Possible transport energy sources for the future

Transport Policy 27 (2013) 1–10

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Transport Policy journal homepage: www.elsevier.com/locate/tranpol

Possible transport energy sources for the future L. De Simio n, M. Gambino, S. Iannaccone Istituto Motori, Italian National Research Council, Via Marconi 8, Naples 80125, Italy

a r t i c l e i n f o

Keywords: Biomass Coal BTL SNG LNG

abstract In the medium to long term, low fossil fuel availability will make it necessary to find alternatives. Mass production of biofuels will not be a practical solution because it requires strong competition for land that is used for growing food. Therefore, it will be necessary to revise the frame of transportation energy sources. The number of pure light- and heavy-duty electric vehicles could increase in urban areas. Instead, it will be hard to find a viable alternative to the internal combustion engine for extraurban transport vehicles, therefore alternative synthetic fuels could be used to compensate for fossil fuel depletion. Aside from a small share obtainable from biomass, most synthetic fuels are expected to be obtained from coal. Among these, synthetic natural gas represents a very good solution. In fact, synthetic natural gas will be advantageous with respect to hydrogen, whose on-board storage will be an unsolved problem in the medium term, and with respect to synthetic liquid fuels, which require more energy in the production phase. Moreover, the carbon content of liquid fuels, which is higher than that of gaseous fuels, will be responsible for higher CO2 emissions from vehicles. Currently, natural gas has poor diffusion in the transport sector, and this paper highlights the motivations for favouring a policy aimed at increasing the share of gaseous fuel-powered vehicles. In addition to the low environmental impact, synthetic natural gas also offers the possibility of optimising the utilisation of future resources. & 2013 Elsevier Ltd. All rights reserved.

1. Introduction Because energy sources are finite and worldwide consumption of these sources is quickly increasing, a reduction in the availability of fossil fuels will be a challenge in the near future, in particular for developing countries. To delay the point at which the energy demand will surpass the availability of energy sources, three steps are necessary:

(1) reduction of energy consumption by discouraging the production of rubbish goods; (2) optimisation of productive processes. The concept of optimising all phases of production from raw materials to goods, including their disposal, must have a wide application; (3) utilisation of new energy sources. Among these, renewable sources, in particular, must be supported. The availability of these types of sources is limited by solar radiation, which can be used directly (photovoltaic and thermal) or indirectly (biomass, hydraulic and wind). Because solar energy use implies land utilisation, competition with land used for food

n

Corresponding author. Tel.: þ39 81 71 77 212; fax: þ 39 81 239 60 97. E-mail addresses: [email protected] (L. De Simio), [email protected] (M. Gambino), [email protected] (S. Iannaccone). 0967-070X/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tranpol.2013.01.006

production must be taken into account. This problem will be more constraining as the world population increases in the future. Based on the factors listed above, selecting the best way to use land for energy and other human needs, together with the optimal utilisation of residual fossil sources, is mandatory. For example, coal could represent an important ‘‘clean’’ energy source in the medium-term through its gasification in syngas. Available technologies allow for the production of syngas (a gaseous mixture containing primarily hydrogen, carbon oxide, methane and carbon dioxide) without the usual environmental impacts caused by coal combustion. The two goals of carbon use are (1) to use large coal reserves with a low level of pollution and (2) to produce synthetic fuels for the transport sector. In fact, syngas can be upgraded to synthetic fuels at a gaseous or liquid state that have a higher energy content density and are more suitable for transport uses. Syngas can also be used to generate electricity with a high efficiency (in combined cycle plants) and a high fuel utilisation rate, provided that district heating is used (cogenerating approach). For the transport sector, the electric energy from solar, nuclear, biomass or coal sources could be stored in batteries and used directly or used to produce hydrogen that could be utilised in conventional reciprocating engines or fuel cells. In urban areas, aside from the increasing share of hybrid vehicles, an increase in

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L. De Simio et al. / Transport Policy 27 (2013) 1–10

Nomenclature BMEP BTL CCS CI CNG CTG CTL DF EGR

Brake mean effective pressure Biomass to liquid Carbon capture and storage Compression ignition Compressed natural gas Coal to gas Coal to liquid Dual fuel Exhaust gas recycling

the number of electric and fuel cell vehicles is feasible if battery performance can be improved and hydrogen utilisation problems can be solved. Therefore, electricity could become a substitute for a large quantity of energy demands for on-road urban transports. For extra-urban light-duty vehicles, and even more so for heavyduty vehicles, alternative fuels are expected to have a positive contribution. Because current transport systems are mainly powered by liquid fossil fuels, a less impactful and immediate solution in the short term could be a progressively increased production of liquid biofuels or synthetic liquid fuels from coal, biomass or natural gas (NG). In the medium to long term, the utilisation of gaseous fuels such as synthetic natural gas (SNG) and H2 in dedicated engines or fuel cells could be increased. The transition from liquid to gaseous fuels should be encouraged by environmental policymakers because of the improved well-to-wheel efficiency of gaseous synthetic fuels. This improved efficiency will achieve the best energy pathway conversion and therefore minimise land use competition (biomass fuels) and coal resource utilisation (coal fuels). In this paper, an analysis of different synthetic fuels has been performed with an exclusive focus on energy rather than an economic evaluation, which can be difficult to perform because of the poorly defined developing technologies involved, the scale factor and the poor diffusion of gaseous vehicles. Because some steps in the production process are similar for the different synthetic fuels analysed, the production costs do not vary substantially between synthetic fuels. 1.1. Biomass as a limited source With thermochemical or biological processes, it is possible to produce biofuels (liquid and gaseous) from wastes and other types of biomass for transport uses. Biofuel production methodologies are usually categorised as first or second generation. For the production of first generation biofuels, a specific feedstock composition is required, which involves a dedicated mono-crop and only uses the edible fraction of the plant rather than the whole plant. The rest of the plant can be utilised as by-products with potential economic value. Second generation biofuels, which are currently under development, are advantageous because they use lignocellulosic material (not just the edible fraction); therefore, any type of organic agricultural or forest residue can be used. In this case, a higher energy balance is obtainable. The use of wastes and biomass in thermochemical and biological processes to obtain biofuels, or heat, leads to a reduction of the greenhouse effect because of an improved CO2 balance (Evans, 2007). In fact, the net heat recovery is CO2 neutral because the amount of CO2 produced by the biofuel is the same as the amount trapped during biomass growth. The net heat is defined as the energy content of biofuels minus the fossil energy used for the biofuel production process. In the case of biomass from dedicated energy crops, it is

FD FT GHG LNG LSNG NG SCNG SI SNG THC TWC

Full diesel Fisher Tropsch Greenhouse gas Liquid natural gas Liquid synthetic natural gas Natural gas Synthetic compressed natural gas Spark ignition Synthetic natural gas Total unburned hydrocarbon Three-way catalyst

necessary to analyse and take into account the fossil energy eventually utilised for land treatment and cultivation. Biomass availability is limited by solar energy radiation and land use competition. Therefore, it is difficult to quantify the exact total amount per year. In fact, a large variability in the biomass potential (2000–100,000 Mtoe/y) has been reported in the technical literature (Moreira, 2006). The lower limit of biomass potential may be considered more realistic based on the following evaluations. Considering all suitable and available land, but excluding the land used for food production, one author has calculated the world electric energy potential per year from solar energy using photovoltaic panels (Avanzini, 2009). This author estimated a terrestrial surface suitable for the production of approximately 1.18 Gha and a total producible global electric energy of almost 300  106 GW h/y. This value was estimated assuming an efficiency of approximately 10% for solar energy radiation conversion. As a very rough estimation, the same land surface can be used to produce biomass with a realistic solar energy conversion efficiency of 1% (photosynthesis efficiency), and the amount of energy stored in biomass per year will be approximately one order of magnitude lower, i.e., 30  106 GW h/y, or almost 2500 Mtoe/y. Only a fraction of this biomass energy content will be available as biofuels (liquid or gaseous) because the remainder will be necessary for biofuel production processes. The biofuel energy potential can be calculated as follows:

Z  2500 Mtoe=y where Z, for second generation biofuels, ranges from 40% to 70%. Therefore, an estimation of the equivalent energy available for transport varies from 1000 Mtoe/y to 1750 Mtoe/y depending on the different process used. This value will be reduced because of land utilisation for other energy sources (i.e., photovoltaic, solar thermal, etc.) and for increased food production due to an increase in the human population. Considering the world energy demand for transport, as shown in Fig. 1, it is evident that biofuel from biomass potential will not be sufficient to satisfy the fuel demand. 1.2. Biofuels from biomass Biofuels from the biomass of major diffusions are generally categorised as either first or second generation. Improved global efficiency and land use competition can be obtained by studying third and fourth generation fuels. First generation biofuels utilise only a small edible fraction of vegetables belonging to specific monocultures, whereas second generation biofuels use all types of lignocellulosic materials. Therefore, by using the whole vegetable, it is possible to utilise a large variety of short rotation forest products as raw materials for the production of these biofuels. For the same yield of biofuels, first generation biofuels require more land than second generation biofuels. Therefore, this paper refers only to second generation processes.

L. De Simio et al. / Transport Policy 27 (2013) 1–10

3500 Production Efficiency, [%]

80

3000 World Energy Demand for Transport, [Mtoe/y]

3

2500 2000 1500 1000

70 60 50 40 30 20 10 0

500

Cellulosic Ethanol

Synthetic Diesel Oil

Synthetic Nat. Gas

Synthetic Hydrogen

0 Fig. 2. Production efficiency of some second-generation biofuels.

2030 80

Table 1 Second generation biofuels production from biomass. Biofuel Bioethanol

Biodiesel

Production process – Advanced hydrolysis – Fermentation – Gasification – Fisher Tropsch synthesis (from CO and H2 to oil) – Fuel conditioning (Separation, hydrocracking)

Biomethane

– Gasification – Methanation (from CO and H2 to CH4) – Fuel conditioning (H2O, CO2 removal)

Biohydrogen

– Gasification – CO water–gas shift (from CO and H2O to H2) – Fuel conditioning (purification)

The main second generation biofuels are reported in Table 1 with their associated production processes. With the exception of second generation ethanol, the production process begins with the gasification of biomass. This technology is currently under development and could benefit from further research on coal gasification plants. In Fig. 2, the efficiency of the production process from biomass is shown for the main second generation biofuels, but the agricultural process and collection methods are not included (Spath et al., 2005; Drift et al., 2005; Tampier et al., 2006). SNG, also called biomethane, shows the highest production efficiency, closely followed by synthetic hydrogen. The reported cellulosic ethanol efficiency (Fig. 2) does not include the energy contained in lignin, which is a by-product of the process. Lignin can be used in the gasification process to obtain other second generation biofuels. In this case, the global process can reach the same efficiency as that of gaseous biofuels. Similarly, for synthetic diesel oil, Boerrigter and Zwart (2004) found the production efficiency to be similar to that of SNG when these two fuels are co-produced. However, to reach the same efficiency of the SNG production process, in the cases of ethanol production with lignin conversion and co-production of BTL and SNG, a more complex and expensive production plant with a second line for lignin conversion or SNG, respectively, is necessary. Therefore, in the

Production Efficiency, [%]

Fig. 1. World energy demand for transport [IEA 2004].

80

70 60 50 40 30 20 10 0

70 60 50 40 30 20

SCNG production + S I engine efficiency

2020

C I engine efficiency

2010

Overall Efficiency (engine), [%]

1971 2002

10 0

Synthetic Diesel Oil (BTL)

Synthetic Nat. Gas (SNG)

Synthetic Diesel Oil (BTL)

Synthetic Nat. Gas (SCNG)

Fig. 3. Biomass to mechanical energy conversion efficiency.

following paragraph, the focus will be on SNG and synthetic diesel oil (also named biomass to liquid, or BTL). Among the fuels in Fig. 2 that are suitable for spark ignition (SI) engines, SNG shows the best production efficiency (45% higher than BTL), whereas BTL is the best fuel for compression ignition (CI) engines. To calculate the efficiency of the whole conversion process, from the biomass energy content to the mechanical energy at the engine shaft, some considerations are necessary. SNG must be compressed to SCNG (synthetic compressed natural gas) to increase its density for utilisation as a transport fuel, and almost 10–15% of the energy content of the produced SNG needs to be consumed. Moreover, BTL is used in CI engines, whereas SCNG is used in SI engines, which have a lower thermal efficiency. Therefore, from the production efficiency reported on the left side of Fig. 3, it is possible to evaluate the overall efficiency (from biomass to the engine shaft) reported on the right side of Fig. 3. The advantage of SNG over BTL in the production phase is significantly reduced when the final mechanical energy available at the engine shaft is considered. The calculation is performed using a global efficiency of 32% for the diesel engine and 27% for SI engine. These values are obtained as the mean values of the efficiency measured on the extra-urban phase of the European transient cycle (ETC). With these global efficiencies, the advantage of SNG over BTL (or CTL) is reduced from 45% (production phase) to 20%. This value is further reduced to approximately 10% in the case of LNG. These results imply better land use or a better utilisation of coal energy for the production of SCNG compared with that of BTL. This comparison is made by considering the

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L. De Simio et al. / Transport Policy 27 (2013) 1–10

induced revertants / kWh

7000 6000 5000 4000 3000 2000 1000 Diesel CNG

TA 1

00

+S

9

TA 1

00

9 TA 98 +S

TA 98

0

Fig. 4. Mutagenicity of exhaust emissions from D (diesel), B20 (20% biodiesel blend), and NG engine: induced revertants/kW h in S. typhimurium strains.

thermal efficiency of CI and SI engines based on data from past experiments conducted on heavy-duty engines at the Istituto Motori. However, the data do not take into account the efficiency improvements that are achievable with developing technologies, which will be more effective for SI engines than for CI engines. 1.3. Coal and synthetic fuels Based on the trend of conventional fossil fuel availability and taking into consideration previous evaluations on the availability of biofuels, it is expected that in the medium to long term, most synthetic fuels will be obtained from coal. In fact, coal will once again become an important source because it can be used with lower emissions than in the past to produce syngas. Moreover, coal gasification plants also constitute the base model for future biomass gasification plants because coal composition is more homogeneous than biomass and therefore is easier to treat to obtain syngas in a gasifier (Prins et al., 2007). The energy consumed in the syngas production process can be justified by the possibility of utilising coal with a lower environmental impact than direct coal burning. Every step that consumes energy and, in turn, reduces the output of the system (power or fuel) or increases the consumption of resources will be referred to hereafter as an energetic cost, which takes into account the amount of energy consumed in a step. Although coal produces the highest CO2 quantities per energy unit among fossil fuels, the gasification process can offer a possible partial solution to the CO2 emission problem. In fact, in a syngas production plant, CO2 capture and storage (CCS, e.g., separating and compressing the gas in geologic cavities or growing algae) can be achieved even if a further energetic cost is necessary because a share of syngas will be consumed to produce energy for CCS. From syngas, in particular synthetic diesel oil (also named CTL, coal to liquid), synthetic natural gas, also named CTG, coal to gas) and pure hydrogen can be obtained. The utilisation of CTL and SNG in transport implies associated CO2 emissions into the atmosphere due to the carbon content of the synthetic fuels. Therefore, only in the case of hydrogen production from coal with CCS can the net CO2 production be zero. In any case, without CCS technology, the use of coals to produce hydrogen or non-carbon-free fuels implies an increase in greenhouse gas (GHG) emissions in the atmosphere. Some

authors calculate that in the case of CTL without CCS, well-towheel GHG emissions will increase by 110% over those from fossil diesel oil. Even considering CCS, emissions from CTL were found to be 25% higher, mostly due to GHG emissions from mining (Vliet et al., 2009). At the parity of energy of the synthetic fuel produced, if the conversion efficiency decreases, a higher amount of CO2 must be captured, and a greater quantity of coal is consumed. Furthermore, the higher the carbon content in the synthetic fuel, the higher the CO2 emission of the vehicle. Therefore, the use of synthetic fuels with the best energy efficiency for production and on-road use should be encouraged. Comparing CTL with SCNG obtained from coal, as already seen when using biomass as the feedstock, SCNG production and the use of SCNG as a transport fuel yield the highest efficiency together and produce a lower carbon dioxide emission from the vehicle. The best solution for CO2 emissions reduction is to produce H2 from coal using CCS. In fact, the efficiency of the production process is as high as that of SNG, and the vehicle CO2 emission is zero. In the long term, if the number of gas-powered vehicles increases, then it may be more possible to have a large diffusion of H2 or H2/SCNG blends in transport.

2. Experimental section 2.1. Natural gas technologies in transports The main gaseous fuels obtainable from biomass or coal are hydrogen and SNG. Hydrogen can be used in thermal engines or in fuel cells. The structural and safety problems of the hydrogen network distribution and on-board storage restrict its use in prototypes. Instead, natural gas technology is mature in transportation and allows for low pollution, high efficiency and reliable engines. In Fig. 4, the toxicity from an EURO II spark ignition NG stoichiometric engine for buses that uses a three-way catalyst (TWC) is shown in comparison with that of an equivalent diesel engine, EURO II (Baldassarri et al., 2006). The data are collected from experiments conducted at the Istituto Motori on a steady state R49 cycle similar to the current European stationary cycle test approval for HD diesel engines. A very low exhaust toxicity was detected after many tests on bacteria strains. Currently, a heavy-duty EURO V diesel engine (with electronic control of the high pressure injection) can reach almost the same low levels of toxicity. This result is possible because of technologies that have been applied to modern diesel engines equipped with particulate filters (to eliminate particulate matter) and oxidation catalysts (to oxidise unburned hydrocarbons and carbon oxide). Moreover, to decrease nitrogen oxide emissions (NOx) to the levels of a stoichiometric SI NG engine with a three-way catalyst, a selective catalytic converter should be added to the diesel exhaust aftertreatment system. However, all these steps that are implemented to limit diesel engine pollution also worsen the fuel consumption. Because the environmental impact of NG engines was already determined to be low several years ago, the research has focused on increasing the SI engine efficiency (i.e., downsizing, variable valve actuation, direct injection, exhaust gas recycling, and cylinder deactivation). Another possibility that might produce a higher thermal efficiency than that of a typical SI engine is offered by dual fuel (DF) technology, which allows a CI engine to be fed mainly with a gaseous fuel, as described in Section 2.3. The above considerations on future developments of diesel and gas engines could slightly change the relative values shown in Fig. 3, where the current engines’ thermal efficiencies have been estimated to calculate the biomass to mechanical energy conversion efficiency. In particular, the advantage of SCNG compared to

L. De Simio et al. / Transport Policy 27 (2013) 1–10

5

Table 2 Main properties of NG, CNG, LNG and diesel oil.

Density [kg/l] Pressure [bar] Him [MJ/kg] Hiv [MJ/l] Flammable limits [%] Stoichiometric ratio [kg/kg] Fuel litres equivalent to 1 l of diesel oil [l/ldiesel equivalent] n

NGn

CNGn

LNGn

DIESEL

0.0007 (15 1C, 1 bar) 1 49 0.035 5–15 17.2 ffi 1000

0.180 (15 1C, 240 bar) 240 49 8.8 5–15 17.2 3.8

0.423 ( 160 1C, 1 bar) 1 49 20.7 5–15 17.2 1.6

0.80 1 42 33.6 1–6 14.7 1

Assumed mainly constituted by methane.

5

25 CNG

Weight of tank Weight of fuel

Overall Efficiency (engine), [%]

[kg/ldiesel equivalent ]

4 3

LNG

2

Diesel

1 0 Steel

Steel & Fibre Aluminum & Composite Cryogenic Tank Fibre

Fig. 5. Tank comparison in terms of the equivalent energy basis of a litre of diesel oil transported.

20

15

10

5

1350

0

1200 1050 900

[l]

Synthetic Diesel Oil (BTL)

CNG LNG Diesel

750

Synthetic Nat. Gas (SCNG)

Liquefied Synt. Nat. Gas (LSNG)

600 450

Fig. 7. Biomass to mechanical energy conversion efficiency.

300

240

150 200

225

250

275

300

[ldiesel equivalent] Fig. 6. Tank comparison in terms of size per litre of diesel equivalent transported.

BTL in terms of the overall efficiency from biomass to the engine shaft could increase. 2.2. Liquefied synthetic natural gas for transports

Curva saturazione LNG di saturation line

180

Temperature, [°C]

175

B

120

CNGIniezione direct injection GNC: diretta

Compressione Compression

60

LNGIniezione direct injection GNL: diretta

A

0

20

C

50

100

150

200

Cylinder pressure, [bar]

-60

Riscaldamento LNG Heating del GNL

-120

A' B' LNGCompressione Compression del GNL

-180

The negligible environmental impact of gas-powered vehicles and their low autonomy range determine the diffusion of NG vehicles in urban applications. Liquefied gaseous fuel could overcome the problem of autonomy and therefore allow the diffusion of SNG in extra-urban uses. At ambient pressure, NG is liquefied at a temperature of almost 160 1C. In Table 2, the main characteristics of methane and diesel oil are compared. Liquid natural gas (LNG) reduces the gap between the gaseous fuel and the diesel oil storage by less than half in terms of the equivalent energy basis of a litre of diesel oil. The possibility of storing natural gas in liquid form reduces the weight (Fig. 5) and size (Fig. 6) of the tank, which becomes comparable to the tank size used for diesel fuel. The energetic cost of liquefying NG is almost 10–30% of the SNG energetic content compared with 10–15% of that for

0

50

100

150

200

250

300

350

400

450

Density, [kg/m3] Fig. 8. Compression process of methane in the temperature–density diagram.

compressing SNG (SCNG) at 200 bar (Wegrzyn et al., 1998). Therefore, a slightly lower overall energetic efficiency from biomass (Fig. 7) is expected, but the autonomy range is almost two times the range of SCNG (last row of Table 2). The qualitative trend shown in Fig. 7 is also valid for synthetic fuels from coal. The overall efficiency for LSNG in Fig. 7 has been calculated considering the liquefying energetic cost for an SNG (mainly consisting of methane). However, if the liquefaction process (to produce LSNG) is integrated into SNG production, substituting

6

L. De Simio et al. / Transport Policy 27 (2013) 1–10

240

B'

200

[ba r]

Curva saturazioneline LNG di saturation NGC: diretta CNGIniezione direct injection

su r e,

150

100

60 A

50

LNG: diretta LNGIniezione direct injection

pr es

120

LNGCompressione del GNL Compression

20

A'

0 0

50

100

Table 3 Main characteristics of IVECO 8360.46R Diesel EURO II engine in FD and DF mode. 6 cylinder in-line turbocharged

Compressione Compression

Cy lin de r

Pressure, [bar]

180

Riscaldamento LNG Heating del GNL

C

B

150

200

Density,

250

300

350

400

450

Displacement Bore  stroke Compression ratio Rated power Maximum torque Specific power Fuel system After treatment EGR system Throttle valve

7.8 l 112  130 mm 17.6:1 166 kW @ 2050 rpm 965 N m @ 1250 rpm 21 kW/l In line mechanical pump (210 bar) Three-way catalyst (in DF mode) High pressure route (in DF mode) Operating only in DF mode

[kg/m3]

Fig. 9. Compression process of methane in the pressure–density diagram. Table 4 Main characteristics of IVECO CURSOR 8 CNG spark ignition EURO V engine.

the fuel conditioning step (H2O, CO2 and N2 removal) could make the overall efficiency slightly higher. Moreover, a high pressure direct injection system could be developed with SNG in the liquid phase. The liquid phase makes it possible to utilise a cryogenic pump for high injection pressure, which consumes less energy when operating on a liquid compared with a gas. A low temperature increases the fuel density, reducing the injection time at the parity of the engine requirement. The temperature–density and pressure–density diagrams of methane are reported in Figs. 8 and 9, in which it is possible to visualise the evolution of the status parameters. According to these diagrams, when taking SCNG from a tank at 20 1C and at an initial pressure of 200 bar, it is possible to inject the fuel at 200 bar with a density of 130 kg/m3, as shown at point C. The density of the injected gas will decrease along the line CB depending on the residual pressure in the tank. For instance, starting from an SCNG tank at 50 bar and 20 1C (point A), a compression at 200 bar increases the temperature of the injected SNG to 120 1C (point B), with a density of approximately 100 kg/m3. In the case of LSNG, the compression up to 200 bar of the fuel in the liquid state (with lower energy consumption), line A0 B0 , is practically at a constant temperature (the storage temperature is about  160 1C) and results in a supercritical fluid at high density. This fluid can be injected at ambient temperature and, therefore, at a density of approximately 130 kg/m3 by heating it from B0 to C (or at a lower temperature and a higher density, by controlling the heating process). The heat required to raise the supercritical fluid temperature (B0 C) can be managed by the engine coolant and interrupted at any point. If the operating conditions make it convenient to inject SNG at a temperature below ambient temperature (with a consequently high supercritical fuel density), then it is possible to do so with this system. By achieving direct injection at a high pressure and density, it is possible to utilise gaseous engines with more complex injection strategies to better control combustion and emissions, increasing both the efficiency and the specific engine power. Moreover, direct injection at closed inlet valves allows for better control of THC emissions because unburned mixture loss can be greatly reduced during overlap, and the volumetric efficiency is no longer limited by the fuel gaseous state, which occurs in the case of port injection. 2.3. The dual fuel approach Dual fuel technology is used to transform a conventional diesel engine to operate with a share of gaseous fuel while retaining the capability to run in full diesel (FD) mode. This technology is not common and mostly used in after-market applications. These

6 cylinder in-line turbocharged Displacement Bore  stroke Compression ratio Rated power Maximum torque Specific power Control system After treatment

7.8 l 115  125 mm 11.0:1 200 kW @ 2100 rpm 1100 N m @ 1250–1650 rpm 26 kW/l Timed multipoint injection Three-way catalyst

after-market applications often aim for cost reduction rather than emission control. In a DF engine, an air/gaseous fuel mixture is ignited by means of a pilot diesel oil injection. The diesel fuel ignites similarly to in a compression ignition engine. The combustion then propagates by means of different flame fronts in an almost homogeneous air/ gaseous fuel mixture. At light load, the lean mixture does not allow a quick flame front propagation (Karim et al., 1993), and a high percentage of gaseous fuel does not burn. In contrast, at high load, knock can occur, caused by the auto ignition of the ‘‘end gas’’ by high combustion temperatures and high levels of substitution of diesel oil with gas (Kubesh, 1992). The advantage of DF technology is the possibility of using a gaseous fuel and achieving an overall efficiency typical of a diesel engine, higher than that of an SI engine, and with lower pollutant emission than that of a diesel engine, especially for particulate matter emission. To obtain these results, different devices, such as a throttle valve, an exhaust gas recycling (EGR) system and a three-way catalyst, can be employed and optimised (De Simio et al., 2007). For extra urban transport, the possibility of using a dual fuel (DF) CI engine that uses both diesel oil and gaseous fuels could be an alternative to an SI full gaseous engine. The data from the experiments conducted at the Istituto Motori on two different commercial, six-cylinder, heavy-duty engines for buses are reported below. The two engines are a diesel EURO II and an NG SI EURO V, whose main characteristics are reported in Tables 3 and 4, respectively. Both the engines were equipped with a three-way catalyst and were tested in stationary conditions. A fossil NG with 86% methane content by volume was used. The EURO II diesel engine was fuelled in FD mode, and it was modified at the Istituto Motori to operate in DF mode with the aim of reducing the pollutants from the engine while retaining a high efficiency in FD mode. To optimise the DF operation, a motorised throttle valve and an EGR system were added to the diesel engine. In FD mode, the throttle valve was set to the wide

L. De Simio et al. / Transport Policy 27 (2013) 1–10

1.0 0.9

NOx, [g/kWh]

BMEP BMEPmax

0.8 0.7 0.6

DF FD - EURO II (+ oxi-cat) NG - EURO V

0.5 0.4 0.3 0.2 0.1 0.5

0.6

0.7

0.8

0.9

10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0.0

1.0

0.4

0.5

0.6

NG - EURO V

0.2

0.3

0.4

0.7

0.8

0.9

1.0

1500 1400 1300 1200 1100 1000 900 800 700 600 500

0.6

0.7

0.8

0.9

1.0

DF FD - EURO II (+ oxi-cat) NG - EURO V

0.1

0.2

0.3

0.4

BMEP BMEPmax

0.5

0.6

0.7

0.8

0.9

1.0

BMEP BMEPmax

Fig. 11. THC emission for DF, FD and full NG engines measured in the condition of Fig. 10.

Fig. 14. CO2 emission for DF, FD and full NG engines measured in the condition of Fig. 10.

7.0

45

Thermal Efficiency, [%]

DF

6.0

FD - EURO II (+ oxi-cat)

CO, [g/kWh]

0.5

Fig. 13. NOx emission for DF, FD and full NG engines measured in the condition of Fig. 10.

CO2, [g/kWh]

THC, [g/kWh]

DF FD - EURO II (+ oxi-cat) NG - EURO V

0.3

FD - EURO II (+ oxi-cat)

BMEP BMEPmax

Fig. 10. Tested points in FD, DF and full NG on the normalized engine map of the two engines.

0.2

DF

0.1

SPEED SPEEDmax

1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 0.1

7

5.0

NG - EURO V

4.0 3.0 2.0 1.0 0.0

40 35 30 DF

25

FD - EURO II (+ oxi-cat)

20

NG - EURO V

15 0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

BMEP BMEPmax

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

BMEP BMEPmax

Fig. 12. CO emission for DF, FD and full NG engines measured in the condition of Fig. 10.

Fig. 15. Thermal efficiency for DF, FD and full NG engines measured in the condition of Fig. 10.

open position, and no exhaust gas was recycled. The TWC had only an oxidant effect (oxi-cat) on THC and CO emissions, and it did not affect NOx emissions because of the high oxygen concentration in the exhaust gas. In DF mode, the throttle valve and the EGR rate were optimised to have a close stoichiometric air/fuel mixture to allow NOx reduction into the TWC. The comparison between the spark ignition EURO V heavyduty engine and the old EURO II heavy-duty diesel engine, operating in FD and in DF modes, highlights the potential of DF

technology. In fact, a EURO V spark ignition natural gas engine, stoichiometric with TWC, represents the best technology for emission abatement. A diesel engine is the best technology for achieving maximum efficiency in EURO V and EURO II engines. Euro standards have a strong influence on emissions but have little effect on the overall engine efficiency. The test points were chosen throughout the whole operating range of the SI engine and of the diesel engine in FD mode. In DF mode, the engine working area did not include very low or

L. De Simio et al. / Transport Policy 27 (2013) 1–10

NG %, [% mas.]

8

100 98 96 94 92 90 88 86 84 82 80

Table 5 Indicative production costs of FT-diesel and SNG from biomass.

DF

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

BMEP BMEPmax

Fuel

Plant size MWof biomass

Biomass Production (LHV 17 MJ/kg) costs $/GJ t/day

Reference

BTL BTL BTL BTL BTL BTL SNG

20 50 100 400 4000 10000 20

100 250 500 2000 20000 50000 100

30–40 ffi 31 20–23 ffi 21 ffi 16 15–16 20–30

SNG SNG

100 150

500 750

ffi 10 15–27

Zhang (2010) Swain et al. (2011) Zhang (2010) Sarkar et al. (2011) Swain et al. (2011) Zhang (2010) Gassner and Mare´chal (2009) Boerrigter et al. (2004) Gassner and Mare´chal (2009)

Fig. 16. Natural gas fraction for DF engines measured in the condition of Fig. 10.

maximum loads because of the high total unburned hydrocarbon (THC) and the knock risk, respectively. Because the two engines have slightly different maximum speeds and maximum brake mean effective pressure (BMEP), in Fig. 10, the tested points are normalised with the rated speed (SPEED/SPEEDmax) and the rated load (BMEP/BMEPmax) of each engine. In the figure, the DF working area (hatched area) is clearly smaller than the diesel working area (dotted area). In Figs. 11–15, the gaseous emissions downstream of the three-way catalyst and the thermal efficiency of the three tested engine configurations are shown. The experimental values were measured at the same speed and load conditions in Fig. 10 and are reported vs. the BMEP/BMEPmax. This choice is justified by the fact that specific emissions and the thermal efficiency are more influenced by load than by speed. For instance, with reference to the NG engine, the three tested points at low load (0.1 to 0.2 BMEP/BMEPmax, Fig. 10) show CO2 emissions in a narrow range (approximately 800–870 g/kW h, Fig. 14) and a thermal efficiency varying from 22 to 24% (Fig. 15). In addition, the THC and CO emissions at these three points were slightly influenced by the speed in the range of 0.1 to 0.2 g/kW h (Fig. 11) and in the range of 0.25–0.75 g/kW h (Fig. 12), respectively. The NOx emissions showed a higher variation because of the engine control strategy, which affects the air/fuel ratio and the TWC conversion efficiency. The above considerations are the same if other groups of test points with a similar BMEP/BMEPmax and a different speed are examined. In Fig. 11, the THC emissions are reported. The lowest values were obtained with the TWC SI engine, except with a few conditions. In DF, the THC resulted in the same order of magnitude as that of the SI engine, especially at high loads, which provide better combustion conditions. As shown in Fig. 12, the CO emissions in DF were close to zero, whereas those of the SI engine were sometimes very high. The fact that an SI NG EURO 5 engine emits high THC and CO in some conditions is because of the control strategy used to optimise the TWC NOx conversion efficiency. For this reason, the NOx emissions in Fig. 13 are generally close to zero for the SI NG engine. The potential of the DF engine can be appreciated at high load (approximately 0.85 BMEP/BMEPmax), where it is possible to obtain very low NOx emissions (Fig. 13) together with low CO (Fig. 12). This result does not occur at decreasing load for a leaner mixture, which reduces the NOx conversion efficiency of the TWC. Developing technology, such as cylinder deactivation, could increase the load in active cylinders (operating close to a stoichiometric mixture), and switching off the passive cylinders could represent a valid solution for controlling NOx emissions at partial load.

At the same engine thermal efficiency, NG, which is mainly composed of methane, allows for approximately 25% lower CO2 emissions than diesel oil. In Fig. 14, the SI NG engine shows lower CO2 emissions than FD, despite the lower thermal efficiency reported in Fig. 15. In addition, the DF CO2 emissions are lower than those for FD but are slightly higher than those for the SI engine, despite the higher efficiency of the DF (Fig. 15), because 10 to 15% of the total fuel flow rate in DF mode is diesel oil (Fig. 16). DF has the potential to use NG or other gaseous fuels as the main fuel in engines with a higher thermal efficiency compared with SI. For NOx control and thermal efficiency, the higher the load, the greater the achievable benefits. In the discussion of Figs. 11–13, no emphasis was placed on the data for FD mode because the tested engine is not representative of modern heavy-duty diesel engines equipped with electronically controlled high pressure injection, an oxidant catalyst (for CO and THC), a particulate filter (for PM), an EGR and sometimes a selective catalytic reduction system for NOx. The very low CO and THC concentrations measured in FD at the Istituto Motori were obtained due to the presence of the TWC (acting only as an oxidant with lean mixtures) and are lower than those of EURO II diesel engines, which do not have an oxidation catalyst. The high NOx emissions are typical of a EURO II diesel engine and are not influenced by the presence of the TWC because TWC cannot reduce NOx when the exhaust gas contains oxygen. Instead, the efficiency of the EURO II diesel engine (Fig. 15) has been emphasised because a EURO V diesel engine shows a similar overall efficiency. In fact, even if the injection technologies of modern diesel engines are able to decrease fuel consumption, the strategies employed to control emissions will limit this improvement. Therefore, limited by its efficiency, the EURO II diesel engine was selected as the reference for DF performance analysis.

3. Discussion of costs The two most promising fuels for internal combustion engines are Fisher Tropsch (FT) diesel and synthetic natural gas. There are no great differences in the plants used to produce these fuels from biomass and/or coal. For both production processes, it is necessary to produce syngas through a gasification process of biomass or coal. The syngas then undergoes catalytic processes to obtain methane, in the case of SNG, or larger hydrocarbon chain molecules, in the case of FT-diesel fuel. The main difference is the higher energetic consumption required to obtain larger molecules starting from a gas mixture of CO and H2 (syngas). The production costs are similar for SNG and synthetic diesel, and these costs strongly depend on the scale of the production plant

L. De Simio et al. / Transport Policy 27 (2013) 1–10

Comventional fuel prices, $/GJ

25

20

Crude oil NG

15

10

5

0 1994 1996 1998 2000 2002 2004 2006 2008 2010 2012 2014 Fig. 17. Prices of crude oil and NG.

Sunde et al. (2011). In Table 5, the indicative production costs of FT-diesel and SNG from biomass are reported. As an estimate, small plants with an input of 20–50 MW of biomass yield a fuel cost production in the range of 20–40 $/GJ, whereas with large plants, the cost decreases to 10–20 $/GJ. The lower values of these ranges are for SNG production. This result is in agreement with the higher amount of synthetic fuel produced in the case of SNG compared with FT-diesel. In fact, using the same feedstock and similar plant costs, a higher synthetic fuel production reduces the cost per unit of energy. To reduce the production costs of synthetic fuels, electricity and fuel could be co-produced (Williams et al., 2009, Mantripragada and Rubin (2011)), even if this reduces the overall yield of fuel produced. Lower costs could be obtained if coal is used instead of biomass because of the lower feedstock price (in the range of 2–5 $/GJ for biomass and 1–2 $/GJ for coal (Ren et al., 2009)). However, in the case of coal, the additional cost of CCS to reduce the CO2 emission with respect to the fossil source could counterbalance the lower feedstock price. The production cost of conventional diesel is almost 20% higher than that of crude oil because of the refining process (http://www.eia.gov), whereas CNG requires an additional cost of approximately 1–1.2 $/GJ (Johnson, 2010) for the compression of NG. Therefore, the current diesel fuel production cost is in the range of 15–20 $/GJ, whereas that of CNG is in the range of 2.5– 4 $/GJ. The cost of conventional diesel fuel is approximately two times lower than the production cost of synthetic fuel (both FT and SNG) from a small scale plant. However, if a large scale plant is considered, then synthetic fuels can compete with fossil diesel oil. Large production plants that require a high availability of feedstock are more feasible if fed with coal than with biomass because of the greater land areas required for the collection of biomass Fig. 17.

4. Conclusions In the future, the challenges posed by fossil fuel availability and global warming will continue to grow. Pure electric vehicles can reduce the local environmental impact, CO2 and fossil fuel consumption if the production of electricity is associated with carbon-free sources. Pure electric vehicles are especially suitable for urban applications because of the short distances that must be covered by vehicles. For extra-urban transport, synthetic fuels will be necessary to compensate for fossil fuel depletion. Synthetic fuels can be produced from biomass and coal. Competing for land and exploiting solar radiation limit the annual biomass

9

potential for the energy demand for transportation. However, biofuels from biomass reduce CO2 emissions. Synthetic fuels from coal have great potential, but the problems related to CO2 will persist. Furthermore, the capture and storage technology of CO2 can be applied to synthetic fuel production plants but not to vehicle emissions. Only in the case of H2 from coal can CO2 emissions be theoretically captured and stored. The production of synthetic gaseous fuels is almost 45% more efficient with respect to synthetic liquid fuels because the production process of a liquid fuel is more complex. However, the advantage of 45% more efficiency is reduced to approximately 20% after considering both the compression of SNG in cylinders and the lower thermal efficiency of an SI engine fuelled with SNG compared with a CI engine fuelled with BTL. Therefore, producing SCNG instead of BTL requires better coal utilisation or land use (if biomass is used). The target of future research is to increase this 20% efficiency. Among the different options to increase the efficiency of gaseous fuels is dual fuel technology, which uses up to 90% of the gaseous fuel in a CI engine instead of an SI engine. Although synthetic gaseous fuels yield the highest overall efficiencies, mass production of these fuels is not justified because of the poor diffusion of gas vehicles. There is a need for an appropriate environmental policy that encourages the diffusion of gas vehicles and exploits the advantages of synthetic gaseous fuels. References Avanzini, P., 2009. Scenari energetici per il XXI secolo. In: Bertoli, C. (Ed.), Energia e Trasporti—Stato Attuale e Prospettive Future Della Ricerca Scientifica. CNR Dipartimento Energia e Trasporti, pp. 3–30. Baldassarri, L., Battistelli, C.L., Conti, L., Crebelli, R., De Berardis, B., Gambino, M., Iamiceli, A.L., Iannaccone, S., 2006. Evaluation of emission toxicity of urban bus engines: compressed natural gas and comparison with liquid fuels. Science of the Total Environment 355 (1-3), 64–77. Boerrigter, H., Zwart, R.W.R., 2004. High efficiency co-production of Fischer– Tropsch transportation fuel and substitute Natural Gas from biomass. ECN Biomass. Boerrigter, H., Deurwaarder, E.P., Kersten, S.R.A., Mozaffarian, M., Zwart, R.W.R. (2004). ‘‘Green gas ’’ as SNG (synthetic natural gas) a renewable fuel with conventional quality. In: Science in Thermal and Chemical Biomass Conversion Conference, 2 September 2004, Victoria, Vancouver Island, BC, Canada. De Simio, L., Gambino, M., Iannaccone, S., 2007. Low-polluting, High-efficiency, Mixed Fuel/Natural Gas Engine for Transport Application, in Urban Transport and the Environment in the 21st Century. WIT Press (Southampton), pp. 493–502. Drift A., Meijden C.M., Boerrigter H., (2005). Milena gasification technology for high efficient SNG production from biomass, ECN Biomass 2005. In: 14th European Biomass Conference & Exhibition, Paris France, 17–21 October 2005. Evans, R.L., 2007. Fuelling Our Future. Cambridge University Press. Gassner, M., Mare´chal, F., 2009. Thermo-economic process model for thermochemical production of synthetic natural gas (SNG) from lignocellulosic biomass. Biomass and Bioenergy 33 (11), 1587–1604. Johnson, C. (2010). Business Case for Compressed Natural Gas in Municipal Fleets Business Case for Compressed Natural Gas in Municipal Fleets, Technical Report NREL/TP-7A2-47919. Karim G., Liu A.Z., Jones W. (1993), Exhaust Emissions from Dual Fuel Engines at Light Load, SAE Paper 932822. Kubesh, J. (1992), Analysis of Knock in a Dual-Fuel Engine, SAE Paper 922367. Mantripragada, H.C., Rubin, E.S., 2011. Techno-economic evaluation of coal-toliquids (CTL) plants with carbon capture and sequestration. Energy Policy 39 (5), 2808–2816. Moreira, R.J., 2006. Global biomass energy potential. Mitigation and Adaptation Strategies for Global Change 11 (2), 313–333. Prins, Mark J., Ptasinski, Krzysztof J., Janssen, Frans J.J.G., 2007. From coal to biomass gasification: comparison of thermodynamic efficiency. Energy 32, 1248–1259. ¨ B., Patel, M.K., Blok, K., 2009. Petrochemicals from oil, natural gas, Ren, T., Daniels, coal and biomass: production costs in 2030–2050. Resources, Conservation and Recycling 53 (12), 653–663. Sarkar, S., Kumar, A., Sultana, A., 2011. Biofuels and biochemicals production from forest biomass in Western Canada. Energy 36 (10), 6251–6262. Spath P., Aden A., Eggeman T., Ringer M., Wallace B., Jechura J., (2005). Biomass to Hydrogen Production Detailed Design and Economics Utilizing the Battelle Columbus Laboratory Indirectly-Heated Gasifier, Technical Report Biomass to Hydrogen NREL/TP-510-37408 Production. Sunde, K., Brekke, A., Solberg, B., 2011. Environmental impacts and costs of woody biomass-to-liquid (BTL) production and use—a review. Forest Policy and Economics 13 (8), 591–602.

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Swain, P.K., Das, L.M., Naik, S.N., 2011. Biomass to liquid: a prospective challenge to research and development in 21st century. Renewable and Sustainable Energy Reviews 15 (9), 4917–4933. Tampier M., Beauchemin P. A., Smith D., Bibeau E., (2006), Identifying Environmentally Preferable Uses For Biomass Resources, British Columbia Ministry of Forests and Range, Ministry of Energy, Mines & Petroleum.U.S. Energy Information Administration. /http://www.eia.govS. Vliet, O.P.R., Faaij, A.P.C., Turkenburg, W.C., 2009. Fischer–Tropsch diesel production in a well-to-wheel perspective: a carbon, energy flow and cost analysis. Energy Conversion and Management 50, 855–876.

Wegrzyn E., Litzke W., Gurevich M., 1998. DOEIBNL Liquid Natural Gas Heavy Vehicle Program, SAE Technical Paper 981919. Williams, R.H., Larson, E.D., Liu, G., Kreutz, T.G., 2009. Fischer–Tropsch fuels from coal and biomass: strategic advantages of once-through (‘‘polygeneration’’) configurations. Energy Procedia 1 (1), 4379–4386. Zhang, W., 2010. Automotive fuels from biomass via gasification. Fuel Processing Technology 91 (8), 866–876.