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Emissions

Paraffinic components, such as HVO and GTL, are practically free of aromatics, polyaromatics, olefins, sulfur and high boiling fractions. Cetane number of paraffinic fuels can be very high (EN 15940 Class A) or moderate (Class B). Therefore, remarkable benefits over standard diesel fuels are achieved, especially if regional diesel fuel contains considerably sulfur and aromatics. A large number of studies with paraffinic fuel components are available. These are carried out mostly with HVO or GTL paraffinic components, because other options, such as bio-origin BTL, are not yet commercially available. Paraffinic components can be different from each other, for example, regarding cetane numbers. In this Chapter, focus is given on high cetane paraffinic HVO/XTL (EN 15940 Class A).

Engine cleanliness and emission control devices

Deposit formation in fuel injectors of engines is a phenomenon that shall be limited to a certain level in order to keep power output and low exhaust emissions constant over the entire life of a vehicle. Paraffinic HVO does not contain constituents that would lead to deposits in fuel injectors. HVO has shown a good performance in injector fouling tests.

Practically all new diesel cars today and heavy duty vehicles in the future are equipped with a particulate filter, which reduces emissions effectively. The lower engine-out particulate emissions of HVO are beneficial for particulate filters: exhaust back-pressure is lower and regeneration cycles to clean up the filter are longer (Kopperoinen et al. 2011). Liebig et al. (2009) reported that GTL fuel reduced particulate formation to such extent that the regeneration cycle was significantly prolonged, by approximately 70% compared with conventional diesel. Low level of ash-forming elements (P, Ca, and Mg) allow ash-free combustion and long life time for exhaust after-treatment systems (Mikkonen et al. 2012).

Power output, fuel consumption, and CO2

Gill et al. (2011) reviewed publications with paraffinic Fischer-Tropsch (FT) fuels. 75% Of the publications claimed that effective power was at the same level with FT fuels and with conventional diesel fuel. 25% of reviewed publications reported of decreased effective power with FT fuels, respectively. Thermal efficiency was reported to be higher with FT fuels than with conventional diesel in 58% of the reviewed publications.

Gill et al. (2011) reported that brake specific fuel consumption was lower for FT fuels than for diesel fuel in 83% of the reviewed publications, whereas higher in 17% of the publications. Paraffinic fuel's higher energy content (MJ/kg) and improved combustion efficiency leads to lower mass-based fuel consumption when compared to regular diesel fuel (Crepeau et al. 2009). However, this does not completely compensate the lower density of paraffinic fuels, and therefore volumetric fuel consumption increases compared especially to high-density summer grade diesel fuels.

Theoretically, to travel the same distance in same conditions the mass-based fuel consumption is approximately 2% lower, whereas volumetric fuel consumption approximately 5% higher for paraffinic fuel (density 0.779 kg/dm3, LHV 44.0 MJ/kg) than for regular diesel fuel (density 0.835 kg/dm3 , LHV 43.2 MJ/kg) (Murtonen and Aakko-Saksa 2009). The difference in volumetric fuel consumption would be lower if compared to low density winter grade diesel fuel. With particulate-filter equipped vehicles, lower back-pressure and longer regeneration cycle lead to fuel consumption benefit for paraffinic fuels when compared to conventional diesel fuel. (Mikkonen et al. 2012).

In the review by Gill et al. (2011), 67% of the studies showed decreased tailpipe CO2 emissions with paraffinic fuels when compared to conventional diesel fuel. This is caused by the higher hydrogen content and by a slightly better engine efficiency for paraffins than for diesel fuel. In the studies with paraffinic HVO, tailpipe CO2 emission was reduced by 4-5% when HVO was compared to EN 590 fuel (Mikkonen et al. 2012). Mizushima and Takada (2014 IEA-AMF Annex 38) assumed that unchanged fuel economy with NExBTL and  reduced CO2 emission when compared with diesel fuel by eco‐driving was due to the area and frequency of use of the engine.

Tailpipe CO2 emissions correspond only to the tank-to-wheels part of a life cycle greenhouse gas emissions, whereas the bio-feedstock plays the major role in this respect.

Regulated emissions

Paraffinic sulfur-free fuels with high cetane number and low aromatic content result in significant emission reductions when compared to conventional diesel fuel. Clean combustion of paraffins is visualized in a simplified manner in Figure 2, which shows a comparison of conventional and paraffinic diesel burning in an open baker (The Alliance for Synthetic Fuels in Europe, ASFE).

Figure 2. Burning of conventional diesel (left) and paraffinic diesel (right). (ASFE).

Paraffinic diesel fuel typically reduces NOx and PM emissions when compared to conventional diesel fuel. Diesel engines are important sources of  these harmful emissions in urban air, while CO and HC emissions are typically low for diesel engines.

Numerous studies on the exhaust emissions with paraffinic fuels have been carried out with paraffinic HVO and/or GTL fuels. In the review by Gill et al. (2011), 79% of the publications reported a decrease in NOx and PM emissions when paraffinic GTL was compared to conventional diesel fuel. 5% of studies reported of increased PM emissions with paraffinic fuels and the rest of the studies did not observe significant changes. 100% of the reviewed publications reported of reduced HC emissions, and 94% of reduced CO emissions when paraffinic GTL fuels are compared to conventional diesel fuel. For the GTL fuel, 8% and 16% reductions in NOx emissions and 23% and a 33% reduction in PM emission have been observed when compared with standard diesel fuel (Clark and Wardle 2009, Alleman et al. 2004).

For paraffinic HVO, substantial reductions in PM, NOx, CO, and HC emissions have been found in a number of studies on paraffinic (IEA Annex 37: Nylund and Koponen Eds. 2012, Erkkilä et al. 2011, Happonen et al. 2010, Kleinschenk et al. 2005, Kuronen et al. 2007, Murtonen et al. 2009, Mäkinen et al. 2011, Nylund et al. 2006, 2011, Rantanen et al. 2005, Rothe et al. 2005 and Sugiyama et al. 2011). Results from the exhaust emission tests with over 36 trucks and buses or their engines, and several passenger cars in vehicle and engine test beds are summarized in Figure 3. The results highlight significant reduction of PM, CO and HC, as well as decrease in NOx (Neste 2016).

Figure 3. Average effect of neat and almost neat (85%) Neste Renewable Diesel (HVO) on tailpipe emissions in EURO II to EURO VI vehicles compared to a sulfur-free EN 590 diesel fuel (Neste 2016).

The effect of paraffinic XTL/HVO on regulated exhaust emissions is summarized in Table 2. Typically PM and NOx emissions reduce with the blending ratio of HVO even at small HVO concentrations

 

Table 2. Comparison of paraffinic fuels and standard diesel fuel with heavy-duty vehicles and light-duty cars. Negative values mean lower emissions for paraffinic fuels than for diesel fuel.

 

Trucks, buses

GTL a/ HVO b

Euro 1 - 4 cars
GTL a

Passenger cars

85% /100% HVO b

Passenger cars

Tens of % HVO b

 

 

 

 

 

CO

-5… -22% / -27%

-40… -86%

-47%/-59%

-20… -40%

HC

-9… -28%/ -32%

-45… -63%

-40%/-57%

-10… -30%

NOx

-5… -26%/ -9%

-10… +5%

-1%/-1%

0… -10%

PM

-18… -38%/ -28%

-12… -42%

-13%/-30%

0… -10%

No of vehicles/ engines tested

/36

 

6/3

 

a Comparison of GTL and standard diesel fuel, Euro I–V heavy-duty (Clark et al. 2009). b Effect of paraffinic HVO fuel compared to sulfur-free EN 590 diesel (Neste 2016).

 

Despite of general reduction of emissions with paraffinic fuels, there has been also spread in the results with individual engines and vehicles. Some truck or bus types have shown reduction of PM emission up to 47% together with a negligible or even slightly increasing effect on NOx. This has been observed with common-rail engines, which do not have volume-controlled injection systems.Vice versa, in some engines NOx is reduced up to 14%, but PM is less influenced. One reason for increased NOx with some engines could be HVO's high cetane number, which may advance the actual start of the combustion and speed up combustion (Nylund et al. 2011). In the IEA-AMF Annex 38, Phase 1, Sato et al. (2012) also assumed that high cetane number of HVO shortens the ignition delay and reduce the rate of premixed combustion, which would be prone to produce NOx. HVO did not increase NOx when compared to diesel fuel in the engine tests and in vehicle operation. In the Phase 2 of Annex 38, Mizushima and Takada (2014) observed that the BTL with an extremely low flash point and low initial boiling point had a tendency to increase the NOx emissions when compared to diesel fuel, while HVO and another BTL batch with relatively high flash points maintained the NOx emissions at the same level as diesel fuel.

Generally, the spread in results may arise from the differences of the type of fuel injection systems, engine calibrations and strategies for controlling exhaust gas recirculation (EGR) (Mikkonen et al. 2012). Kitano et al. (2007) considers that distillation range of GTL fuels is a significant factor as regards engine-out PM emission, whereas cetane number is important for HC and CO emissions. These fuel properties are not considered to affect significantly the NOx emissions. Clark et al. (2006) observed with an EGR equipped car  a slight NOx disadvantage for GTL. This was deemed to be due to lower EGR rate for low-density fuel whereas most cars showed low NOx with GTL fuel.

In the study by the Environment Canada, negligible effect of HVO on the NOx emissions was found in the EPA 1998 platform buses, while NOx emissions reduced by 38% with HVO when compared to diesel in the 2010 platform buses. For the EPA 2007 platform buses, HVO yielded lower particle emissions than diesel fuel. PN and PM emissions were too low for conclusions for the EPA 2010 buses. For the EPA 1998 platform, varying fuels didn’t have much effect on particle emissions, but HVO gave slightly higher nucleation mode particles. (IEA-AMF Annex 37: Nylund and Koponen Eds. 2012).

Unregulated emissions

A number of studies with paraffinic fuels has covered unregulated emissions. Generally, paraffinic fuels reduce emissions of aldehydes, benzene, 1,3-butadiene, polyaromatic hydrocarbons, and Ames mutagenicity.

Due to high cetane number of paraffinic diesel fuel,  low aldehyde emissions can be expected,  however, methyl groups of branched paraffins may also act as  formaldehyde precursors. In many studies, lower aldehyde emissions have been observed for paraffinic Fischer-Tropsch diesel than for conventional diesel (Schubert et al. 2002, Nord & Haupt 2002 and Alleman et al. 2003). Formaldehyde emissions reduced by 15-18% when paraffinic HVO and GTL fuels were compared to diesel fuel with two heavy-duty engines without after-treatment, but no significant changes were observed with four buses and one non-road engine (Murtonen and Aakko-Saksa 2009). With the EPA 1998 bus, HVO produced less carbonyls compared to other test fuels in the IEA-AMF Annex 37 (Nylund and Koponen Eds. 2012). Nylund et al. (2011) reported of a slight increase in formaldehyde and acetaldehyde emissions when HVO was compared to diesel fuel for Euro III vehicle. Munack et al. (2005) also reported of increased carbonyl emissions for NExBTL in comparison with diesel fuel. Rantanen et al. (2005) reported that formaldehyde and acetaldehyde emissions were 30-40% lower for 85% HVO blend than for diesel fuel by with three passenger cars.

Methane, benzene, and toluene emissions reduced when paraffinic HVO was compared with regular diesel fuel in the tests with heavy-duty vehicles, whereas ethene and propene emissions increased.

1,3-Butadiene results were not unambiguous in a study by Nylund et al. (2011), while with three passenger cars, 1,3-butadiene and benzene emissions decreased by 30-50% when 85% HVO blend was compared to EN590 diesel fuel in a study by Rantanen et al. (2005).

With today’s diesel vehicles and cars, greater part of PM is soot, while  very low share of soluble organic fraction (SOF, e.g. 5-13%), sulfates and nitrates are present (Murtonen et al. 2009). Paraffinic HVO and GTL fuels are capable to effectively reduce PM, both soot and SOF portions of PM.

The general trend in literature shows substantial reduction in PAH emissions when paraffinic fuels are compared to regular diesel fuel (IEA-AMF Annex 31: Larsen et al. 2007, Munack 2010 in IEA-AMF Annex 37). Particulate associated priority PAHs reduced 32-76% when HVO and GTL were compared to diesel fuel with four buses and three heavy-duty engines (Euro IV, EEV, one non-road Stage 3A) (Murtonen and Aakko-Saksa 2009). For passenger cars, priority PAHs were reduced with 85% HVO blend when compared to EN590 diesel fuel (Rantanen et al. 2005). In today's heavy-duty vehicles, emission level of priority PAHs is often below detection limit and differences between fuels cannot be distinguished. This was the case in the study with paraffinic and conventional fuels for the Euro III, Euro IV and EEV vehicles (Nylund et al. 2011).

Extremely low mutagenic activity of particulate extracts with the Ames strains are observed for modern buses and most heavy-duty engines. When mutagenic activity was high enough to differentiate fuels, lower activity was observed with HVO and GTL fuels than with EN 590 diesel fuel (Murtonen and Aakko-Saksa 2009). For Euro III heavy-duty engine, Münack et al. (2010 in IEA-AMF Annex 37) reported reduced mutagenic activity (Ames test) of particulate extracts and semivolatile phase with paraffinic HVO fuel. With passenger cars, Ames mutagenicity of particulate extract was reduced with 85% HVO blend when compared to EN 590 diesel fuel (Rantanen et al. 2005). Krahl et al. (2007) did not find significant differences in mutagenicity of particulate extracts between GTL and diesel fuel.

Paraffinic fuels efficiently reduce the number of particles in a size class higher than 80 nm, but the number of smaller particles is not necessarily reduced (Murtonen et al. 2009). Heikkilä et al. concludes that particle size and concentration imply a similar formation mechanism of the particles for paraffinic GTL and EN 590 regular diesel fuel (Heikkilä et al. 2009).

Paraffinic HVO fuel has shown many benefits at cold temperatures: faster and easier cold start, less cold start smoke, less engine noise after a cold start (Mikkonen et al. 2012). In a study by Nylund et al. (2011), PM, CO, and HC emissions increased when using conventional diesel fuel when moving from +23 to  -20 °C, while these emissions were at the same level at both test temperatures when using  paraffinic HVO fuel. In the cold start tests at ‐5 °C in the IEA-AMF Annex 38 (Mizushima et al. 2012), a vehicle operated smoothly with HVO during the period of engine starting to an increase in engine coolant temperature. HVO had the equivalent cold startability as diesel fuel.

Optimizing engines for HVO

Studies with paraffinic diesel fuels are mainly conducted with engines and vehicles using factory settings. However, further benefits in engine performance, exhaust emissions and fuel consumption could be achieved by adjusting engine parameters to utilize properties of paraffinic fuels (Aatola et al. 2008). HVO can help with "PM – NOx" and ”NOx– fuel consumption” trade-off phenomena, which are challenges for engine designers (Figure 4). Favorable engine-out NOx to PM ratio with paraffinic fuels may give additional benefit in reduction of PM emissions with after-treatment devices (Nylund et al. 2011). In a study by Aatola (2008), NOx emission reduced remarkably with retarded timing, while fuel consumption increased. With advanced fuel injection timing, energy and fuel consumption were reduced, but NOx benefit was not obtained. The optimum combination could be advanced timing together with NOx reduction technologies. This issue is studied also by Kind et al (2010) by using paraffinic GTL fuel. “Diesel- flexible fuel vehicles (FFVs)” could be designed adapting themselves automatically in a similar way as FFV-cars today for gasoline blends. Another possibility could be changing engine mapping for fleet vehicles, which are using neat HVO. (Mikkonen et al. 2012).

 

Figure 4. NOx / PM trade-off curves of diesel fuel, a blend with 30% Neste Renewable Diesel (HVO) and neat HVO. A modern direct injection heavy-duty engine, common rail fuel injection with different fuel injection advance settings (Aatola et al.2008 in Neste 2016).

A recent work within the IEA-AMF Annex 45 concluded that HVO positively affects the combustion and emissions even if standard ECU settings are used. Moreover, HVO-adapted ECU settings showed a decreased fuel consumption without exceeding current emission limits. (Stengel and Vium 2015).

Crepeau et al. (2009) mentions that engine calibration to reduce engine-out NOx emission by taking advantage of low PM emission with paraffinic fuels may lead to excessive PM emissions with regular diesel fuel. This is not a problem for engines equipped with diesel particulate filters. However, adaptive systems, closed-loop control etc. would be needed when running with different fuels.

Field trials

Various field trials with paraffinic GTL and HVO fuels were conducted before wide-scale market introduction. For example, field trials with GTL conducted with buses in  Shanghai , Beijing and the Netherlands  showed no need for extra maintenance during the trials and no fuel-related problems. No changes in driveability compared to standard diesel were observed. Most drivers registered the lack of diesel smell inside and/or outside of buses. (Clark et al. 2009).

Extensive field trials have been carried out on paraffinic HVO fuel in Finland, Germany and Canada (Nylund et al. 2011, Mikkonen et al. 2012). The fuel has performed well, both at 100% content and a variety of blending ratios even during severe winters. There have not been any problems or need for extra maintenance in fuel filters, fuel systems, fuel hoses, seals in fuel systems, engines or exhaust after-treatment devices. The same has applied for fuel logistics: no differences between paraffinic and fossil diesel fuel were observed regarding water, microbiological growth, storage stability or material issues.

HVO is used in for example in Finland, California, Sweden and Austria. In Finland, HVO containing diesel has been available since 2007, and today practically all outlets sell ”Neste Pro Diesel” containing at least 15% of HVO. This fuel meets the WWFC (2013) Category 5 specification for the most modern vehicles (Neste 2016). Wood-based paraffinic diesel, called UPM BioVerno, has been on market in Finland since 2014, produced by the Finnish pulp and paper company, UPM. The fuel production is based on using crude tall oil, a wood-based residue of pulping process. (Laurikko et al. 2014). HVO blends and neat HVO in Finland have been in place all year round including also severe winters for 10 years without technical modifications to vehicles, pipelines, storage tanks, tanker trucks and service stations. This experience has proven that HVO is a safe high-quality fuel option for the use without requiring any changes to fuel logistics or vehicles.

Summary

Overall, several regulated and unregulated emissions are reduced when substituting regular diesel fuels with paraffinic fuels:

  • NOx emission reduces generally up to 16% but with some engine technologies and adjustments a slight increase up to 5% may occur.
  • PM emission reduces quite consistently by 12-45% even when compared to sulfur-free diesel fuel.
  • CO and HC emissions reduce substantially, approximately by 20-80%. However, these emission species are very low for diesel engines already in general.
  • Formaldehyde and acetaldehyde emissions reduce in most cases.
  • Methane, benzene, and 1,3-butadiene emissions reduce in most cases, whereas ethene and propene emissions may increase.
  • PAH emissions and Ames mutagenicity reduce.
  • Cold start smoke and emissions at cold temperatures reduce.