Impacts of Biodiesel Applied to the Transportation Fleets in the Greater Houston Area

Houston, the fourth largest metropolis in the US, currently experiences severe air pollution. Major pollutants, such as VOCs, CO, NO x , PM, SO x , CH 4 , and CO 2 , are released from the transportation fleets. To decrease fossil fuel use and greenhouse gas emissions from fleet vehicles, more and more biodiesel is used in vehicles in the Houston metropolis. The GREET model was used for simulating the fuel cycle emissions of diesel vehicles using different biodiesel blends inHouston.The fuels examinedwere dieselbiodiesel blends of B0, B5, B20, B50, B80, and B100. The energy and water use and emissions from vehicles fueled with the blends were investigated. The study shows that the reductions in GHG emissions are significant at the Well-to-Pump stage, and all the emissions, except GHGs and NO x , reduce at the Pump-to-Wheel stage. The overall Well-to-Wheel analysis shows that biodiesel is beneficial for both passenger cars and heavy duty trucks. However, the benefits are more pronounced for passenger cars compared to heavy duty vehicles. When 50% of diesel passenger cars and HDDTs are switched to B20 in the Greater Houston area in 2025, the daily GHG emissions will be reduced by 2.0 and 712.1 CO 2 -equivalent tonnes, respectively.


Introduction
Dependence on imported crude oil and concerns of global climate change due to greenhouse gas emissions from fossil fuels are increasingly impacting the national security strategy and economic and environmental plans of many countries.With the increase in the world population, the demand for transportation vehicles is increasing, which leads to the increased use of fossil fuels.Some of the estimates suggest that there is about a 10% annual increase in vehicle population in some developing countries and even more (15%) in China [1].Another study indicated that world energy demand would rise by about 60% by 2030, and fossil fuel would continue to dominate energy supplies in the coming decades [2].Transportation fleets using fossil fuels release a large amount of greenhouse gases (GHGs) and other harmful air pollutants, which prove being hazardous to the environment and human health.The study of global energy consumption trends showed that CO emissions from fossil fuel-driven vehicles contribute to more than 70% of the total emissions from all the sources, and the corresponding CO 2 emissions contribute to 19% [3].As the fossil fuels are exhausted every day, there is a great need to find alternative fuels to satisfy the energy demand of the world and reduce the GHGs and pollutant emissions.
Biodiesel is one of the best developed biofuels, which can replace diesel fuel used in internal combustion engines equipping the transportation vehicles and boilers [4].Houston, the fourth largest city in the US, has a very heavy transportation load.It is known to experience severe air pollution episodes due to these automobile emissions in addition to oil refineries and other industries that support these petrochemical businesses.Regarding biodiesel development, Houston owns the largest biodiesel production facility in North America, RBF Port Neches Facility [5].In order to decrease the amount of fossil fuel use and greenhouse gas emissions from fleet vehicles in the Greater Houston area, biodiesel derived from resources, such as soybean, rapeseed, animal fat, and kitchen fat waste.The biodiesel is not only blended into the regular diesel, but also has been directly tested in diesel engines [6].
Among the available bioresources for biodiesel production, soybean is abundant in Texas [7].
With the increasing use of biodiesel in transportation, the impacts of biodiesel on combustion characteristics, engine performance, and exhaust emissions were widely studied based on passenger cars, light duty trucks, and heavy duty vehicles [8][9][10][11][12][13][14][15].Kousoulidou et al. investigated the emissions of different biodiesel blends used in different types of passenger cars and found that the influence of biodiesel on tailpipe emissions primarily depends on the biodiesel blend ratios and secondly on the engine technology [11].The vehicle tests demonstrated that the use of biofuel in the transportation sector can reduce the GHG emissions from the diesel vehicles.Additionally, the impacts of biodiesel can be evaluated by environmental life cycle assessment (LCA), which is outlined by the International Organization of Standardization's 14040 series [16].Most LCA studies of biodiesel have focused on environmental impacts of biodiesel derived from different bioresources, e.g., soybean, rapeseed, and algae [17][18][19][20][21].The effects of algae-derived biodiesel used in the passenger cars could be net positive or negative, depending on the algae cultivation and conversion processes from algae to biodiesel [18].Some researchers quantified the regional impacts of soybeanderived biodiesel for transportation fleets in Pennsylvania by using the real fleet data and pointed out that only the instate produced B5 is preferable with its lowest environmental impacts due to the state's limited area of farmlands [21].
Recently, we have studied the impacts of bioethanol on the Houston's transportation sector, focusing on light duty vehicles such as gasoline passenger cars and gasoline light duty trucks 1 and 2 [22].To further investigate the impacts of biofuels on the metropolitan environment, we conducted LCA of biodiesel applications in diesel vehicles in the Greater Houston area.The biodiesel can be derived from locally produced soybean in Houston's neighboring counties with the mature production procedures.The study examined the emissions of GHGs, VOCs, SO  , NO  , CO, PM 10 , and PM 2.5 from the production of biodiesel fuels and their use in vehicles.Furthermore, various blends of biodiesel and diesel used in the vehicles were analyzed to study the environmental influence of higher blending ratios.The GREET (Greenhouse gases, Regulated Emissions, and Energy use in Transportation) Fuel-Cycle software was used to simulate the GHGs and pollutant emissions.The Vehicle Miles Traveled (VMT) for the Greater Houston area were obtained from the reports of the US Department of Transportation [23].

Materials and Methods
The GREET model [24] was developed by the Argonne National Laboratory and is regularly updated.It currently includes three major components: Fuel Cycle Model GREET 1, Vehicle Cycle Model GEET 2, and an app version GREET .net.GREET 2 is specially used for the life cycle assessment of vehicles from materials manufactured for vehicles to disposal of vehicles.GREET 1 is perfect for fuel cycle used in different vehicles and the 2017 Excel-based version was used in our study.The fuel cycle emissions were analyzed using three phases: Well-to-Pump, Pump-to-Wheel, and Well-to-Wheel.
Low-sulfur diesel (B0) and five fuel blends of the lowsulfur diesel and biodiesel derived from soybean were used, including B5, B20, B50, B80, and B100 (where the number indicates the percentage of biodiesel in the blend).Diesel vehicles equipped with compression ignition direct injection (CIDI) engines including passenger cars, light duty trucks (LDT1 and LDT2), and heavy duty trucks (long-haul) were taken into account in this study.The emissions of CO 2equivalent greenhouse gases, primarily CO 2 , methane (CH 4 ), and nitrous oxide (N 2 O), and other pollutants such as VOCs, CO, NO  , particulate matter (PM) including PM 2.5 and PM 10 , and SO  were analyzed with respect to the abovementioned three pathways, and the targeted years were 2018 and 2025 for running the simulations.
The daily VMT records for Houston, Texas, can be downloaded from the US Department of Transportation/Federal Highway Administration [23] for the years of 2014-2016.However, the VMT reports of the years 2017 and 2018 and the future year 2025 are not available.The VMT data for the years 2018 and 2025 in Houston are estimated based on the economic conditions, the rise in population, and vehicle demands in Houston.The daily VMT mix [25] is taken into account for the various vehicle classes.According to the different ratios of vehicle classes in the vehicle population in the Greater Houston area [25], the VMT of the different types of diesel-based vehicles including passenger cars, LDT1, LDT2, and long-haul heavy duty diesel trucks (HDDTs) are calculated for the Greater Houston area and are listed in Table 1.The statistical data of the daily VMT mix showed that the long-haul HDDTs were the majority, and the diesel passenger cars took the second place in the diesel-powered transportation fleet of Houston.Houston is located in the subtropical zone, which makes it much easier to use highblend biodiesel in vehicles compared to the north region of the US throughout the whole year.
The default emission rates for B0 (i.e., the regular diesel), provided by the GREET model, were used in the simulations.The relative emission rates for B5, B20, B50, B80, and B100 during vehicle operation were determined based on the experimental emission reports of diesel engines using biodiesel blends [12,26].With respect to MPG (mile per gallon) in the GREET model, the base vehicles of the simulated light duty diesel vehicles including passenger cars, LDT1, and LDT2 using the biodiesel blends are the corresponding light duty gasoline vehicles using regular gasoline.The relative MPG of the light duty diesel vehicles using regular diesel is expressed as 120% in the GREET model because the MPG of regular diesel is 1.2 times the regular gasoline.The relative MPG at B5, B20, B50, B80, and B100 for the light duty diesel vehicles were adjusted based on the engine experiments fueled with different biodiesel blends [12,26].The relative emission rates of passenger cars, LDT1, and LDT2 during operation were determined from the same experimental results.In the GREET model, for heavy duty vehicles fueled with the biodiesel blends including long-haul heavy duty trucks, short-haul heavy trucks, and school buses, their base vehicles are themselves using B0 when the relative MPG are calculated.The relative emission rates of HDDTs during vehicle operation were also directly determined from the  same experimental reports.The electricity grid involved in this study is specified as the mixed electricity generation in Texas Regional Entity.

Results and Discussion
The relative MPG and emission rates of diesel vehicles using different biodiesel blends were determined from the abovementioned experimental results reported in the literature [12,26] and integrated to the GREET model.Table 2 shows the relative mile per gallon (MPG) and emission rates of B0, B5, B20, B50, B80, and B100 for passenger cars, LDT1, and LDT2, which were used in the simulations for the target years 2018 and 2025.The MPG slightly decrease with the increase of biodiesel in the blends.VOCs, CO, PM 10 , and PM 2.5 emissions significantly decrease with the increase of biodiesel in the blends.NO  emissions have slightly increased up to 10.3% for B100.The life cycle assessment of energy and water use and emissions associated with the different biodiesel blends was performed using the GREET model following the three phases: Well-to-Pump, Pump-to-Wheel, and Well-to-Wheel.First, the Well-to-Pump analysis mainly focuses on the feedstock of soybean and production of biodiesel from soybean.The first stage addresses the energy and water use and pollutant emissions released in soybean farming, transportation from field to biofuel refinery, biodiesel production, and biodiesel transportation.Second, the Pump-to-Wheel emissions analysis was used to address the energy and water use and pollutant emissions during vehicle operation fueled with the different biodiesel blends.The Well-to-Wheel analysis adds up the first and second analyses and thus provides the overall estimates.
Tables 3 and 4 list the daily energy and water use and emissions of pollutants from diesel-powered passenger cars and long-haul HDDTs during the stage of Well-to-Pump for the years 2018 and 2025.Based on the calculations evaluated per km, all of the energy and water use and the pollutant emissions significantly decrease for all the biodiesel blends in the future year 2025 for both passenger cars and longhaul HDDTs compared to those in 2018.The improvements in energy efficiency and emission rates can be attributed to continuous improvement in the process of low-sulfur diesel and biodiesel production in the future.However, in Tables 3 and  4 there exist some increases in these terms of energy, water, and emissions, more often for the long-haul HDDTs, because of the increasing VMT in the future year.Figure 1 shows the percentage reductions of energy use and pollutant emissions from passenger cars during the Well-to-Pump stage in Houston for the target year 2018.The results obtained for B0 are used as the base to estimate other blends.The positive values show the reductions of GHG emissions and the negative percentages show the increases in the particular pollutant emissions and energy use compared to the corresponding base values.The GHG emissions significantly reduced with the increase of biodiesel percentage in the blends, and other emissions VOCs, CO, NO  , SO  , PM 10 , and PM 2.5 and energy use increased as the biodiesel proportion increased in the blends.The increases in energy use and pollutant emissions are associated with the biodiesel production which consumes more energy and resources than the production of conventional diesel, and more emissions would be released when the fuel blends containing more biodiesel are produced.The study shows that the different emissions have different trends for the years 2018 and 2025.The simulations applied to the LDT1 and LDT2 show the same trends of energy use and emissions as those of the passenger cars during the Well-to-Pump stage.Our study also shows that the energy use and emission trends for the long-haul HDDTs are almost the same as those for passenger cars.Figure 2 shows the increase in water consumption during Well-to-Pump for long-haul HDDTs in Houston for the target years 2018 and 2025.The water use significantly increased with the increase of biodiesel percentage in the blends, even higher in the future year compared to each baseline.The tremendous increase of water use [27,28] for high biodiesel ratio in the blends is related to the increased water consumption for agricultural production of soybean and biodiesel derivation from soybean in the biorefinery.Tables 5 and 6 list the energy and water use and emissions of pollutants from diesel passenger cars and long-haul HDDTs during the Pump-to-Wheel stage for 2018 and 2025.During the Pump-to-Wheel stage, all of the energy use and the pollutant emissions significantly will decrease at all the biodiesel blends in the later year 2025 for both passenger cars and long-haul HDDTs compared to those in 2018.The improvements in energy efficiency and emission rates are attributed to the enhancement of vehicle fuel efficiency in the future.Figure 3 shows the percentage reductions of pollutant emissions from pump to wheels in Houston for the target years 2018 and 2025.Similar to the Well-to-Pump stage, the positive values show the reductions of the exhaust emissions and the negative percentages show the increases in the particular pollutant emissions and energy use.The SO  , VOCs, CO, PM 2.5 , and PM 10 emissions reduced with the increase of biodiesel percentage in the blends.The emission reductions for SO  , VOCs, and CO are more significant at higher blend ratios.However, the emissions of GHGs and NO  and energy use slightly increased with the increase of biodiesel percentage in the blends.The emission trends of passenger cars, LDT1 and LDT2, and HDDTs are mostly similar, and the deviations are insignificant.The study also shows that the emission trends for different biodiesel blends are the same for the target years 2018 and 2025.Figure 4 shows the percentage reductions of pollutant emissions for the passenger cars and long-haul HDDTs during the Well-to-Wheel stage in the Houston metropolis for the target years 2018 and 2025.For the passenger cars, the GHG, VOCs, and CO emissions reduced with the increase of biodiesel percentage in the blends; energy use and the emissions of NO  and SO  increased; and the emissions of PM 10 and PM 2.5 did not change significantly for the different blends.The percentage increases of SO  emissions are much more in the future year at the higher blend ratios compared to the B0 emissions for the same future year.The emission trends have slight differences for different years.
For the long-haul HDDTs, significant reductions of GHG and CO emissions were observed at the higher blend ratios; the reduction degrees of CO emissions were slightly lower than those of passenger cars; VOC emissions varied with the biodiesel blend ratio; the energy use and the emissions of NO  and SO  increased similarly to those of the passenger cars with the increase of biodiesel percentage in the blends.
The SO  emissions of HDDTs share the same trend as that for passenger cars.The emission reductions for VOCs of HDDTs are relatively obvious at B20, B50, and B80 in 2018 and at B20, B50, B80, and B100 in 2025.The emissions of PM 10 and PM 2.5 reduced with the increase of biodiesel percentage in the blends, and this would show more reductions at higher blends in 2025.Based on two types of passenger cars and HDDTs, the Well-to-Wheel analysis also reveals that biodiesel has more emission reductions for passenger cars than HDDTs at the higher biodiesel blend ratios per km because of higher MPG of passenger cars.The prevalent biodiesel blend that is currently used in the Greater Houston is B5.If more biodiesel can be used in the transportation fleets, Houston can share more benefits from bioenergy to reduce metropolitan pollution.To study the real impact of biodiesel blends in the Houston metropolis, two hypothetical scenarios are taken into our calculations, where 25% and 50% of all passenger cars and HDDTs are switched to the B20 and B50 fuels in 2025.Through our calculations from well to wheels, it can be found that the daily reductions of GHGs, VOCs, and CO emissions from 25% of all the possible diesel passenger cars using B20 would be 0. It can be seen that when 50% diesel vehicles of the transportation fleet are taken into account, the daily reductions of these emissions will be more than the double of the 25% case.The emissions of NO  will slightly increase for both passenger cars and HDDTs in both 2025 scenarios.The emissions of SO  will also increase for both passenger cars and HDDTs in both scenarios, and the increases in SO  emissions during the Well-to-Pump stage will cause the overall increases of SO  emissions.

Conclusions
The life cycle energy and water use and emissions of GHGs, VOCs, CO, NO  , SO  , PM 10 , and PM 2.5 for biodiesel blends with regular diesel were studied in Houston by using passenger cars and long-haul HDDTs with the GREET model.Three major pathways, Well-to-Pump, Pump-to-Wheel, and Well-to-Wheel were used in LCA for the years 2018 and 2025.The Well-to-Pump analysis shows that only GHG emissions significantly decrease with the increase of biodiesel blends, while other emissions and water and energy use increase.
The Pump-to-Wheel analysis shows that total energy use and the emissions of GHGs and NO  increase, and other emissions of SO  , VOCs, CO, PM 10 , and PM 2.5 decrease for higher biodiesel fuel blends.The emissions of SO  , VOCs, and CO significantly reduce with the increase of the biodiesel blend ratios.Overall, the Well-to-Wheel analysis of biodiesel applications in transportation vehicles in Houston shows that an increase in the biodiesel blend results in the increases in energy and water use and all the pollutant emissions except GHGs, VOCs, CO, and PM emissions from the diesel vehicles.For the long-haul HDDTs, the reductions of VOCs emissions are not significant as those of passenger cars at the corresponding blends.It is also seen that the emission trends will change in the future year 2025.The reduction of VOCs emissions will alleviate the secondary pollutant O 3 which often occurs in spring and summer in Houston because VOCs are one major precursor which produce ozone.The research confirms that biodiesel applications are environment-friendly for both passenger cars and long-haul HDDTs, and the environmental impacts depend not only on the fuel blends but also on the vehicle types.The comparison of the overall energy use and emissions between passenger cars and HDDTs shows that biodiesel is more beneficial for passenger cars than HDDTs per km.

Figure 1 :Figure 2 :
Figure 1: Percentage reductions in energy use and emissions at the Well-to-Pump stage in 2018.

Figure 3 :
Figure 3: Percentage reduction in energy use and emissions for (a) passenger cars and (b) and long-haul HDDTs during the Pump-to-Wheel stage.

Table 1 :
Daily VMT of four types of diesel vehicles in the Greater Houston area (units: km).

Table 2 :
The MPGs and emission rates of diesel vehicles.
Note.MPG: mile per gallon.TBW: tire and brake wear.

Table 3 :
Well-to-Pump daily energy and water use and emissions of pollutants associated with passenger cars.

Table 4 :
Well-to-Pump daily energy and water use and emissions of pollutants associated with long-haul heavy duty trucks.

Table 5 :
daily energy and water use and emissions of pollutants from passenger cars.

Table 6 :
Pump-to-Wheel daily energy and water use and emissions of pollutants from long-haul heavy duty trucks.