

**Engine** 

**Simulation**

**Combustion**

The ICE was simulated as a gas turbine in this paper. The process parameters are shown in Table 7 and discussed in Section 4.7 for the reference case of hazelnut shells and olive pruning feeding. The gas turbine engine was fed with syngas at an ambient temperature (30 ◦C) that was compressed by up to 20 bar pressure before entering the turbine [39,40].


**Table 7.** Gas turbine's cycle operating conditions.

#### **4. Results and Discussion**

**3. Internal** 

In this simulation, we considered 1 MWth as the input size and the HHV of each of the four biomass wastes analyzed and the feed was fixed in this way: for the hazelnut shells, the input flow settled at the constant flow rate of 180 kg/h; for the olive pruning, the input flow settled at the constant flow rate of 170 kg/h; for the olive pomace, the input flow settled at the constant flow rate of 153 kg/h; and for wheat straw, it was settled at the constant flow rate of 179 kg/h.

In the first configuration with air, the gasification agen<sup>t</sup> considered was at the constant flow rate of 159 kg/h at 25 ◦C and 1 bar.

Focusing on the syngas composition out of the gasifier, a sensitivity study was carried out by varying:

• The gasifier operating temperature to verify the influence of gasification temperature on the syngas composition, from 785 to 870 ◦C, in case of air as oxidant agent;


$$\eta\_{CG} = \frac{M\_{\text{syn}} \cdot LHV\_{\text{syn}}}{M\_{\text{binomass}} \cdot LHV\_{\text{biomass}}},\tag{1}$$

where *Msyn* and *Mbiomass* are the mass of the produced syngas and the original biomass, respectively; *LHVsyn* and *LHVbiomass* are the LHV of the produced syngas and the original biomass, respectively.

• The steam to biomass (S/B) ratio, in the configuration of Figure 3, to study the possible improvements of the plant efficiency when more steam was delivered to the gasifier.

#### *4.1. Syngas Composition*

At the gasification temperature of 800 ◦C and with the input flow rate declared above, with air as the gasifying agent, the simulation was conducted in Aspen Plus, as shown in Figure 2. The compositions of the product syngas for each biomass waste analyzed are shown in Table 8.


**Table 8.** Composition of the syngas in %dry mole fraction.

#### *4.2. Effect of Gasification Temperature*

The syngas composition, in the stream GASRAW as defined in Figure 2, was obtained by varying the gasification temperature between 785 and 870 ◦C. The sensitivity analysis conducted for the hazelnut shells is shown in Figure 4a, for the olive pruning in Figure 4b, for olive pomace in Figure 4c, and for wheat in Figure 4d.

 **Figure 4.** *Cont*.

**Figure 4.** (**a**) Effect of gasification temperature on the syngas composition from hazelnut shells. (**b**) Effect of gasification temperature on the syngas composition from olive pruning. (**c**) Effect of gasification temperature on the syngas composition from olive pomace. (**d**) Effect of gasification temperature on the syngas composition from wheat straw.

From Figure 4a–d, it can be observed that the concentrations of CO and H2O increased with an increase in temperature, instead the concentrations of CO2 and CH4 decrease with increasing in temperature. Similar trends were reported in [55]. The endothermic reactions (3) and (6) reported in Table 4 favor their forward reaction with increasing gasification temperature and will result in an increase of the concentration of CO and H2 and a decrease of CO2 and CH4. However, the decrease of CH4 is mostly determined by the effect of steam methane reforming, which is prevalent at high temperature.

#### *4.3. Effect of ER*

The effect of ER on syngas composition was investigated. Figure 5a–d show the trend of syngas composition by varying ER from 0.2 to 0.6 and maintaining the gasification temperature at 800 ◦C.

**Figure 5.** *Cont*.

**Figure 5.** (**a**) Effect of air equivalent ratio on the syngas composition from hazelnut shells. (**b**) Effect of air equivalent ratio on the syngas composition from olive pruning. (**c**) Effect of air equivalent ratio on the syngas composition from olive pomace. (**d**) Effect of air equivalent ratio on the syngas composition from wheat straw.

The trend obtained showed good agreemen<sup>t</sup> with the results in the literature. With the increase in ER, the yields of CO2 and H2O increased, and the yields of H2 and CO decreased. In order to evaluate the thermodynamic balance into the gasifier, Figure 6 shows the gasifier heat required and the LHV of the syngas produced (stream GASRAW), using olive pruning as an example because the others showed a similar trend. The heat required and LHV decreased as the ER increased, as foreseen from the previous figures and from the increase in the oxidant. The LHV varied between 5 and 4 MJ/Nm3, while the heat demand Q varied between 257 and 185 MJ/h. In the example of olive pruning, given the similar results for the other biomass wastes, the gas yield was 1.7 Nm3/kg and the biomass inlet was 170 kg/h, so the variation of 1 MJ/Nm<sup>3</sup> of the LHV corresponded to a variation of 100 MJ/h while the Q variation was 72 MJ/h. As the LHV decreased faster than Q with the increase of the ER and a loss of LHV accounted for more than a decrease of heat demand, the optimum value was lowest at ER = 0.2, when considering the overall energy balance. Indeed, at ER = 0.2, the corresponding values of Q and

LHV were the highest (260 MJ/h and 5 MJ/Nm3, respectively), and with an increase in the ER, there was a decrease in efficiency given that the lower LHV was not compensated for by the decrease in the heat demand.

**Figure 6.** Gasifier heat demand and LHV vs. ER.

#### *4.4. Cold Gas Efficiency and LHV vs. Gasification Temperature and ER*

In Figure 7a–d, it can be seen that the value of the cold gas efficiency, named CGEFF, and the LHV (MJ/Nm3) on the y-axis, was obtained by varying the gasifier temperature corresponding to the parametric curves representing the ER.

**Figure 7.** *Cont*.

**Figure 7.** (**a**) Cold gas efficiency and LHV for hazelnut shells. (**b**) Cold gas efficiency and LHV for olive pruning. (**c**) Cold gas efficiency and LHV for olive pomace. (**d**) Cold gas efficiency and LHV for wheat straw.

As shown in Figure 7a–d, the cold gas efficiency and the LHV decreased as the ER increased; according to Figure 6, they showed increasing behavior as the temperature rose, so higher values of ER are not useful because the lower value of LHV means that lower heat can be generated through gas combustion, which leads to lower net power from the turbines. The best combination of LHV and cold gas efficiency for each biomass waste was:


However, the necessary ER calculated for the total combustion considered an excess of air of 10%, which was equal to 0.27. Therefore, the best values of LHV and cold gas efficiency obtained by moving the parametric line representing ER = 0.27 in Figure 7a–d are:


#### *4.5. Effect of Steam to Biomass (S/B) Ratio*

Considering the configuration shown in Figure 3 where the oxidant was only steam, a sensitivity analysis was carried out by varying the S/B parameter between 0.2 to 1.35. The S/B ratio is important to identify the quantitative effects of the addition of steam on the performance of the gasifier. Figure 8a–d show the effect of the S/B ratio on the syngas composition at a gasification temperature of 800 ◦C for the biomass wastes analyzed.

**Figure 8.** (**a**) Effect of the S/B ratio on the syngas composition from hazelnut shells. (**b**) Effect of the S/B ratio on the syngas composition from olive pruning. (**c**) Effect of the S/B ratio on the syngas composition from pomace olive. (**d**) Effect of the S/B ratio on the syngas composition from wheat straw.

It was observed that the concentration of H2 increased with the increasing S/B ratio until it reached a maximum; then the concentration decreased. The hydrogen peak was almost at the beginning, which was due to the absence of air and the use of a variable external source of heat. In particular, Figure 8 shows that there was a lower regime of steam to biomass in order to reduce the heat demand, which, as shown in Figure 8, increased with the increase of S/B.

In order to evaluate the thermodynamic balance into the gasifier, Figure 9 shows the gasifier heat required and the LHV of the syngas produced (stream GASRAW). This has been shown only for the example of hazelnut shells as the other sources showed a similar trend. The heat required Q and LHV increased as the S/B increased, as foreseen from the previous figures and from the increase in the oxidant. The LHV varied between 6.5 and 9 MJ/Nm<sup>3</sup> while the heat demand Q varied between 550 and 1550 MJ/h. In the case of the hazelnut shells, which was similar to that of the other waste sources, the gas yield was 1.56 Nm3/kg and the biomass inlet was 180 kg/h, so the variation of 1 MJ/Nm<sup>3</sup> of the LHV corresponded to a variation of 350 MJ/h. Moreover, as shown in Figure 9, the curve representing the LHV had a higher slope and was always stronger with respect to the heat demand Q. For this reason, the optimum had the lowest value of S/B after the intersection point of the two curves. Considering the overall energy balance, a good value of S/B could be 0.2. However, as S/B increased, the LHV also increased. Therefore, each time, a careful evaluation is needed in order to determine the aim of the research. If, for example, the goal was to improve the H2 production or the increment of the LHV value, grea<sup>t</sup> heat required for the gasifier could be accepted.

**Figure 9.** Gasifier heat demand vs. S/B considering hazelnut shells.

#### *4.6. Cold Gas Efficiency and LHV vs. Gasifier Temperature and S/B*

Referring to the configuration shown in Figure 3 where the gasifying agen<sup>t</sup> is steam, the values of the cold gas efficiency and the LHV obtained by varying the gasifier temperature corresponding to the parametric curves representing the S/B are shown in Figure 10a–d.

**Figure 10.** *Cont*.

**Figure 10.** (**a**) Cold gas efficiency and LHV for hazelnut shells. (**b**) Cold gas efficiency and LHV for olive pruning. (**c**) Cold gas efficiency and LHV for olive pomace. (**d**) Cold gas efficiency and LHV for wheat straw.

Considering that the simulation was conducted assuming that S/B = 0.33 and that the increase of S/B means an increase of the heat required, we chose to stay with a low value of steam to biomass. Figure 10a–d show a decrease in the cold gas efficiency and the LHV with the increase in temperature and decrease of the S/B ratio. A comparison between Figure 10a–d shows that the cold gas efficiency was higher for hazelnut shells than for the othr biomass wastes and its maximum value was 58% at 870 ◦C with a S/B = 0.33. The highest value of LHV and cold gas efficiency for each biomass waste type was:


#### *4.7. Internal Combustion Engine Performance*

As a result of the consideration explained in Sections 4.4 and 4.6 by taking into account the highest value of LHV and cold gas efficiency, we chose to analyze the ICE behavior using the example of olive pruning for the configuration of air gasification and the example of hazelnut shells for the configuration of steam gasification. For the two cases under observation, the following Table 9 quotes the electrical efficiency and the cogeneration efficiency, by bringing the exhaust fumes at the utilization temperature of 80 ◦C and a pressure drop in the turbine of 10 kPa.

The cogeneration efficiency is defined as follows:

$$
\eta\_{CHP} = \frac{N\_{TILRB} + Q\_{EXCH} + Q\_{EX}}{LHV\_{BIOM} \cdot M\_{BIOM} + Q\_{INPLT}},
\tag{2}
$$

where *NTURB* is the effective electrical power of the turbine, *Q*EXCH is the heat of the exchangers, *QEX* is the heat produced to bring the exhausted fumes to 80 ◦C, *LHVBIOM* is the lower heat value of the biomass, *MBIOM* is the mass of the biomass and *QINPUT* is the heat associate to the Gibbs reactor.

The electrical efficiency is defined as:

$$
\eta\_{el} = \frac{N\_{TILRB}}{LHV\_{BIOM} \cdot M\_{BIOM}}.\tag{3}
$$



#### **5. Conclusions**

An Aspen Plus model was developed for the gasification of biomass waste and for power generation from syngas. The most available biomass wastes on Italian soil were investigated to select those most suitable for the gasification process. The main parameters governing the gasification process of biomass waste in a bubbling fluidized bed gasifier using air and steam as the oxidizing agents were discussed. The effect of gasification temperature, ER, and S/B ratio was analyzed and the results showed that it was more useful to work at high temperature, low ER, and with a S/B of around 0.33. The value of the cold gas efficiency and the LHV achieved for each biomass waste in different configurations and operative conditions were studied. The best syngas compositions to feed the ICE were:


These results confirm that the gasifier/ICE is an attractive technique when considering the environmental benefits and the electrical efficiency obtained.

**Author Contributions:** E.B.: concept, methodology and revision of the simulation results; A.C.: biomass analysis in particular regarding the agricultural biomass waste typologies, properties and availability; V.M.: software simulation, draft writing; M.V.: managemen<sup>t</sup> of the resources, data curation, validation, writing, review and editing.

**Funding:** This research was chiefly funded by "HBF2.0" research project within *Ricerca di Sistema* of Italian Ministry of the Economic Development, gran<sup>t</sup> number CCSEB\_00224 and partially funded by MIUR (Italian MInistry for education, University and Research), Law 232/2016, "Department of excellence".

**Conflicts of Interest:** The authors declare no conflict of interest.
