beformed through partial destruction of a molecule, such as propane, to form anunstable intermediate, which then must combine with other carbon molecules tocreate a six-carbon benzene ring. Thermodynamic principles suggest that such achain of events is improbable. Nevertheless, benzene, toluene, and xylene (BTX)were included in the testing, because natural gas contains BTX in varyingamounts. Although it was anticipated that BTX in the exhaust would beattributed solely to failure of the engine to bum BTX in the fuel completely, the data proved otherwise. Based on the theory that combustion-productformation is inversely proportional to product complexity, a family ofcompounds known as polycyclic aromatic hydrocarbons (PAH's) are the leastprobable PIC's of natural gas fuel. Fig. 2 shows that naphthalene probable PIC's of natural gas fuel. Fig. 2 shows that naphthalene is the simplest of themultiring structures.