Pollutant Formation during Utilization of Renewable Natural Gas Containing Trace Ammonia Impurities

Abstract

Renewable natural gas (RNG), produced from biogas upgrading, is an important alternative to fossil fuels. Unfortunately, RNG contains several trace contaminants, one being NH 3 , of particular concern for RNG produced from farming operations. The presence of NH 3 in RNG, particularly if it is injected into the natural gas (NG) pipeline network, could have serious consequences, ranging from damage to the NG infrastructure, to corrosion of the end-user equipment, and increased pollutant formation during combustion. This paper is part of a broader investigation studying all such impacts, the focus here being pollutant emissions. Two common end-user equipment were tested: An internal combustion engine and a household water heater, both operating with NG injected with NH 3 with concentrations ranging from 0 to 500 ppmv. Emissions studied included unburned hydrocarbons (UHC), CO, and nitrogen oxides (NO x ), the latter being of key concern. The presence of NH 3 resulted in increased NO x emissions for both equipment. For the water heater, the relationship between the amount of NO x formed and the NH 3 concentration was quantitative: For every molecule of NH 3 fed to the water heater, one molecule of NO x was additionally produced. However, for the engine a lower quantity of NO x was formed, to that corresponding to complete conversion. The results with both devices are significant, indicating the need for thorough RNG purification prior to its injection into the NG network.

Impact of Trace Ammonia Impurities on the Utilization of Renewable Natural Gas

“…In summary, the research in this section of the paper focused on the issue of pollutant emissions associated with the end-use of ammonia-containing RNG and complements a prior study by the group. 32 Between the two studies, three common enduser pieces of equipment were tested in total: a small IC engine, a common household water heater, and a wall furnace.…”

“…It is evident from Figure 3 that the presence of NH 3 in NG directly correlates with an increase in NO x emissions for the wall furnace as it did with the other two combustion devices that we previously studied. 32 On the same figure, we also include (for visual purposes) a line passing through a number of "calculated" points. These are calculated on the assumption that each nitrogen atom in the NH 3 in the gas is converted into one atom of NO x .…”

“…In a previous study published elsewhere, 32 results of testing a household tankless water heater and a small, commercial IC engine were presented. In this study, the previous investigations were complemented by studying a wall furnace acquired from the Williams Furnace Company (model P323011, illustrated in Figure S1 in the Supporting Information section) and used in the experiments without any further modification.…”

“…It consists of three sections: (i) the gas delivery and metering section, which includes standard regulators and metering devices used by SoCalGas, their integrity after exposure being part of the study; (ii) the glass corrosion chamber, inside of which the specimens were placed for testing; and (iii) a water heater that runs on the NH 3 /NG mixture exiting the corrosion chamber, thus eliminating the need to vent the NH 3 /NG mixture into the hood, but which is also part of the study. 32 For the experiments with humidified NH 3 /NG mixtures, the gas delivery section was modified by adding a Harvard PHD 2000 infusion syringe pump and a home-made heating coil made out of stainless steel (SS) tubing heated using a heating tape connected to an Inkbird PID temperature controller. The syringe pump was used to pump deionized (DI) water at a controlled flow rate through the heated SS coil to generate water vapor that was then intermixed with the NG stream fed into the corrosion chamber.…”

“…However, they offer no insight into the impact that trace NH 3 impurities may have during RNG combustion. In a prior study, 32 pollutant emissions during the use of NH 3 -containing RNG for power generation were investigated. Two common end-user pieces of equipment were tested: an IC engine and a household water heater, both operating with NG containing NH 3 with concentrations from 0 to 500 ppm v .…”