Methane (natural gas, biomethane)
Natural gas is used for many different applications such as power generation, domestic use, transportation, and as a feedstock for the production of ammonia-based fertilizers. It is the dominant alternative road transport fuel in addition to ethanol. Since natural gas contains mainly methane, biomethane can be used as a substitute.
In road transport vehicles, methane is mostly used in compressed form (compressed natural gas, CNG, or compressed biogas, CBG), but there is also some interest for liquefied form (liquefied natural gas, LNG, or liquefied biogas, LBG). For long distance travel, natural gas is usually shipped as LNG, and then re-gasified in coastal terminals to be injected into the natural gas grid. In all pathways, the composition of the natural gas has high variability. Biomethane can be produced locally. Therefore, it is not as dependent on gas grid or shipping as natural gas.
Methane is traditionally used in the Otto engine, either under stoichiometric or lean-burn conditions. In recent last years other engine technologies have been developed, e.g. dual-fuel compression-ignition engines. Energy efficiency is higher for lean-burn than for stoichiometric gas engine, but the stoichiometric engine can control emissions efficiently with a three way catalyst; also NOx emissions that are problematic for lean-burn natural gas engines. Dual-fuel engines need to be equipped with similar aftertreatment technology to diesel engines to meet emission legislation in many regions. All natural gas engines deliver low PM emissions. Methane emissions from natural gas vehicles are substantial, but many of the other unregulated emissions have been reported to be lower for natural gas vehicles than for gasoline or diesel vehicles.
Recently, AMF Annex 51 Key Messages pointed out that "The use of methane in transport is predicted to increase, especially in liquid form in heavy-duty road transport and in the marine sector. Methane reduces, e.g., particulate emissions, and holds a promise to reduce CO2 emission up to 20…25 %. However, almost all methane fuelled engines have methane slip that could nullify the CO2 benefit. AMF Annex 51 shows that technologies exist to mitigate this problem."
- Legislation, standards and properties
- Methane engines
- Emissions and efficiency (bus study)
- Methane emissions
- Non-regulated emissions
Natural gas is used as an energy source and as an automotive fuel to respond the need for reduction of oil dependence and thus to increase in the security of energy supply. Natural gas is a colorless, odorless, and clean-burning fuel that has been and is currently used for many different applications such as power generation, domestic use, transportation, and as a feedstock for the production of ammonia-based fertilizers. Natural gas is the dominant alternative road transport fuel in addition to ethanol. The global fleet of natural gas vehicles is approximately 22.4 million according to the NGV Journal (http://www.ngvjournal.com/worldwide-ngv-statistics/, entered 17.7.2016). With number of light-duty and heavy-duty vehicles, their mileages and fuel consumptions, global natural gas consumption in road transport would be in maximum 60 Mtoe. Synthetic diesel fuel is produced from natural gas by liquefaction (Gas-to-Liquid, GTL). According to the IEA Headline Energy Data 2015, global use of natural gas in the transport sector in 2013 was 96.2 Mtoe, which indicates potential use of natural gas as GTL in the range of 30 Mtoe.
As mentioned, a part of natural gas is converted to GTL for use in road transportation. However, there are also other conversion pathways for natural gas. Natural gas can be converted to methanol or synthetic gasoline, which are liquid fuels, or it could be converted to other type of gaseous fuels, such as DME or LPG. Hydrogen can be produced from NG via methane reforming, and electricity can be generated at a NG -powered plant for on-road vehicles. For natural gas-derived fuels to be chosen for implementation, they would need to be produced, delivered and used in vehicles at prices competitive with traditional fuels. In addition to cost, emphasis must also be placed on the environmental benefits, energy use, and energy security that each fuel pathway can offer to a particular nation. In the AMF Annex 48, the feasibility of the different natural gas pathways used in motor vehicles were assessed to determine the advantages and disadvantages of each option. To demonstrate how differently each factor can weigh in, case studies were conducted in six different countries spanning three continents. (AMF Annex 48: Sikes et al. 2015).
The term biomethane refers to methane from a renewable origin. It is produced by the anaerobic digestion of organic matter (dead animal and plant material, manure, sewage sludge, organic waste, etc.), which is stored in air-tight tanks in order to reproduce the best possible conditions for the anaerobic microbes producing gas during the digestion process. It can also be produced by anaerobic degradation of organic matter in landfills, and this is referred as landfill gas. The raw gas is known as biogas, mainly consisting of methane and CO2 plus some minor trace components which greatly depend on the feedstock. Biomethane is known as the upgraded form of biogas, and its final quality/composition is dependent on the operational parameters of the final use, and on the upgrading technology used. I.e. biomethane is a methane-rich gas derived from biogas. The third route to biomethane is via gasification of biomass. A great benefit of biogas/biomethane is that it can be produced from a great variety of sources: basically, all types of biomatter can be used for this purpose, such as waste. However, not all substrates behave in the same way regarding the biogas production efficiency or have the same emissions saving potential. Since methane is the main constituent of natural gas, biomethane can be used in natural gas vehicles without need for modifications.
Fossil methane is traditionally natural gas trapped beneath the surface of the earth. When formed, fossil natural gas tries to reach the surface as it is a fluid with low density. The gas is then trapped into the different geological formations made of layers of sedimentary porous rock, topped by an impermeable formation that acts as a roof. The gas is extracted by drilling through the impermeable rock and the gas released is usually under pressure. After extraction NG must go through some processes, basically, to eliminate oil, water, and some other trace components from the raw extracted gas.
In addition to traditional fossil methane, today unconventional sources of fossil methane are also important. Fossil unconventional methane can originate from several sources. 1) Shale gas is a form of natural gas trapped into shales, which are fine-grained and organic-rich rock formations. The estimation of its potential and existing reserves has substantially changed over the last years, enhancing the worldwide natural gas market. This can be explained by the progress in extraction technology, the hydraulic fracturing and the horizontal drilling techniques. 2) Coal-bed gas is a form of natural gas that can be found in a near-liquid state adsorbed into a solid matrix of coal. Free of H2S, coal-bed gas contains lower amounts of heavier hydrocarbons such as ethane, propane, and butane than conventional natural gas. 3) Tight gas (or tight sandstone gas) is natural gas found trapped within impermeable rock and non-porous sandstone or limestone formations. Thus, its extraction is more complicated and is usually performed by fracturing or acidizing. The classification of tight gas as conventional or unconventional natural gas can vary, so it is often considered falling in-between the two classes. 4) Methane hydrates are solid compounds in which methane is trapped within a crystal water structure, forming a solid structure similar to ice. Substantial reserves of methane hydrates have been found under the sediments beneath the ocean floors. Commercial-scale production of gas from these formations has never been accomplished, but several trials and field tests have been made over the last years. One recent method was based on the injection of CO2 into the hydrates, which then replaces and releases the methane molecules locked-up in “the ice”.
Standards are relevant, as design of engines should base its work on a known fuel composition and its potential variability.
ISO 15403:2006 defines natural gas as a gas with
- more than 70% volume/mole of methane and
- a higher calorific value of 30–45 MJ/Nm3.
It also recommends limits for
- moisture and dust, 3 vol-% for both, and
- for CO2, O2, and H2S a limit of <5mg/m3.
The following European specifications for natural gas and biomethane automotive market fuels were published in 2016/2017:
- EN 16723-1 Natural gas and biomethane for use in transport and biomethane for injection in the natural gas network - Part 1: Specifications for biomethane for injection in the natural gas network (Under Approval)
- EN 16723-2 Natural gas and biomethane for use in transport and biomethane for injection in the natural gas network - Part 2: Automotive fuel specifications (Under Approval)
Requirements of EN 16723-2:2017 are for example:
- Methane number over 65
- Total volatile silicon ≤ 0.3 mgSi/m3
- Hydrogen ≤ 2 wt%
- Total sulfur ≤ 30 mg/m3
The methane number is also an important property of natural gas. This value, which is calculated using an approach by the South West Research Institute, indicates the knocking resistance of the fuel; a methane number of 80 gives the same knocking behavior as a mixture of 20% hydrogen and 80% methane. EN 437:2003 “Test gases â€• Test pressures â€• Appliance categories” presents Wobbe index ranges of “Test Reference Gases”. The Wobbe Index is an indicator of the interchangeability of fuel gases (the higher heating value divided by the square root of specific gravity). However, both standards are of limited use when looking at natural gas vehicles (NGVs) performances, fuel economy, emissions, and consumer’s price safeguard.
UNECE Regulation No. 83 defines the emissions standards for light duty vehicles, including NGVs. The regulation defines the reference gas specifications (G20 and G25) to be used during the testing, and they are supposed to be representative of the different existing market qualities. The UNECE Regulation No. 49 defines the Type Approval procedure for heavy-duty engines and, Regulation 83 provides reference fuel specifications for heavy-duty NGVs. In order to cover the expected variability of natural gas quality across Europe, the regulation presents relevant differences/performances for gases deviating from pure methane (G20) to specified GR-G23 (for H-gas range) and G23-G25 (for L-range).
An essential problem is the extent to which the reference fuels used in emission tests have properties similar to the properties of the fuel in the real-world situation. The following summary and Table 1 summarize some critical information and provides relevant correlations:
- Biomethane, particularly in its liquid form (LBM) is the nearest to the G20 reference test-gas (pure methane). Liquefaction eliminates CO2, sulfur, and metals that are present in raw biogas.
- Over 95% of LNG typically has higher grades than G23 test-gas and high grade pipeline gas. LNG contains very little nitrogen, while G23 is generated by diluting methane with ~ 7.5% nitrogen.
- Low grade pipeline gas is simulated by G25 test-gas, which is generated by adding ~ 14% of nitrogen to methane. However, L-gas has higher contents of C2, giving higher Wobbe Index and lower methane number than G25.
- The high C2-contents of the Rich-LNG is simulated by GR test-gas by adding 13% of ethane to methane. However Rich-LNG has higher contents of C3+, giving higher Wobbe Index and lower methane number than GR
A broad conclusion is that G23, G25, and GR reference test-gas formulations are not really representative of the actual composition available in the gas-pipeline and LNG markets. In the test-gases methane is diluted with either nitrogen or ethane, while methane in actual gas is “diluted” by both ethane (and C3+) and/or inerts (N2 and CO2), depending on source.
Table 1. Test reference NG vs. typical NG/biomethane compositions (NGVA Europe’s LNG Position Paper. A. Nicotra - 2012.).
a Refers to raw biogas with less than 500 mg/m3 of H2S.
b The methane loss depends on operating conditions. These figures are guaranteed by manufacturers or given by operators.
c The quality of biomethane is a function of operational parameters.
d Raw gas compressed to 7 bar.
e Number of references reviewed.
f CarboTech./Quest Air.
g Malmberg / Flotech.
Table 2 reflects the variations that can be found for some relevant components between different pipeline gas specifications (for some countries these are the typical values found within the natural gas pipeline transmission system) in Europe.
Table 2. Summary of European natural gas pipeline specifications (mandated and typical values are mixed) for some relevant components. (Source: GASQUAL Project).
Natural gas is a mixture of different hydrocarbons, methane being the main constituent (usually 87–97%). It can also contain some impurities such as nitrogen or CO2. For natural gas, main variations include:
- calorific value
- methane number
- sulfur content
- content on inerts (nitrogen and carbon dioxide)
- content of higher hydrocarbons
Biomethane is upgraded from biogas, which mainly consists of methane and CO2 plus some minor/trace components which greatly depend on the feedstock (Table 3).
Table 3. Examples of biogas compositions from different sources (Kajolinna et al. 2015).
Final quality/composition of biomethane depends on the operational parameters of the final use and on the upgrading technology used (Table 4). Depending on the source, several trace components have to be closely controlled when using biomethane as a vehicle fuel, including:
- Siloxanes (risk of abrasion and increased probability for knocking)
- Hydrogen (risk of embrittlement for the metallic materials)
- Water (risk of corrosion and driveability problems)
- Hydrogen sulfide, H2S (corrosive in the presence of water could affect after-treatment devices, and combustion products could create problems by sticking the engine valves)
Table 4. Comparison of selected parameters for common upgrading processes (Urban et al. 2008).
LNG suffers from the great variety of import sources and final uses. Figure 1 shows how methane number and Wobbe index vary for imported LNG in Europe:
Figure 1. Methane number vs. Wobbe index for imported LNG in Europe. (GIIGNL 2010 / E. ON Ruhrgas).
The relation between temperature and pressure for saturated LNG is seen in the Figure 2.
Figure 2. LNG (as pure CH4) saturated pressure/temperature relation (NGVA Europe. Data from NIST).
All in all, as a consequence of the multi-use nature of NG as an energy carrier together with the different import sources, the natural gas market is characterized by a substantial variation in the gas composition. This is an important factor when talking about automotive fuels.
Biomethane can be produced locally and therefore distribution is different from natural gas in many respects. However, both bio-origin and fossil methane is used in compressed or liquefied state for storage and for transportation purposes.
- Compressed methane (CNG, CBG): natural gas or biomethane has been compressed after processing; mainly used for vehicles and typically compressed up to 200 bar.
- Liquefied methane (LNG, LBG): natural gas or biomethane has been liquefied after processing. Temperature is about -161.7 °C at atmospheric pressure and, when used as an automotive fuel, it can be stored in on-board cryogenic tanks (vacuum-isolated stainless-steel vessels), which have different operating pressure ranges.
Natural gas is transported over long distanced as compressed by pipeline or as liquefied by ships. Natural gas pipeline pressures in Europe typically range from 2 to 80 bar. The trend nowadays is to increase the pressure in the international connecting pipelines in order to reduce the transport cost. Pressure of the pipelines laid down at the bottom of the sea has to be sufficient, as it is not possible to install intermediate compression stations. Natural gas is transported as liquefied by ship, for example, when the distances to the consumption point are long (above 4.000 km), for example, when transporting large volumes over the seas. Usually most of the LNG gets gasified and injected into the NG grid. However, part of it can be directly used as LNG, and then usually transported by LNG road tankers.
Gaseous and liquefied natural gas pathways canot be clearly separated from each other, as many of the imported LNG is re-gasified in coastal LNG terminals so that it can be injected into the NG grid. It should be emphasized that both pathways are affected by the fact that the composition of the natural gas transported has high variability.
Figure 3 provides a visual comparison of the volumetric equivalence between diesel, CNG, and LNG for a given energy content.
Figure 3. CNG/ LNG/ Diesel energy and volume equivalence (NGVA Europe).
A well-known practice within the natural gas sector is the addition of odorants to help identify NG in case of a leak. Historically, this has been done in different ways as practically each European country has followed its own national code/standard on this subject. For years, the most used odorants have been tetrahydrothiophene (THT) and mercaptan, both sulfur-based odorants. During the last 10-15 years, several European countries have started to run technical programs in view of the substitution of THT and mercaptans with sulfur-free odorants. Countries like Germany, in which the odorization practice is governed by the DVGW standard G 280-1 ‘Gas Odorization’, started in 1995 developing a sulfur-free odorant for gas distribution networks and, already in 2007 more than 40 gas distributors in Germany, Austria and Switzerland had changed their odorization practices with THT or mercaptans to sulfur-free odorants like Gasodor™ S-Free™. The situation is still not balanced in Europe, however, as there are still countries using THT and mercaptans when running odorization practices. The level of sulfur derived from the addition of THT and mercaptan is linked to the exact positioning of the measurement equipment, as sulfur is more concentrated the closer the measurement is performed to the odorization point. According to E.ON Ruhrgas AG (and though different amounts are used in different countries), these could be indicative values:
- Mean sulfur content before odorization: 3.5–6 mg/m3
- THT is generally adding 5–10 mg/m3 (measured as S)
- Mercaptan is generally adding 1–3 mg/m3 (measured as S)
The use of sulfur-free odorants would mean further reduction of already low sulfur content of natural gas. Sulfur is known for its negative effect on the proper functioning of engine exhaust after-treatment systems leading to reduction of the conversion efficiency over time.
Oil carry-over and water/humidity control at refueling stations
Natural gas refueling stations can either be CNG, LNG, or LCNG stations, which can offer compressed, liquefied or both types of natural gas. LCNG stations are supplied with LNG, and CNG is produced with a vaporizer. Apart from this, CNG stations can either be fed from the natural gas grid directly, or fed from LNG which is then vaporized and put under pressure in order to get it settled to 200 bar. During the compression phase in a natural gas refueling station, some contaminants like water and oil can squeeze into the final compressed gas interfering with the proper functioning of NGVs. Some of the contaminants can come from the grid-distributed gas, and some others, like oil, can come from the compressor lubricants themselves. For those stations directly fed from the grid and also for stations being fed from natural gas mobile storage units, it’s typical that the gas is processed at the refueling site in order to make two main adaptations for its use in the vehicles:
- Drying: NG must be dry enough not to cause corrosion and drivability issues when put under high pressure. Water content values of 5 mg/kg are achievable and are currently good enough to guarantee the proper operation of the vehicles and their systems.
- Filtering: there is no existing suitable method for the measurement of particles in the gas, but for the protection of NGV systems (engines and associated components) it is necessary in order to ensure proper and durable functioning. There are several CNG coalescing filter suppliers that can be used today. According to the suppliers their products are able to remove 99.995% of the aerosols in the size of 0.3 to 0.6 microns when installed in series with other pre-filters. It’s generally recommended to use two filters after the compressor (and before the storage system) and another fine-mesh filter before the CNG dispenser.
Some other factors to consider are: how good are filters in removing aerosols and what is the need to have a proper maintenance program for the filtration systems. Experience has proven that, if not controlled, these two aspects can have important negative effects to the vehicles e.g. in form of corrosion of the CNG metallic tanks. This could affect safety in the long term, drivability problems due to water precipitation in connection with the expansion cooling that occurs when the gas flows from the storage tank to the engine inlet and could create ice plugs, abrasion of the mechanical parts due to solid particles entering the system, oil deposition in the engine’s distribution system, etc.
When talking about using methane as a fuel for automotive applications, the main engine technology to date has been the Otto engine, either under stoichiometric or lean-burn conditions of the air-fuel mixture. Nevertheless other engine technologies have also been developed, e.g. dual-fuel engines, which are compression-ignition based engines.
The difference in the functioning principle causes also relevant differences in the pollutant emissions derived from these engines, and thus also significant differences to the after-treatment strategies. Some of the main differences:
- Stoichiometric spark-ignition (SI) natural gas engines: characterized by a homogeneous air-fuel mixture, the air-fuel ratio being controlled via an oxygen sensor (or lambda sensor) installed in the exhaust stream.
- Lean-burn spark-ignition NG engines: characterized by a stratified air-fuel mixture. These engines usually require indirect fuel injection or a direct fuel injection with induced turbulence. The indirect fuel injection requires the fuel to be injected in a pre-chamber designed, to keep the air-fuel mixture at stoichiometric conditions until it is suctioned into the combustion chamber. Excess of oxygen concentration in the exhaust is controlled via a linear oxygen sensor.
- Dual-fuel compression-ignition NG/Diesel engines: dual-fuel engines differ from dedicated engines in their capability to burn two fuels at the same time. Dual fuel engines use diesel as the main ignition source for the natural gas-air mixture. Diesel substitution ratios can vary depending on the dual fuel engine technology and also depending on the operation of the engine itself.
Theoretically, energy efficiency is higher and engine-out emissions are lower for lean-burn engines than for stoichiometric engines. However, the stoichiometric engine is able to control emissions efficiently via a three way catalyst (TWC), which oxidizes CO and HC while reducing NOx. Due to the excess oxygen, TWCs can’t be used for lean-burn engines. Instead, oxidation catalysts are used to oxidize CO and HC, but without an effect on NOx. For the dual-fuel engine, the current and future emission legislations (EURO V and EURO VI) require that the engine is equipped with similar after-treatment technology as diesel engines, which means installation of a selective catalytic reduction (SCR), an oxidation catalyst and a diesel particulate filter (DPF). NGVs equipped with TWCs meet even EURO VI NOx emission requirements.
Regulated emissions, namely CO, HC, NOx and PM, depend on the sophistication of the engine and the exhaust control system. Methane as a motor fuel can provide exhaust emission advantages over diesel, especially for CNG vehicles equipped with stoichiometric SI engines and TWC. As a drawback, the energy efficiency of spark-ignited gas engines is worse than that of diesel engines. Therefore CNG vehicles have higher energy consumption (MJ/km) than diesel vehicles. However, energy consumption of methane fuelled cars is lower than that of gasoline fuelled cars (Karlsson et al. 2008).
Carbon intensity of methane is better than that of diesel fuel because of the higher hydrogen-carbon ratio of methane (CH4) when compared to diesel (C15H28) or gasoline (C7H15). This generally leads to less or comparable tailpipe CO2 emissions with CNG as for diesel and gasoline engines depending on engine.
In the AMF Annex 37, fuel and technology alternatives for buses were studied, and one bus comparison is shown in Figure 4. Here it is noted that tailpipe carbon dioxide emissions are only a part of lifecycle greenhouse gas emissions.
Figure 4. Tailpipe CO2e emission results for European heavy-duty vehicles. Methane emissions are taken into account with factor of 21. (Nylund and Koponen 2012).
Hesterberg et al. (2008) reviewed 25 studies on emissions from CNG and diesel fuelled heavy-duty vehicles and light-duty cars. When equipped with exhaust aftertreatment devices, most emissions were at similar levels with diesel and CNG buses. However, NOx emissions were substantially lower with TWC equipped CNG buses than with diesel buses. For the NGVs are equipped with spark ignition engines, also the tailpipe NOx and PM emissions are typically lower than those of diesel engines.
A study with buses, AMF Annex 37 reported by Nylund and Koponen (2012), showed that natural gas in combination with stoichiometric combustion and TWC delivers low regulated emissions, whereas lean-burn natural gas engines are characterized by high NOx emissions (Figure 5). All natural gas engines, independent of combustion system, deliver low particulate matter emissions, i.e. equivalent to particulate filter equipped diesel engines.
For diesel engines certain aftertreatment devices increase tailpipe NO2 emissions, which is also the case for lean-burn CNG engines, whereas stoichiometric CNG engines are low emitters of NO2. The share of NO2 of NOx was 35–70 % when engines were equipped with NO2 producing aftertreatment devices, but in other cases below 5% in Nylund and Koponen (2012). As an example, a mean NO2/NOx ratio for typical heavy-duty (EEV classified) diesel engines is in the range of 0.4 to 0.6, while typical values for respective natural gas engines is in the range of 0.01 to 0.05 (Kytö et al. 2009).
Figure 5. NOx and PM emission results for European vehicles (Nylund and Koponen 2012).
AMF Annex 39 (Enhanced Emission Performance and Fuel Efficiency for HD Methane Fuelled Engines) reported by Olofssen et al. (2014) aimed at investigating how the development level of the methane fuelled engines for heavy duty vehicles and the potential to reach high energy efficiency, sustainability and emission performance. Annex 39 included a literature study (Broman et al. 2010) and testing in Sweden, Finland and Canada. Vehicles tested were spark ignited (SI) dedicated gas engines and vehicles equipped with dual-fuel NG/Diesel engines. Methane used was CNG and sometimes mixed with biomethane.
Testing in Canada with dual-fuel concept High Pressure Direct Injection (HPDI), where diesel is a small amount of diesel is injected just for igniting the mix of methane gas and diesel. Methane used as fuel was Liquefied Natural Gas (LNG). The tested dedicated SI gas buses operates only on gas, while the NG/methane concepts could use only diesel fuel, or variable mix of diesel and methane gas. However, the truck using HPDI-technology could only operate properly when both methane and diesel fuel is available.
Test result from testing heavy duty vehicles equipped with dedicated SI methane engines show low emissions. In Sweden, energy efficiency of these engines is not in the same range as for the heavy duty vehicles (~ 18% vs. ~ 33%). In Finland, the tested bus with SI engine was well in line with Euro VI emission requirements and is considered as “best in class”.
The results from testing heavy duty vehicles equipped with NG/diesel technology showed that the theoretical diesel replacement capacity of 75-80% was difficult to achieve, particularly at low engine loads. Furthermore, to reach Euro V/EEV and Euro VI emission levels, obviously advanced combustion control and thermal management are needed.
New dual fuel technology (HPDI 2.0) is under development and expected to meet Euro VI and EPA 2014 emission requirement is under development. In addition, a newly developed dual fuel systems using the “fumigation” technology meeting Euro IV and V emission requirements was presented February 2014 (Gas Enhanced Methane Diesel, GEMDi). It is estimated that the average ratio for diesel replacement will be about 60%.
AMF Annex 39 included also limited tests with retrofit systems where older HD Euro III vehicles have been converted to use NG/Diesel technology. The emission performance will be dramatically negatively affected, except for the PM emissions. However, a possible advantage might be to reduce operating costs for the vehicle.
In the US, emission regulations handle dual-fuel technology based on diesel/gas ratio (without implementation of methane emissions). In Europe, work was started to modify the present regulations to include approval procedure for NG/Diesel dual-fuel technology.
AMF Annex 39 highlighted the following outcomes:
- NG/Diesel dual fuel concepts:
o Difficult to meet Euro V/VI emission standards with technology available by 2014
o Suitable only for OEM applications (not for retrofitting)
o Diesel replacement depends on load and is lower than expected
o Total GHG emissions might be higher for NG/diesel than for diesel vehicles
- Dedicated spark ignited engines (SI)
o No problem to meet Euro V/EEV emission requirements
o Lower engine efficiency when compared to diesel especially for lean-mix applications (18% vs. 33%)
o Lean-mix concept operating mostly on Æ›1
Similarly as for regulated emissions (Table 5), exhaust aftertreatment devices reduce most unregulated emissions to extremely low levels (Table 6), however, methane emissions from CNG vehicles are substantial. In the older studies, methane emissions from a CNG bus were around 150 mg/km (Murtonen and Aakko-Saksa 2009) and even as high as 2750 mg/km (review, Hesterberg et al. 2008). Karlsson et al. (2008) noticed that when using biomethane, methane represented 74% of hydrocarbon emissions at normal temperature.
Recently, in AMF Annex 51 the following findings were reported:
- The key mechanisms behind unburned methane emissions have been identified as: misfire/bulk quenching, wall quenching, crevice volumes, post oxidation and valve timing/overlap. All of these mechanisms are important factors, but wall quenching becomes more important as the engine operation becomes leaner.
- The major problems with unburned methane formation are associated with medium speed 4-stroke dual fuel engines.
- Since the unburned methane emissions origin from areas near the combustion chamber walls,the sensible way forwardis towards direct injection of natural gas/bio-methane in order to reduce emissions.
- Addition of hydrogen to natural gas in a test with a stoichiometric operated Euro 4 CNG vehicle showed significant reductions in THC and NOx emissions. Hydrogen blending could be an interesting option for processes with diluted mixture formation (lean or EGR operation).
- A series of Rh/zeolite catalystdesignsfor oxidation of exhaust CH4 were tested. 1 wt.% Rh/zeolite catalyst had higher activity compared with the commercial catalyst under the same operation condition. Amore efficient regeneration method is still being sought.
- Another case investigated the Pd based catalyst performance. In this study, some key factors were found, which enhancedthe activity and durability of current Pd-based CNG catalysts.
- Regeneration of the catalysts used is an important issue, and a regeneration method withhydrogen was studied. With a catalyst aged to a conversion efficiency of 37%, it was possible to maintain this level and even increase the efficiency after regeneration and ageing again applying regeneration gases containing 2,5% hydrogen.
- A number of vehicles were tested for tailpipe methane emissions as well as other methane emissions. The result of this investigation indicates that the major contribution of methane originates from slip during driving, i.e. tailpipe emissions.
Formaldehyde, acetaldehyde, 1,3-butadiene, and benzene emissions have been reported to be lower for CNG vehicles, particularly for dedicated CNG vehicles, than for gasoline or diesel vehicles. Karlsson et al. (2008) has studied bi-fuelled biomethane car in comparison to gasoline car and observed quite small differences in emissions at -7 °C.
Ammonia emission tends to be high with gasoline cars equipped with three-way catalysts (Aakko-Saksa et al. 2012). This was the case also for a TWC equipped CNG bus shown in Table 6.
Low emissions of carcinogenic emissions, such as 4-ring polycyclic aromatic hydrocarbons, have been observed with CNG fuelled vehicles and cars. In addition, Ames mutagenicity of particulate matter was lower for CNG vehicles when compared to diesel vehicles in a study by Murtonen and Aakko-Saksa (2009).
Mean numerical results with transit buses, trucks, and cars running on diesel with trap or CNG with TWC from a review by Hesterberg et al. (2008) are shown in Table 5. Summary of selected studies on the unregulated emissions with CNG, diesel and gasoline is presented in Table 6.
Table 5. Review of regulated emissions (Hesterberg et al. 2008).
Table 6. Summary of selected studies on the unregulated emissions with CNG, diesel and gasoline.
Non-optimized lean-burn CNG vehicles may emit high exhaust emissions, whereas exhaust emissions for optimized CNG vehicles are low (Table 6, Nylund et al. 2004).
Table 6. Exhaust emissions from three studies (Nylund et al. 2004).
CNG delivers very low particle number emissions, almost two orders of magnitude lower than diesel technologies (Figure 6). However, diesel vehicles equipped with DPF produce particle numbers comparable to CNG. The highest particle numbers were clearly observed for diesel buses without DPF in a study by Nylund and Koponen (2012). Karlsson et al. (2008) observed lower particulate mass and particle number emissions for a biomethane fuelled car than for a gasoline car.
Figure 6. Particle number size distribution for a number of technology alternatives (indicative). (Nylund and Koponen 2012).
Car emissions at cold temperature
From natural gas fuelled cars, CO, HC, and NOx emissions are low, which was observed also in the AMF Annex 22 reported by Aakko and Nylund (2003) (Figure 7). In addition, this dedicated, monofuel CNG car is quite insensitive towards ambient temperature, whereas CO and HC emissions from gasoline cars increase as ambient temperature decreases down to -7 °C. Karlsson et al. (2008) reported of difficulties in switching fuel from gasoline to biomethane (CBG) for a bi-fuel NGV after the cold start at -7 °C. In this case, car fuelled with compressed biomethane showed higher CO emission, but lower NOx and particulate matter emissions than gasoline car at -7 °C.
Figure 7. Regulated emissions from diesel cars (TDI and IDI), gasoline fuelled cars (MPI and G-DI), E85, CNG, and LPG cars (Aakko and Nylund 2003).
Aakko-Saksa, P., Rantanen-Kolehmainen, L., Koponen, P., Engman, A. and Kihlman, J. (2011) Biogasoline options – Possibilities for achieving high bio-share and compatibility with conventional cars. SAE International Journal of Fuels and Lubricants, 4:298–317 (also SAE Technical Paper 2011-24-0111). Full technical report: VTT report W187.
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