2.4.1. Carbon Capture and Storage
Carbon Capture and Storage is the technology used in industry, whereupon carbon dioxide is captured and disposed at a collection site and later isolated from the atmosphere.
As stated by Bernstein et al [
15], CCS is a technology encompassing the capture of CO
2 either before or after the occurrence of the combustion phenomenon, transportation of CO
2 to a disposal site and finally its disposal by a method that will permanently isolate it from the atmosphere. One of the best examples of this would be, where CO
2 is extensively being used for enhanced oil recovery (since 1986) where 24 million tonnes of CO
2 have been accumulated. The long-term usage of this technology delivers assurance that this technology is feasible. CCS is practiced at industrial level, however, the technology can be miniaturised in order to be installed in vehicles. In an on-going project by Aramco [
16], the vehicles are equipped with CCS, where they were able to capture 10% CO
2 initially and are aiming to capture 85–96% in coming years. As per the IPCC Special Report on Carbon Dioxide Capture and Storage [
17] the currently available technology can capture about 85–95% of the CO
2 processed in any capture plant. This is located in a power plant resulting in the reduction of CO
2 emission to the atmosphere by 80–90% approximately compared to a plant without CCS. As said by Ajay Pal Singh [
18], the report from Intergovernmental Panel on Climate Change (IPCC) also determined that carbon capture and storage can contribute approximately 15–55% of the aggregate emission reduction towards 2100. This will play a major role in curbing carbon dioxide by implementing several technologies to be implemented to address climate change.
CCS comprises of a chain of processes to capture CO
2 having three main carbon capturing options such as Post-combustion CO
2 capture, Pre-combustion CO
2 capture and Oxyfuel combustion CO
2 capture processes. Wherein in Post-combustion process the CO
2 can be captured from the gas emitted after burning the fossil fuel or biogas (the latter) widely used in coal plants and now being proposed and tested in internal combustion engines. In Pre-combustion process the fossil fuel, biogas is reformed to separate the hydrogen and carbon dioxide before burning them. The fuel conversion steps are sophisticated and expensive the high concentration of CO
2 in the gas with high pressure makes the separation easier. The pre-combustion process is being used in manufacturing fertilizers and in hydrogen production, as this process, in the end, produces hydrogen and CO
2 from which CO
2 is separated. In the Oxyfuel combustion process, highly pure oxygen is extracted from the air which demands higher energy requirements and is used in the combustion process of fossil fuel or biogas. This results in a higher concentration of CO
2 in the emitting gas that makes it easier to separate CO
2 from the emitting gases as shown in [
19].
Post-combustion CO
2 capturing is a more economically attractive process in comparison, which captures CO
2 from the emission of flue gases. The tailpipe emissions from an ICE emit a significant amount of exhaust gases and heat dissipated from the engine. This heat can be reused in the process of CO
2 extraction from the exhaust by the means of a CO
2 capturing device [
20]. This can be installed in a vehicle and CO
2 can be captured alongside generating electricity from a Thermal Electric Generator (TEG) built inside the heat exchanger. The TEG in this heat exchanger uses the principle of a temperature difference to generate the electricity. The flow diagram for this system is shown in
Figure 4, below.
The heat exchanger works on the principle of heat conduction, as the exhaust gas passes through the heat exchanger, heat is extracted from the exhaust gas to cool down the exhaust gas that exits the heat exchanger. The first TEG is arranged to be coupled thermally with the exhaust gas chamber to the CO
2 absorber fluid chamber in a manner effective to heat the fluid by the exhaust gas. This releases CO
2 from the fluid and generates electricity in response to a temperature difference between the exhaust gas chamber and the absorber fluid chamber. Due to the temperature difference between the engine coolant chamber and the CO
2 absorbing fluid the heat exchanger may advantageously include a second TEG configured to thermally couple the engine coolant chamber to the absorber fluid chamber. In a manner effective to heat the CO
2 absorbent fluid by heat from the engine coolant to further release CO
2 gas from the CO
2 absorbent fluid and generate electricity in response to a temperature difference. The system fitted in a car would need an electrical converter which can be used in converting the required electrical energy from the TEG in a suitable form of electric energy to be used for the electrical systems in the vehicle, for example vehicle electronics or electric-assisted boosting systems. A basic version of this heat exchanger can be configured, wherein only CO
2 can be captured from the exhaust gases and the facility of electricity generation is not required in this device [
21].
Carbon Capture and Storage in India: Carbon Capture and Storage technology is still under development in India and there are in total three plants [
22], that have implemented CCS to curb CO
2 emissions which are urea producing plants [
23]:
These plants have CO2 absorption capacity of 450, 150 and 450 Tonnes per day, respectively, and they use post-combustion technology to capture CO2.
CCS in vehicles could be a revolutionary step for India towards curbing CO
2 tail pipe emissions. As per the article “India Seeking Ways to Limit Climate Change after IPCC Report” [
24], the experts at a climate change meeting accepted the implementation of CCS in India. If there were no new technologies to help reduce emissions significantly, they think CCS is still commercially unviable and a very challenging technology. Being a developing country there are numerous ways that the new technologies can be implemented to overcome the emissions.
However, India has good CCS opportunities as expressed by Kapila and Haszeldine [
25] in that CCS can be implemented in the fertilizer producing plants where there is a shortage of CO
2 as they tend to use all of the CO
2 which is generated from ammonia producing urea plants. It has been observed that additional CO
2 generating units were built to supply urea production. Secondly, the Government of India already has a plan for Enhanced Oil Recovery (EOR) offshore and onshore, where a facility at Hazira port, Gujarat is developed for EOR onshore site 70 km away and is estimated that 1200 tonnes of CO
2 would be transported to this oil field on a day-to-day basis, which would maintain the pressure in the oil field. Implementing CCS in the Coal and Power sector, carbon saving could start with efficient generation technologies and it would, also, be possible to design new generation power plants in India such as Ultra Mega Power Projects (UMPPs) to be “carbon capture ready” empowering a future retrofit of CCS.
Exporting CO2 for foreign EOR activities could be a big business for the country, where the captured CO2 from vehicles and large industries can be transported to the neighbouring gulf countries from the planned UMPP projects situated on the coasts of India. Nations such as Qatar are major gas producers as they have established a huge LPG tanker traffic, these tankers maybe converted to take return loads of CO2 to be injected in the oil fields.
2.4.2. Waste Heat Recovery
Waste heat recovery is being investigated in the automotive industry to increase the efficiency of ICEs, as well as to reduce CO
2 emissions by converting the thermal energy to electrical energy by using either thermal fluid systems via Organic Rankine Cycle (ORC) or Thermoelectric Generators (TEG). Avaritsioti [
26] observed that the conversion of 20% of exhaust waste heat into electricity might increase fuel efficiency up to 10%. It was then showcased that the use of exhaust heat recovery by replacing the conventional alternator was a cost-effective way to reduce the greenhouse gases in the heavy-duty vehicles.
As observed in a Sankey’s energy flow diagram barely 25% of the fuel energy is used in the vehicle operation and the remaining 75% of energy is dissipated to the atmosphere. The other half is rejected by the coolant, lubrication and by the means of the charge-air heat exchangers [
27]. It is also notable that even though it had been said that approximately 2/3 of the fuel energy is transformed into the waste heat, almost all this emitted thermal energy is an outcome of the limitation of the thermodynamic cycle of an engine. And hence, as a result, it is not feasible to reclaim all the thermal energy for the useful work without the laws of thermodynamics being violated. Some of the WHR options are discussed below.
Electric Turbocompounding: As the name suggests this technology helps in generating electricity from the energy extracted from the residual kinetic energy of the exhaust gases downstream the turbocharger typically in Diesel engines, by coupling an electric generator to a turbine. An additional electricity-generating turbine is installed downstream of the main turbocharger turbine as suggested by [
27], however, due to low pressure gas at the end of the exhaust, the second (power) turbine has to recover relatively efficiently at low-pressure. Therefore, the residual amount of pressure is used to generate electricity. The power turbine then generates electricity to be used by the vehicle components or to be stored directly to the battery of the vehicle.
The electric turbo-compounding technology exhibited results of fuel economy gains between 3–10% depending upon the application. This system is typically installed on heavy duty diesel engines although increasingly electrified powertrains of any size can use this technology.
Thermo Electric Generators: As discussed earlier TEGs were used to generate the electricity required in vehicle components by the means of temperature difference. The technology can be implemented for Waste Heat Recovery systems where the generators convert exhaust heat energy directly to electric energy using the Seebeck principle in electronics in which electricity is generated between two semiconductors because of temperature difference. As suggested by [
28], the implementation of TEGs in ICEs shows good results in reducing fuel consumption. However, this technology is still under research and development and is expensive compared to other technologies. The research in this technology so far has yielded up to 2% of the fuel efficiency improvement.
In a very special case of a hybridised boosted optimised system with turbo-compounding (HyBoost project [
27]), a 1.0 L, four-cylinder downsized direct-injection gasoline engine was developed to deliver 116 kW and 240 Nm, offering 35% reduction in fuel consumption and CO
2 emissions. The system was able to reduce carbon emissions from 169 g/km to 99.7 g/km on the new European Driving Cycle matching the 2.0 L engine performance, i.e., 50% downsizing was demonstrated. It was equipped with an electric supercharger for transient lag mitigation, electric turbocompound, efficient liquid charged air cooler and advanced knock mitigation technologies.
2.4.5. Advanced Combustion
When compared to the other engine types the Diesel engines are the most efficient due to their higher compression ratios and overall fuel-lean combustion, which provides low carbon monoxide and hydrocarbon emissions when compared to gasoline engines. The main focus is on reducing nitrous oxides and particulate matter emissions because of high temperatures and fuel-rich regions respectively, as discussed in [
36]. Four general methods are extensively being developed to satisfy emission standards and to overcome the NOx and PM emissions i.e., improvement of the combustion process, usage of alternative fuels, introduction of advanced combustion concepts and exhaust after-treatment devices. From the diesel combustion perspective, to reduce NOx and PM emissions, high temperature stoichiometric and fuel-rich regions should be averted, concurrently. Consequently, one of the most effective approaches is Low Temperature Combustion (LTC) which shows improvement in fuel atomization, lower equivalence ratios and decreased combustion temperature which decreases NOx and PM emissions instantaneously. This maintains high thermal efficiency complying with the emission standards and increasing in fuel demands.
Low Temperature Combustion: The concept of LTC reduces the flame temperature and permits adequate homogenous air-fuel mixture, leading to a simultaneous reduction of PM, NOx and smoke emissions [
37]. As claimed by the [
38], LTC was able to attain a very low NOx emission (<35 ppm) by using exhaust gas recirculation and PM (<0.05) by means of advanced fuel injection timing. Hence the technology has become a very popular research topic. LTC is an advanced combustion technology that is achieved by early fuel injection in the combustion chamber that improves the air-fuel mixing before the start of combustion inside the cylinder. By injecting the fuel nearer to the top dead centre (TDC) with EGR controlled combustion, dual fuel injection, the resultant air-fuel mixing avoids fuel-rich regions that lower the temperature below 2100K, which in turn reduces NOx and PM emissions. This type of combustion increases the engine efficiency and reduces emissions.
LTC is achieved through a variety of methods such as Homogenous Charge Compression Ignition (HCCI), Premixed Charge Compression Ignition (PCCI), Partially Premixed Combustion (PCC), Reactivity Controlled Compression Ignition (RCCI), Gasoline Compression Ignition (GCI), High Efficiency Clean Combustion (HECC) and Spark Assisted Compression Ignition (SACI). However, we will be briefly looking into HCCI and PCCI methods in this literature. Currently, numerous researchers are concentrating on reducing the use of conventional fuel, forming new alternative fuels and reformulating the fuels to reduce emissions such as a project on the effects of bio-diesel and binary blends in a turbocharged diesel engine [
39].
Homogeneous Charge Compression Ignition (HCCI): The HCCI method was developed in 1979 using a gasoline engine to increase the combustion process stability for a 2-stroke engine. In HCCI a well premixed fuel-air mixture gets ignited at the end of the compression stroke devoid of any spark assistance. In this process, the combustion occurs at multiple places because of auto-ignition of the mixture which achieves its chemical initiation energy. Due to compression pressure and hot temperatures in the cylinder, the mixture ignites at different hot-spots inside the cylinder resulting in auto-ignition of the air-fuel mixture. This process reduces high flame fronts and maintains the in-cylinder temperatures at lower levels. In a study of HCCI and low temperature combustion with the aid of optics [
40], it was demonstrated that near-zero NOx emissions were feasible due to low combustion temperature and high EGR attenuation. The overall homogenous lean air-fuel mixture also tends in avoiding the soot formation in emissions. Therefore, in HCCI engine technology the air-fuel mixture ignition is controlled by its composition and the in-cylinder temperatures. However, to attain homogenous mixture, several methods such as high fuel injection pressure with small nozzle holes, high boost pressure, and higher swirl ratios are used to increase the homogeneous mixture properties of the air-fuel mixture injected in the combustion chamber.
Premixed Charge Combustion Ignition (PCCI): PCCI was developed from HCCI due to the latter’s limiting operating range, high rise in pressure and inability of controlling the combustion parameters such as SOC [
36]. This depends on several factors such as fuel auto-ignition qualities, fuel mixture homogenous characteristics, in-cylinder temperature and pressures etc. These challenges motivated the researchers to develop PCCI. This method is capable of reducing NOx and soot emissions simultaneously with improved engine efficiency. It was observed by the authors that there was 31% reduction in CO
2 emissions with an increase in 14% of BSFC with the aid of diesel and ethanol-blended composition of the fuel. As per the authors in a review paper based on LTC for Bharat Stage VI standards [
41], fuel mixing time is an important building block in low emission policy. As the mixing is increased there are lower possibilities of the formation of PM, HC, CO and NOx due to the increase in engine efficiency. The PCCI method of combustion has a requirement to obtain premixed charge before the start of combustion, achieved by ignition delay before SOC of high-temperature reactions. Some of the ignition delay methods are as discussed in HCCI, however, use of low cetane fuel with low compression ratio can also aid towards delaying in the ignition. The key in achieving successful implementation to PCCI lies in harmonizing the mixing and chemistry times, where prolonging the mixing time improves particulate matter emissions, for example. The time of mixing must be extended so that the ignition cannot occur before the desired level of mixing. Therefore, if the mixing rates can be increased by utilizing mixing related design constraints such as spray targeting techniques, swirl geometry, the necessary mixing timing and ignition delay can be reduced as a consequence of gaining control over the phase of combustion. In the case of PCCI, there are typically two types of injection strategies, that are adopted depending on when the fuel injection occurs relative to TDC [
42].
2.4.6. Regenerative Braking
This technology was invented back in 1886 to convert kinetic energy into electric energy and store it in batteries or super-capacitors. The modern form of regenerative braking such as the Kinetic Energy Recovery System (KERS) uses the same principle to capture the kinetic energy from braking. KERS is being widely researched and implemented in hybrid and electric powertrain vehicles. It is successfully implemented in F1 and LMP1-H hybrid cars.
A new concept developed, known as electric KERS can store energy and provide it back to other systems by being connected to traction batteries or other electronics such as an electric turbocharger or an electric pump in the engine [
43]. The technology of KERS was introduced in Formula 1 during the 2011 season and was mandated by the FIA in all the cars [
44]. As per [
43], most of the F1 and LMP1 hybrid-powertrain car companies such as Toyota and Porsche have shown a deep interest in harvesting the energy in every way possible.
Vehicles implementing this type of technology are more fuel-efficient and the fuel consumption reduction can reach 30–50% by a regenerative braking technology in any hybrid electric vehicle [
45]. The technology is integrated with the powertrain of hybrid and electric cars such as Toyota Prius and Nissan Leaf. In passenger vehicles, there are very low decelerations and implementing KERS does not put any constraint on driving style. KERS has been integrated with petrol-powered engines in large-sized sedans to provide improvements in fuel economy of about 20–25% and improvements in fuel consumption may be achieved by coupling KERS in smaller diesel turbocharged engines [
46]. The author stated that the results of the car fuel economy on the New European Driving Cycle validated vehicle model with a 1.6 L turbocharged direction injection engine which would normally require 0.0.35 kg of fuel to cover full cycle (4.7 L/100 km in urban sector cycle and 3.8 L/100 km in full cycle), required 3.5 L/100 km (0.12 kg) of fuel to cover the urban cycle and 3.2 L/100 km (0.29 kg) of fuel to cover full cycle, improving fuel efficiency by 25% and 17%, respectively. Therefore, it was concluded that KERS implemented with the high-speed flywheel with a CVT concept permits efficient regenerative braking and torque through considerable improvements in fuel efficiency, consequently reducing fuel consumption and CO
2 emissions. Clark et al [
47] suggests a fuel efficiency improvement of 29.4% in the rural driving cycle and 51.7% in urban conditions by implementing regenerative braking technology.
For road commercial heavy goods vehicles, claimed reduction of fuel consumption and CO
2 by 15–25% have been published [
48]. A case in India where regenerative braking is being implemented at the Delhi metro, which became the first rail project in the world to receive 0.4 million tonnes of carbon credits for the consecutive period of ten years from 2007 to 2017. By implementing regenerative braking in locomotives, the Delhi Metro was able to lower its energy usage by approximately 30% [
49].