**3. Natural gas**

2 Will-be-set-by-IN-TECH

*<sup>φ</sup>* <sup>=</sup> <sup>1</sup>

Equation 5 defines the indicated mean effective pressure (imep), an engine parameter which evaluates the work obtained by an engine cycle, *p dV*, divided by the engine displacement. The Coefficient of Variation of imep, COV*imep*, is the ratio of the standard deviation of the indicated mean effective pressure and the average imep over a representative number of

> *Vd*

*COVimep* <sup>=</sup> *<sup>σ</sup>imep*

In case the effect of mechanical efficiency has to be taken into account, the brake mean effective pressure (bmep) is considered. In 4-stroke engines, the bmep is calculated from the torque

*bmep* <sup>=</sup> *<sup>T</sup>* · <sup>4</sup>*<sup>π</sup>*

where *α* + *β* = 1. The quantities *α* and *β* represent the mole per each species in the blend, and it is immediate to observe that the reduction of the C/H ratio, compared to pure methane,

The burning velocity represents a main property for the combustion characteristics of the fuels and is defined as the velocity at which unburned gases move through the combustion wave in the direction normal to the wave surface (Glassman & Yetter, 2008). The laminar burning velocities can be obtained using the following equation 9 (Mandilas et al., 2007) being S*<sup>s</sup>* the unstretched flame speed, *ρ<sup>b</sup>* and *ρ<sup>u</sup>* the burned and unburned gas densities. Equation 10 relates the unstretched flame speed, the stretched flame speed S*n*, the stretch rate *κ* and the

*ul* = *Ss*

The stretch rate *κ* is calculated from the position of the flame front, *R* = *R*(*t*), with the

The Markstein length characterizes the variation in the local flame speed due to the influence of external stretching and determines the flame instability with respect to preferential

*<sup>κ</sup>* <sup>=</sup> <sup>1</sup> *R dR*

*ρb ρu*

*Ss* − *Sn* = *κLb* (10)

*dt* (11)

The stoichiometric reaction equation of a methane-hydrogen blend reads as:

*Vd*

(*O*<sup>2</sup> + 3.76 *N*2) → *αCO*<sup>2</sup> + (2 *α* + *β*) *H*2*O* +

*imepavg*

*imep* <sup>=</sup> <sup>1</sup>

measured at the engine shaft, according to equation 7:

 <sup>2</sup> *<sup>α</sup>* <sup>+</sup> *<sup>β</sup>* 2 

brings about a theoretical reduction of the CO2.

cycles, equation 6.

(*α CH*<sup>4</sup> + *β H*2) +

Markstein length L*b*.

following equation 11 (Chen, 2009):

diffusion (Markstein, 1964).

*<sup>λ</sup>* (4)

*p dV* (5)

 <sup>2</sup> *<sup>α</sup>* <sup>+</sup> *<sup>β</sup>* 2  (6)

(7)

3.76 *N*<sup>2</sup> (8)

(9)

The main natural gas constituent is methane and the composition is strictly dependent on the origin gas field. Table 1 shows the composition of a natural gas sample obtained by the Italian distribution network, determined by means of gas chromatographic analysis.

Natural gas has been widely investigated as fuel for road vehicles because of its lower impact on the environment than gasoline and more widespread resources.


Table 1. Example of natural gas composition.

Ristovski et al. (2004) performed an experimental activity on a passenger car converted to operate either on gasoline or on compressed natural gas (CNG). Fuelling the engine by CNG, both regulated (CO, NOx and HC) and unregulated emissions (PAHs and formaldehyde) were lower than gasoline.

Prati, Mariani, Torbati, Unich, Costagliola & Morrone (2011) tested a bifuel passenger car fuelled alternatively by gasoline and natural gas on a chassis dynamometer over different driving cycles, in order to evaluate the effects of fuel properties on combustion, exhaust emissions and engine efficiency. The results showed that gasoline produced CO emissions higher than NG over the real world Artemis driving cycles, as a consequence of mixture enrichment during load transients. A detailed description of the driving cycles is reported in Barlow et al. (2009). Over the type approval New European Driving Cycle (NEDC), NG involved higher HC emissions compared to gasoline as a consequence of the higher light-off temperature for the catalytic oxidation of CH4, which is the major constituent of HC when the vehicle is fuelled by NG, while there were no differences over the Artemis driving cycles which were performed after a warming up conditioning of the vehicle. NOx emissions were higher for gasoline over all the test cycles. CO2 emissions for CNG showed a reduction between 21% and 29% over the tested driving cycles as a consequence of the reduced carbon content of the fuel and the lower fuel consumption on mass basis. A 5% fuel consumption reduction, expressed in MJ/km, is observed over the NEDC for the CNG respect to gasoline, while for the Artemis the reduction ranges between 10% and 22%. The higher gasoline consumption is the consequence of the mixture enrichment during transients. Particulate emissions referred to gasoline were higher than NG ones over the NEDC and comparable over the Artemis. Particle number observed was also higher for gasoline, with the exception of the Artemis Motorway.

of electrolysers the electrolyte is an ion conducting membrane that allows H<sup>+</sup> ions to be transported from the anode to the cathode side to recombine forming hydrogen. They are known as Proton Exchange Membrane (PEM) electrolysers (Barbir, 2005). However, water electrolysis powered by renewable energy sources is not competitive considering the current

A Review of Hydrogen-Natural Gas Blend Fuels in Internal Combustion Engines 21

The nowadays most economical sources of hydrogen are coal and natural gas, with significant experience in the operation of these types of plants, which will continue to be built and operated. The fuel reforming is a process in which hydrocarbon fuels, such as natural gas, are converted into a hydrogen-rich reformate gas. A reformer accomplishes the task by thermo-chemically processing hydrocarbon feedstock in high temperature reactors with steam and/or oxygen. Effective reformers should efficiently produce pure hydrogen with low pollutants emission. The methane steam reforming global reaction is reported as an example

The reformate gas is composed of 40% − 70% hydrogen by volume and carbon monoxide, carbon dioxide, water, nitrogen and traces of other compounds. The water-gas shift conversion removes CO and increases hydrogen content. Shift step takes place at high temperatures of about 350 − 480◦*C*, followed by a low-temperature shift (180 − 250◦*C*).

Hydrogen has been recognized as an ideal energy carrier but it has not yet been widely employed in the transportation sector. The lack of an efficient storage prevents its application, in particular as fuel for transportation. Because of the low density of hydrogen at ambient conditions, it is a challenge to store enough energy on-board to allow for an acceptable vehicle range. The density can be increased by pressurizing or liquefying hydrogen. High-pressure gaseous hydrogen, up to 700 bars, is considered a potential safety hazard due to problems of material resistance. For vehicle application, cylinders are made of composite fibre due to weight considerations. Indeed, tanks add a relevant weight to the vehicle, much greater than the stored fuel, which is the 3% of the total weight (cylinder plus fuel) for a 700 bars approved

Liquid hydrogen storage requires refrigeration to a temperature of about 20 K, and the liquefaction process requires at least 15.1 MJ/kg. The on-board storage pressures for the liquid hydrogen are only slightly above the atmospheric, with typical values around 6 bars. The vessel for storing liquid hydrogen consists of several metal layers separated by highly insulating materials. The main drawback is the hydrogen boil-off from the storage caused by the need to control tank pressures by venting valves. Boil-off usually starts after a dormancy

As an alternative, even more challenging options have been proposed and investigated. Most attention is paid to storage in solid materials and especially metal hydrides. Here, hydrogen gas is fed to a tank containing a metal powder and is absorbed as hydrogen atoms in the metals crystal lattice to form a metal hydride. In metal hydrides, hydrogen can be stored with energy densities up tp 15000 MJ/m3, higher than that of liquid hydrogen, which is 8700

period and then proceeds at a level of 3% − 5% per day (Sørensen, 2005).

*CH*<sup>4</sup> + *H*2*O* → *CO* + 3 *H*<sup>2</sup> + Δ*hr* (13)

energy costs but it may become more economical in the future (Bartels et al., 2010).

in the following reaction 13.

**4.2 Hydrogen storage**

system (Sørensen, 2005).

Fig. 1. Flame speeds of methane and iso-octane versus equivalence ratio *φ* (Mandilas et al., 2007).

One of the drawbacks of the NG fuel is the laminar burning velocity lower than gasoline, as shown in Figure 1 (Mandilas et al., 2007) requiring, as a consequence, a higher spark advance.

### **4. Hydrogen production and storage**

#### **4.1 Hydrogen production**

The production of hydrogen is an important aspect since it is not present as a free chemical species in nature. Hydrogen can be produced in several ways, but reforming from fossil fuels or partial oxidation and electrolysis are the most employed from an industrial point of view.

The electrolysis consists in splitting the water molecule in hydrogen and oxygen as indicated in the next reaction equation:

$$H\_2O + \Delta h\_r \to H\_2 + \frac{1}{2} \text{ O}\_2 \tag{12}$$

If the energy for water electrolysis is provided by renewable energy sources, hydrogen production is an environmental friendly process, without green-house gas emissions. Two main types of industrial electrolysis units are used today, which differ in the type of electrolyte adopted. The first type of electrolysers is characterised by an alkaline aqueous solution of 25 − 35% in weight of potassium hydroxide (KOH) to maximise the ionic conductivity, in which the hydroxide ions (OH−) are the charge carriers (Ulleberg, 2003). In the second type of electrolysers the electrolyte is an ion conducting membrane that allows H<sup>+</sup> ions to be transported from the anode to the cathode side to recombine forming hydrogen. They are known as Proton Exchange Membrane (PEM) electrolysers (Barbir, 2005). However, water electrolysis powered by renewable energy sources is not competitive considering the current energy costs but it may become more economical in the future (Bartels et al., 2010).

The nowadays most economical sources of hydrogen are coal and natural gas, with significant experience in the operation of these types of plants, which will continue to be built and operated. The fuel reforming is a process in which hydrocarbon fuels, such as natural gas, are converted into a hydrogen-rich reformate gas. A reformer accomplishes the task by thermo-chemically processing hydrocarbon feedstock in high temperature reactors with steam and/or oxygen. Effective reformers should efficiently produce pure hydrogen with low pollutants emission. The methane steam reforming global reaction is reported as an example in the following reaction 13.

$$\rm{CH}\_4 + \rm{H}\_2\rm{O} \rightarrow \rm{CO} + \rm{3} \,\rm{H}\_2 + \Delta h\_r \tag{13}$$

The reformate gas is composed of 40% − 70% hydrogen by volume and carbon monoxide, carbon dioxide, water, nitrogen and traces of other compounds. The water-gas shift conversion removes CO and increases hydrogen content. Shift step takes place at high temperatures of about 350 − 480◦*C*, followed by a low-temperature shift (180 − 250◦*C*).

#### **4.2 Hydrogen storage**

4 Will-be-set-by-IN-TECH

Fig. 1. Flame speeds of methane and iso-octane versus equivalence ratio *φ* (Mandilas et al.,

One of the drawbacks of the NG fuel is the laminar burning velocity lower than gasoline, as shown in Figure 1 (Mandilas et al., 2007) requiring, as a consequence, a higher spark advance.

The production of hydrogen is an important aspect since it is not present as a free chemical species in nature. Hydrogen can be produced in several ways, but reforming from fossil fuels or partial oxidation and electrolysis are the most employed from an industrial point of view. The electrolysis consists in splitting the water molecule in hydrogen and oxygen as indicated

*H*2*O* + Δ*hr* → *H*<sup>2</sup> +

If the energy for water electrolysis is provided by renewable energy sources, hydrogen production is an environmental friendly process, without green-house gas emissions. Two main types of industrial electrolysis units are used today, which differ in the type of electrolyte adopted. The first type of electrolysers is characterised by an alkaline aqueous solution of 25 − 35% in weight of potassium hydroxide (KOH) to maximise the ionic conductivity, in which the hydroxide ions (OH−) are the charge carriers (Ulleberg, 2003). In the second type

1

<sup>2</sup> *<sup>O</sup>*<sup>2</sup> (12)

2007).

**4. Hydrogen production and storage**

**4.1 Hydrogen production**

in the next reaction equation:

Hydrogen has been recognized as an ideal energy carrier but it has not yet been widely employed in the transportation sector. The lack of an efficient storage prevents its application, in particular as fuel for transportation. Because of the low density of hydrogen at ambient conditions, it is a challenge to store enough energy on-board to allow for an acceptable vehicle range. The density can be increased by pressurizing or liquefying hydrogen. High-pressure gaseous hydrogen, up to 700 bars, is considered a potential safety hazard due to problems of material resistance. For vehicle application, cylinders are made of composite fibre due to weight considerations. Indeed, tanks add a relevant weight to the vehicle, much greater than the stored fuel, which is the 3% of the total weight (cylinder plus fuel) for a 700 bars approved system (Sørensen, 2005).

Liquid hydrogen storage requires refrigeration to a temperature of about 20 K, and the liquefaction process requires at least 15.1 MJ/kg. The on-board storage pressures for the liquid hydrogen are only slightly above the atmospheric, with typical values around 6 bars. The vessel for storing liquid hydrogen consists of several metal layers separated by highly insulating materials. The main drawback is the hydrogen boil-off from the storage caused by the need to control tank pressures by venting valves. Boil-off usually starts after a dormancy period and then proceeds at a level of 3% − 5% per day (Sørensen, 2005).

As an alternative, even more challenging options have been proposed and investigated. Most attention is paid to storage in solid materials and especially metal hydrides. Here, hydrogen gas is fed to a tank containing a metal powder and is absorbed as hydrogen atoms in the metals crystal lattice to form a metal hydride. In metal hydrides, hydrogen can be stored with energy densities up tp 15000 MJ/m3, higher than that of liquid hydrogen, which is 8700

**5.1 Combustion characteristics**

hydrogen-methane blends up to 100% hydrogen.

Since hydrogen laminar combustion speed is about eight times greater than methane, it provides a reduction of combustion duration when mixed with natural gas in small concentrations. Many studies have been carried out to measure the flame speed of hydrogen-methane air mixtures at different hydrogen concentrations and equivalence ratios. Ilbas et al. (2006) performed the measurements at ambient temperatures with

A Review of Hydrogen-Natural Gas Blend Fuels in Internal Combustion Engines 23

Fig. 2. Flame speed of different fuels versus equivalence ratio *φ* (Ilbas et al., 2006).

were also widened as the hydrogen content increased in the mixtures.

dependence of this property on hydrogen percentage.

Figure 2 shows the flame speed for methane and a 50% hydrogen-methane blend plotted versus the equivalence ratio. The maximum flame speed for the blend is 0.69 m/s while the maximum for methane is 0.39 m/s for an equivalence ratio *φ* = 1.1. The flammable regions

Figure 3, where the flame speed is plotted versus hydrogen content, shows the non-linear

Mandilas et al. (2007) performed experiments in a spherical stainless steel vessel at initial temperatures up to 600 K and initial pressures up to 1.5 MPa to study the effects of hydrogen addition on laminar and turbulent premixed methane-air flames. The burning velocity, u*l*, was found using equation 9. Methane can be ignited for 0.6 ≤ *φ* ≤ 1.3, with the peak burning velocity occurring at *φ* = 1.0. The addition of H2 extends the ignition limits to the range 0.5 ≤ *φ* ≤ 1.4 and increases the values of u*<sup>l</sup>* at lean equivalence ratios, while u*<sup>l</sup>* does not increase for rich equivalence ratios. The authors also compared the turbulent velocity u*tr* for methane and a blend with 30% of hydrogen. As in the laminar case, the addition of hydrogen

MJ/m<sup>3</sup> (Sørensen, 2005). The main disadvantage, however, is the weight of the storage alloys. Furthermore refuelling times are affected by absorption rates.

Other storage options are under investigation but still at prototypal stage (Bakker, 2010).
