Alternative fuels
Natural gas (compressed and liquefied)
Around 23 million vehicles worldwide run on natural gas (more than 175,000 vehicles in the USA)[1]. Natural gas vehicles (CNG vehicles) are a good choice for fleets with high mileage and centralised fuel supply because they can provide sufficient travel range in a region with the availability of CNG filling stations (CNG stations). For vehicles that travel long distances, liquefied natural gas (LNG) is better, which has a higher energy density than CNG, so its range is more comparable to that of conventional fuel.
CNG and LNG are considered alternative fuels in the US under the 1992 Energy Policy Act. The power, acceleration and cruising speed of natural gas vehicles are comparable to those of vehicles running on conventional fuels.
Scientific evidence on the environmental friendliness of CNG:
a) CNG vehicles have lower greenhouse gas emissions per vehicle kilometre compared to petrol, but higher emissions compared to diesel engines (in the operational phase);
b) in order to ensure low emission of pollutants (above Euro-3 level), CNG-powered vehicles require installation of emission cleaning systems, first of all in order to reduce emissions of nitrogen oxides, but the cleaning systems can be simpler than for diesel engines[2];
c) to provide optimal ecological characteristics special CNG combustion engines are being developed, both compression-ignition and spark-ignition, with their stoichiometric ratios, fuel injection methods, emission purification systems
d) dual-fuel petrol/gas engines of ecological classes 5 and above have better environmental performance when operated with CNG than with petrol in terms of particulate matter (black carbon) and hydrocarbon emissions, but similar or higher in terms of nitrogen oxide emissions (all substances of priority in terms of health impacts)
e) when petrol vehicles are retrofitted to use CNG as a motor fuel[3] emissions of particulate matter and hydrocarbons are reduced, but emissions of carbon monoxide and nitrogen oxides may increase.
In general, the potential for improving the environmental performance of transport by using CNG/LNG is limited and dependent on specific conditions (e.g. urban air pollution). The most feasible conversion to CNG/LPG seems to be for diesel trucks.
Biofuels (liquid — ethanol, methanol, biodiesel; gaseous — biogas, biogenic hydrogen).
Transport biofuels exist mainly as ethanol and biodiesel. In 2014 ethanol accounted for 74% of the transport biofuel market, biodiesel for 23% (mainly in the form of fatty acid methyl esters) and hydrogenated vegetable oil (HVO) for 3%. Biofuels are mainly used in the form of a fuel blend — a small additive (up to 10%, e.g. B7 is the maximum blend allowed by the Fuel Quality Directive for sale in the EU, where B7 specifies a maximum biodiesel content of 7%).
These fuels are produced from food raw materials. Ethanol is produced from sugar cane (61%) and from grain (39%). The main feedstocks for biodiesel production are soy and rapeseed. The use of second-generation raw materials in the production of biofuels, i.e. oilcake, agricultural waste, waste oils and the like, is environmentally friendly.
The use of biofuels on second-generation feedstocks reduces СО2 emissions, and there is also evidence of reduced pollutant emissions from biofuels, apparently due to fewer impurities compared to fossil fuels. For example, biodiesel emits 65-90% less CO2 than fossil diesel[4], for every kilogram of biodiesel, CO2 emissions are reduced by about 3 kilograms[5]. Engines using biodiesel also emit significantly less pollutants, including particulate matter, carbon monoxide and hydrocarbons[6]. In addition, biodiesel has extremely low sulphur content[7] and naturally self-lubricating properties, which reduce metal emissions associated with engine wear[8], reducing wear on emission cleaning systems.
Hydrogen
The use of hydrogen as a vehicle fuel can be achieved by using
- hydrogen itself;
- hydrogen together with conventional petroleum fuels;
- hydrogen as a fuel in fuel cells (to be considered separately).
Hydrogen is used in engines that run on conventional petroleum fuels and also in combination with alternative fuels such as alcohols (ethyl, methyl) or natural gas. It is possible to use hydrogen in combination with synthetic fuels, fuel oils and so on.
Hydrogen can be a fuel for diesel engines with a number of modifications; it significantly improves environmental performance when added to gasoline as an additive; there are scientific papers in RF[9] which tested the possibility of producing a hydrogen fuel mixture from methanol directly on board a car with good environmental performance (better than that of gasoline, using domestic cars as an example). For modern petrol engine designs it is most efficient to use hydrogen as an additive to the petrol-air mixture. This does not require major changes in the design of the fuel system and the engine system as a whole.
The absence of carbon in the hydrogen fuel results in a near absence of carbon oxides (CO and CO2) and unburned hydrocarbons (CnHm) in the exhaust gases. The small amount of these products in the exhaust gases is due to the burnout of the lubricants entering the combustion chamber. When the mixture is stoichiometric, the higher combustion temperature of the hydrogen-air mixture results in twice the nitrogen oxides emitted by the petrol engine. Mixture depletion leads to a rapid reduction in nitrogen oxides, and at α = 1.8 they are virtually absent in the exhaust gases. Nitrogen oxides are also neutralised in catalytic converters.
There is also the problem of detonation combustion. It is possible to use the anti-detonation properties of water to eliminate the detonation of a hydrogen engine.
Vehicle engines and emission control systems
Pollutants are emitted from the ICE as a result of incomplete combustion of fuel, additives and engine oils, as well as from oxidation of air nitrogen (nitrogen oxides). The amount of generated pollutants depends on such factors as type of injection and the presence of an injection control system, stoichiometric ratio, engine shape, engine operating mode[10].
In order to gradually reduce emissions, states are setting emission standards for new types of vehicles (the first of these were the Euro standards). Current regulations (Euro-5 in the Eurasian Economic Union, Euro-6 in the European Union) require vehicle manufacturers to equip vehicles with sophisticated engine management and control systems and associated emission control systems (e.g. fuel timing control, on-board emission control system, exhaust gas recirculation system, catalytic converters, particulate filters, selective catalytic reduction systems and others).
The presence of complex systems increases the cost of production of vehicles, affects their dynamic performance and requires more complex maintenance (for example, for Euro 6 diesel engines — to purchase urea), which for some car owners causes a desire to disable some of the functions and leads to a significant increase in pollutant emissions. For gasoline engines there is a degradation of the three-component catalytic converter, it needs to be replaced after 100-150 thousand kilometres.
Thus, in the course of renewing the city’s vehicle fleet, the importance of controlling pollutant emissions during operation, the completeness of emission control systems and the correctness of on-board control systems is significantly increasing.
Alternative energy engines
Electricity
Electric vehicles use the energy stored in the batteries to drive. There are no harmful emissions or greenhouse gases from the vehicle itself when it is in use. Indirect emissions depend on the way electricity is generated. Disadvantages: charging time (currently at least 10 minutes for electric buses in Moscow as recharging between trips, at least 20 minutes for a personal electric vehicle with “fast” charging up to 80% of the battery charge); risk of battery overheating and combustion; loss of charge in cold seasons; limited inter-charging mileage for most modern models; battery recycling; need for rare-earth elements for battery production. Overall, electric vehicles (predominantly cars and light trucks) can be a good solution for improving air quality in large cities, where the demand for mobility cannot be fully met by public transport and micro-mobility, while providing environmentally friendly ways to generate electricity.
Hydrogen fuel cells
Hydrogen fuel cell vehicles are essentially the same as electric cars, in which electricity is generated on board using fuel cells made from hydrogen. The resulting exhaust consists of water vapour. Compared to electric cars they have a longer range and a shorter refuelling time (up to 5 minutes). It is possible to produce hydrogen directly at the gas station if there is an available source of water by electrolysis (environmentally friendly by using RES in electrolysis). Disadvantages: higher requirements for a hydrogen tank (easily leaking gas), underdeveloped and expensive infrastructure, lower energy efficiency compared to an electric vehicle (additional hydrogen production and compression step). Hydrogen production methods are actively developing, but the most widespread production is from methane (90.5%) and from water by electrolysis (about 2.5%)[11]. In world practice, the main types of hydrogen according to the type of production technologies are usually designated by colours, where green is hydrogen produced from water using RES (carbon-neutral), and brown — hydrogen from methane. In case of production from methane hydrogen is not carbon-neutral, but its carbon footprint can be reduced by application of technologies of CO2 capture and storage and only in this case CO2 emissions per unit of produced energy in life cycle of hydrogen are lower than by simple combustion of natural gas[12].
According to some Russian experts[13], introduction of hydrogen is expected in city buses, commercial vehicles including taxis, long-haul trucks, and on certain sections of railways where centralised refuelling is possible.
The technology is developing, there are still many problems, in particular transportation (corrosion resistance and metal density requirements for hydrogen pipelines). However, in the long term, hydrogen (if transportation problems are solved and production is cheaper) could become the basis of “carbon-neutral” energy, including energy storage for renewables, fuel for power and transport and raw materials for a number of industries (hydrogen’s versatility).
In general, the use of hydrogen in the medium term may be interesting for long-haul transport, certain fleets (where “local” hydrogen production plants can be installed), and coastal areas (water sources).
[1] https://afdc.energy.gov/vehicles/natural_gas.html.
[2] Recent international publications show that emissions of particulate matter from CNG engines of Euro-6 exceed the standards for engines of similar types of diesel-powered vehicles (tested for trucks), due to temporarily less stringent cleaning requirements — only from 2023 must CNG engines of Euro-6 be equipped with particulate matter filters. Thus, CNG engines must be equipped with emission control systems in order to meet the high emission requirements of Euro 6. On cleaning systems: Victor Lejona, Dedicated to Gas: Assessing the Viability of Gas Vehicles; https://www.cenex.co.uk/app/uploads/2019/11/Dedicated-to-Gas-Assessing-the-Viability-of-Gas-Vehicles.pdf.
[3] M.I. Jahirul, H.H. Masjuki, R. Saidur, M.A. Kalam, M.H. Jayed, M.A. Wazed, Comparative engine performance and emission analysis of CNG and gasoline in a retrofitted car engine, Applied Thermal Engineering, Volume 30, Issues 14–15, 2010, Pages 2219-2226, https://doi.org/10.1016/j.applthermaleng.2010.05.037.
[4] JEC Well-To-Wheels Report v5, JRC, 2020.
[5] Economic and Social Aspects of Applying Biodiesel Fuel in Road Transport, M. Skočibušić et al., 2010.
[6] N. Travis, Biofuels, 2012, 3, 285.
[7] M. S. Graboski and R. L. McCormick, Prog. Energy Combust. Sci., 1998, 24, 125.
[8] A. K. Agarwal et al., Renewable Sustainable Energy Rev., 2011, 15, 3278.
[9] Search for ways of entering certain types of alternative energy resources in the sphere of perspective development of hydrogen energy in domestic transport, Ph.D. Prof. V.M. Fomin, Ph.D. Associate Professor N.A. Hripach, Izvestiya 74 MSTU “MAMI” No. 1(19), 2014, vol. 1, pp. 74-83; Fomin V. M., Bendik M. M., Sidorov M. I., Gerasimenko S. A. Hydrogen power engineering for road transport: origin and current state // GIAB. 2006. No. 12.; Kutenev V.F., Kamenev V.F. Prospects of using the hydrogen fuel for the automobile engines (in Russian) // Conversion in engineering industry. — No 6. — Pp. 73-79, quoted in ref. 29.
[10] Also the quality of engine fuel, additives and engine oil.
[11] The remaining 7% is a by-product of manufacturing.
[12] Sebastian Timmerberg, Martin Kaltschmitt, Matthias Finkbeiner, Hydrogen and hydrogen-derived fuels through methane decomposition of natural gas — GHG emissions and costs, Energy Conversion and Management: X, Volume 7, 2020, 100043, ISSN 2590-1745, https://doi.org/10.1016/j.ecmx.2020.100043
[13] A hydrogen strategy for a climate-neutral Europe, p. 10.Also: Rustam Abulmambetov, Head of Department of Sectors of Economy, Ministry of Economic Development (source: http://rosacademtrans.ru/concept_electrotransport/ ); opinion is given in the Glonass NP review “Hydrogen Transport: Current Status and Prospects”, 2021, p. 14.
Cover photo: Zuma / TASS
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