Yard tractors: Their path to zero emissions
Introduction
Meeting federal air quality standards is important for both legal and human health reasons. In this respect, numerous airborne pollutants are known to harm human health (Manisalidis et al., 2020, Schraufnagel et al., 2019, Vellaiyan and Amirthagadeswaran, 2017). Ports area sources with high levels of criteria pollutants contribute to the degradation of local air quality and increased health risks for communities located close to port facilities (Bailey and Solomon, 2004, Giuliano and Linder, 2014, Mousavi et al., 2018, Mueller et al., 2011, POLA, 2018, POLB, 2018). Port activities typically include a wide range of diesel engine sources, including ocean going vessels (OGVs), harbor craft, cargo handling equipment (CHE), locomotives, and trucks (Liu and Ge, 2018, Martinez-Moya et al., 2019, Sim, 2018). Over time, the percentage contribution of criteria emissions from port sources to local inventories has increased significantly, and has led to new regulations with lower emission limits from diesel engines used in ports.
The development of emissions inventories for port sources is an important element of the planning process in order to reduce their environmental impacts on local communities. For example, in the San Pedro Bay, both the Port of Los Angeles (POLA) and the Port of Long Beach (POLB) have carried out extensive studies to quantify the emissions contribution from various sources. Their emissions inventories show that OGVs contribute the most to the port emissions inventories, from 46% to 62% for nitrogen oxides (NOx) and from 52% to 61% for particulate matter (PM) (CAAP, 2017). Similarly, the ports of Oakland, Everglades, and Richmond also showed that OGVs represent from 71% to 81% of NOx emissions and from 62% to 84% of PM emissions for these ports (Agrawal et al., 2009, CARB, 2019). Agrawal et al. (2009) estimated the impact of OGVs on air quality in the LA basin using vanadium as a tracer for OGV fuel use. They found that the OGVs represented 1.4–8.8% of the primary PM, depending on the distance the monitoring station was from the port.
CHE is another important source of port-related emissions (Clean Air Action Plan (CAAP), 2007, CARB, 2015, Zhang et al., 2020, Zhang et al., 2017, Zhou et al., 2020) that contributes about 6% and 10% of the NOx and PM emissions, respectively, at the POLA/POLB. At POLA and POLB, YTs are the most common CHEs used for moving containers at the terminals. According to 2016 Air Emissions Inventories, there were over 1800-yard tractors operating at the POLA/POLB with diesel engines as their primary power source (CARB, 2015, POLB, 2018). For the 2017 base year, over 74% of both NOx and the PM emissions from CHEs were from 2006 and older diesel yard tractors (YTs) (CARB, 2015). The extent of emissions reductions achieved with the latest technology yard tractors was examined by Yu et al. (2017). More recently, Zhu et al. (2020) demonstrated that NOx emissions from a yard tractor equipped with a natural gas engine certified to the CARB optional low NOx standard of 0.02 g/bhp-hr were below the certification limit during operation over a number of different yard tractor driving cycles.
The POLA and the POLB have established the San Pedro Bay Ports Clean Air Action Plan (CAAP) to reduce port-related emissions (CAAP, 2017). In their plan, the POLB/POLA explored various options to reduce and control emissions from diesel engines associated with YTs. One option is to convert engines in existing yard tractors from diesel to natural gas (NG), resulting in a reduction of NOx emissions by 65% and PM emissions by 80% (CARB, 2015). On the horizon are zero-emission and near-zero (NZ) emission YT engine platforms that achieve even greater emission reductions. These emerging technologies include both engines operating on NG and battery electric technologies (Kim et al., 2012, Martinez-Moya et al., 2019, Vujicic et al., 2013, Zhu et al., 2020). Until such technologies become widely available, emission data on existing YTs with various controls are needed to quantify the emissions benefits of potential mitigation methods. Over the past few decades, the POLA and POLB have evaluated various control technologies to reduce emissions from YTs, including: engines with lower emissions, cleaner burning liquid and gaseous fuels, and added emissions and aftertreatment controls. This paper provides an overview of emissions testing that has been undertaken in support of POLA and POLB to characterize the effectiveness of using a range of mitigation strategies for YT emissions reductions. This includes chassis dynamometer emissions measurements of 12 different yard tractors representing a range of different on-road and non-road standards, of yard tractors with a water emulsified fuel, and of NG and LPG YTs. These data have formed the fundamental basis for the development of emissions inventories for ports and for the development and assessment of port environmental plans, both currently and in the past, and, to the authors knowledge, these data represent the largest source of emissions results from in-use YTs available. While this information has been utilized in port emissions inventories that have shown important reductions over time, the underlying input data and research behind these estimates has not been extensively documented in the literature. As such, this research not only provides an important historical perspective on how emissions reductions have been achieved at ports that have been very progressive in environmental mitigation, but these data should also be helpful to other ports that are looking to reduce YT emissions from their existing diesel engines YT fleets as they transition to lower and zero emissions technologies, or for ports for which a rapid transition to emerging lower and zero emission technologies is not as feasible.
Section snippets
Test vehicles
The YTs tested in this study were tested over a period of over a decade and were designed to be representative of the evolution of diesel YTs to lower emissions technologies, as well as to evaluate the potential effectiveness of YTs powered by alternative fuels, such as NG, LPG, and water emulsified fuels. YTs were selected from the in-use fleet to be representative of the in-use engines, technology solutions and applicable regulatory limits at the time of purchase. The importance using engines
NOx emissions
The NOx emissions results for all YTs are shown in Fig. 1. The results in this and the other graphs represent the averages of all tests for the specific test points, and are generally presented on a wheel horsepower/hour (whp-hr) basis. The error bars represent one standard deviation of the average. Also included on the Figures are adjusted representations of the certification standards. The adjusted values represent the standard plus an additional 25% to account for losses that occur between
Conclusions and implications
This paper reviewed various advanced fuel and engine technologies to mitigate the emissions from yard tractors used to move freight in ports. These results present a historical account of the changes in emissions for the San Pedro Bay ports over the past 20 years, and offer a portfolio of opportunities for other ports to implement in order to reduce the effects of port emissions on nearby communities. The results show that as ports around the world begin the process of implementing advanced
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors acknowledge funding from the California Air Resources Board (CARB), the Port of Long Beach, and the Port of Los Angeles. We acknowledge Mr. Don Pacocha, Mr. Eddie O’Neal, and Mr. Joe Valdez of the University of California, Riverside for their contributions in conducting the emissions testing.
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