3. Process analysis

#### 3.1. Influence on syngas composition

As already underlined, syngas composition plays a fundamental role in methanol synthesis step. Therefore, the optimization of the waste-to-methanol process requires analyzing the composition of syngas directly obtained from gasification unit and to investigate how the RDF composition and/or the gasifier-operating conditions can affect the syngas composition. In other words, it is interesting to investigate if a proper selection of RDF or an optimal choice of the gasifier-operating conditions can significantly improve the overall waste-to-methanol process efficiency.

#### 3.1.1. Effect of RDF composition

According to its definition, waste is a solid mixture composed of variable quantities of refused materials belonging to different product classes [26]. However, its variable composition can be restricted to a reasonable limited range, as shown in Table 2; indeed, the waste composition can be defined in terms of three main mass fractions: the combustible fraction (CHO), the moisture fraction (MOI) and ash plus inert fraction (Ash&In). According to reasonable approximations, assuming in the combustible fraction, a carbon to hydrogen and a carbon to oxygen ratios, respectively, equal to 7.5 and 2, and a fixed composition of the Ash&In fraction. As reported in Table 2, waste ultimate analysis can be uniquely gathered from its composition in terms of CHO, MOI and Ash&In.

It is important to underline that the waste composition strongly affects the lower heating value (LHV) of the RDF; as evidenced in Figure 7, in particular, LHV is mainly dependent on the CHO fraction content of the waste. In this work, we assume an RDF with LHV in the range of 14 and 18 MJ/kg; therefore, only waste with composition in the highlighted color region in Figure 7 is analyzed with our simulation tool.

The simulation of gasification unit has been carried out for several waste compositions derived from a fine discretization of the range depicted in Figure 7. As could be expected, syngas composition is influenced by waste composition and LHV variation. Bearing in mind the requirements for methanol synthesis, it is useful to represent methanol module and carbon ratio—CO2/(CO + CO2)—variation as a function of waste LHV (Figure 8). Indeed, each LHV

Figure 7. The lower heating value of waste as function of combustible, moisture and ash and inert contents.

CHO 50–80% C/H = 7.5

Ash&In 10–25% Cl = 0.75%

MOI 10–25%

C/O = 2

S = 0.15% N = 1%

Table 2. Waste composition range considered for feedstock characterization in the simulation tool.

Ash = (Ash&In-Cl-S-N) SiO2 = 35.79%

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C 40–55% H 5–7.5% O 20–27.5%

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CaO = 35.89% Al2O3 = 13.32% Fe2O3 = 15%

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Table 2. Waste composition range considered for feedstock characterization in the simulation tool.

3. Process analysis

152 Gasification for Low-grade Feedstock

process efficiency.

MOI and Ash&In.

3.1.1. Effect of RDF composition

Figure 7 is analyzed with our simulation tool.

3.1. Influence on syngas composition

Figure 6. Methanol synthesis and purification scheme.

As already underlined, syngas composition plays a fundamental role in methanol synthesis step. Therefore, the optimization of the waste-to-methanol process requires analyzing the composition of syngas directly obtained from gasification unit and to investigate how the RDF composition and/or the gasifier-operating conditions can affect the syngas composition. In other words, it is interesting to investigate if a proper selection of RDF or an optimal choice of the gasifier-operating conditions can significantly improve the overall waste-to-methanol

According to its definition, waste is a solid mixture composed of variable quantities of refused materials belonging to different product classes [26]. However, its variable composition can be restricted to a reasonable limited range, as shown in Table 2; indeed, the waste composition can be defined in terms of three main mass fractions: the combustible fraction (CHO), the moisture fraction (MOI) and ash plus inert fraction (Ash&In). According to reasonable approximations, assuming in the combustible fraction, a carbon to hydrogen and a carbon to oxygen ratios, respectively, equal to 7.5 and 2, and a fixed composition of the Ash&In fraction. As reported in Table 2, waste ultimate analysis can be uniquely gathered from its composition in terms of CHO,

It is important to underline that the waste composition strongly affects the lower heating value (LHV) of the RDF; as evidenced in Figure 7, in particular, LHV is mainly dependent on the CHO fraction content of the waste. In this work, we assume an RDF with LHV in the range of 14 and 18 MJ/kg; therefore, only waste with composition in the highlighted color region in

The simulation of gasification unit has been carried out for several waste compositions derived from a fine discretization of the range depicted in Figure 7. As could be expected, syngas

Figure 7. The lower heating value of waste as function of combustible, moisture and ash and inert contents.

composition is influenced by waste composition and LHV variation. Bearing in mind the requirements for methanol synthesis, it is useful to represent methanol module and carbon ratio—CO2/(CO + CO2)—variation as a function of waste LHV (Figure 8). Indeed, each LHV

Figure 8. Methanol module and carbon ratio—CO2/(CO + CO2)—obtained with Aspen Plus simulation as functions of waste LHV (MJ/kg).

3.1.2. Effect of operating condition

3.1.2.1. Effect of temperature

methane and tar content in syngas.

3.1.2.2. Effect of steam introduction

controlled oxygen stream.

Figure 10.

Usually, in gasification processes, the main examined operating conditions are operating pressure, temperature and gasification agent. In this case, the gasifier outlet temperature and

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The syngas composition and methanol synthesis parameters trends as a function of gasifier outlet temperature (i.e. equilibrium temperature of RG3 referred to Figure 3) are illustrated in

The represented trends show that both the methanol modulus and the ratio CO2/(CO + CO2) are improved at a lower gasification temperature, where the hydrogen content is higher. A reduction of the R-G3 temperature also reduces the oxygen consumption. However, a temperature higher than 1100�C must be provided in order to avoid dioxin formation and limit

Steam is a possible oxidant [27], which can be additionally introduced in the gasifier with a

The results of the sensitive analysis made for different steam temperatures are depicted in Figure 11. Indeed, the steam injection results in two opposite effects: (1) an increase in H2 production in the gasifier due to the shift reaction and (2) an increase in the heat required to maintain the top gasifier temperature equal to 1100�C, which in turn results in an increase in the oxygen consumption for exothermic reactions, including hydrogen combustion. These

the introduction of a supplementary steam stream have been deeply investigated.

Figure 9. Syngas yield and efficiency obtained with Aspen Plus simulation as functions of waste LHV (MJ/kg).

value can be obtained from different waste compositions (i.e. from waste with similar combustible fraction but different moisture or ash content) and can result in different syngas compositions; therefore, in each plot, a fixed LHV value corresponds to a range of MM or CO2 content, actually depending on the MOI or Ash content. In the plots, the colored symbols '◊' represent the mean values of carbon ratio at LHV equal to 14, 16 and 18 MJ/kg. From the left panel of Figure 8, it is evident that for different waste compositions, with same LHV, methanol module values are very similar (i.e. MOI and Ash&In contents do not significantly affect the MM value), while a large variability is observed for the CO2 to CO ratio (see the panel on the right). The strong correlation between methanol module and LHV is significant and supports the choice of LHV as a characterizing parameter for the feedstock, also for the analysis of the effects of RDF variability on the downstream process behavior.

Furthermore, as reported in Figure 9 (left panel), the higher the waste LHV is, the higher the syngas yield is obtained, even if some variability related to the MOI and Ash content is observed. Finally, it is worth considering a gasification unit thermal efficiency defined as

$$\frac{LHV\_{Syngas} \cdot kg\_{Syngas}}{LHV\_{RDF} \cdot kg\_{RDF} + LHV\_{CH\_4} \cdot kg\_{CH\_4}}$$

where the heating value of the obtained syngas is compared with the total heating value of RDF and supplemental CH4 fed to the gasifier. From the figure reported in the right panel of Figure 9, it is evident that the efficiency is strictly correlated to the LHV of RDF.

Comparing the obtained results with the technical requirements for methanol synthesis, it is evident that despite suitable CO2/(CO + CO2) ratio that is always obtained, the MM values are always too low, even when RDF with a high heating value is used. That is why a conditioning step is required.

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Figure 9. Syngas yield and efficiency obtained with Aspen Plus simulation as functions of waste LHV (MJ/kg).

#### 3.1.2. Effect of operating condition

value can be obtained from different waste compositions (i.e. from waste with similar combustible fraction but different moisture or ash content) and can result in different syngas compositions; therefore, in each plot, a fixed LHV value corresponds to a range of MM or CO2 content, actually depending on the MOI or Ash content. In the plots, the colored symbols '◊' represent the mean values of carbon ratio at LHV equal to 14, 16 and 18 MJ/kg. From the left panel of Figure 8, it is evident that for different waste compositions, with same LHV, methanol module values are very similar (i.e. MOI and Ash&In contents do not significantly affect the MM value), while a large variability is observed for the CO2 to CO ratio (see the panel on the right). The strong correlation between methanol module and LHV is significant and supports the choice of LHV as a characterizing parameter for the feedstock, also for the analysis of the

Figure 8. Methanol module and carbon ratio—CO2/(CO + CO2)—obtained with Aspen Plus simulation as functions of

Furthermore, as reported in Figure 9 (left panel), the higher the waste LHV is, the higher the syngas yield is obtained, even if some variability related to the MOI and Ash content is observed. Finally, it is worth considering a gasification unit thermal efficiency defined as

> LHVSyngas∙kgSyngas LHVRDF∙kgRDF þ LHVCH<sup>4</sup> ∙kgCH<sup>4</sup>

where the heating value of the obtained syngas is compared with the total heating value of RDF and supplemental CH4 fed to the gasifier. From the figure reported in the right panel of

Comparing the obtained results with the technical requirements for methanol synthesis, it is evident that despite suitable CO2/(CO + CO2) ratio that is always obtained, the MM values are always too low, even when RDF with a high heating value is used. That is why a conditioning

Figure 9, it is evident that the efficiency is strictly correlated to the LHV of RDF.

effects of RDF variability on the downstream process behavior.

step is required.

waste LHV (MJ/kg).

154 Gasification for Low-grade Feedstock

Usually, in gasification processes, the main examined operating conditions are operating pressure, temperature and gasification agent. In this case, the gasifier outlet temperature and the introduction of a supplementary steam stream have been deeply investigated.

#### 3.1.2.1. Effect of temperature

The syngas composition and methanol synthesis parameters trends as a function of gasifier outlet temperature (i.e. equilibrium temperature of RG3 referred to Figure 3) are illustrated in Figure 10.

The represented trends show that both the methanol modulus and the ratio CO2/(CO + CO2) are improved at a lower gasification temperature, where the hydrogen content is higher. A reduction of the R-G3 temperature also reduces the oxygen consumption. However, a temperature higher than 1100�C must be provided in order to avoid dioxin formation and limit methane and tar content in syngas.

#### 3.1.2.2. Effect of steam introduction

Steam is a possible oxidant [27], which can be additionally introduced in the gasifier with a controlled oxygen stream.

The results of the sensitive analysis made for different steam temperatures are depicted in Figure 11. Indeed, the steam injection results in two opposite effects: (1) an increase in H2 production in the gasifier due to the shift reaction and (2) an increase in the heat required to maintain the top gasifier temperature equal to 1100�C, which in turn results in an increase in the oxygen consumption for exothermic reactions, including hydrogen combustion. These

operating parameters of the conditioning section. Here, a controller is set to maintain MM equal to 2.1 at the inlet of the methanol synthesis reactor, by varying the percentage of syngas sent to the shift reactor. Consequently, the superheated stream to add to the shift reactor R-HTS, the methanol yield and CO2 produced will be affected by this variation, depicted in Figure 12. As reported in Section 3.1.1, syngas composition depends on the RDF LHV. In particular, methanol module exhibits a linked correlation with LHV. For this reason, three main syngas compositions corresponding to RDF with LHV equaling to 14–16–18 MJ/kg have been selected, with the aim of analyzing the influence of feedstock variation on methanol production. Figure 12 shows how the LHV values affect the main operating parameters of the

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When waste LHV varies, also syngas flow rate is influenced and as a consequence methanol productivity. However, to better compare consumptions that occur for waste with different LHVs, it has been taken into account to obtain a fixed amount of methanol; in particular, when LHV decreases, a higher quantity of RDF to gasify is required. As represented in Figure 12, the values of operating parameters increase with the decrease in LHV. Then, we refer as design

The Italian municipal solid waste generated, expressed in kg per person per year, is equal to 529 [2]. With this number in mind, and considering that RDF represents a third of MSW quantity, a defined RDF quantity to gasify has been considered. A gasification line has a period of planned and unplanned maintenance. To avoid a plant arrest, three gasification lines working in parallel were adopted, so that when one of them stops, the other two work at their maximum capacity, providing only an 80% of turndown. For the design case, a gasification line with a normal capacity of 7.5 t/h is adopted, so that the available RDF (with 14 MJ/kg) is equal

Referring to Figure 13, a utility consumption has been performed for a further detailed economical evaluation. As shown in Figure 13, a 540 t/d feed is required to produce 225 t/d of

case to the process converting waste with lowest LHV of 14 MJ/kg.

Figure 12. The variation of main operating parameters with a lower heating value.

3.3. Optimized process: mass and energy balance

conditioning section.

to 540 t/d.

Figure 10. In the left panel, H2, CO and CO2% of syngas from gasifier as a function of its outlet temperature—third Gibbs reactor temperature (in Aspen Plus simulation). In the right panel, methanol module and carbon ratio as a function of syngas outlet temperature.

Figure 11. Methanol module, hydrogen % in syngas and O2 consumption to RDF (t/t) as a function of steam to RDF (t/t) value, for different steam inlet temperature '+' 130�C, '◊' 210�C and 'o' 300�C.

mentioned factors explain the maximum hydrogen content, at steam to RDF ratio near to 0.35 and the increase in CO2 content in syngas; on the whole, steam injection results in a reduction of methanol module. As results show, the increment of steam temperature is not enough relevant. On the other hand, steam might be able to destroy tar at a lower temperature and it could decrease the burner's outlet temperature which causes corrosion of refractory reactor covering. These possible benefits cannot be quantified with the support of the illustrated simulation that assumes thermodynamic equilibrium hypothesis. Obviously, a kinetic and fluid-dynamic model of the gasifier should be developed in the future to better analyze and optimize the process.

#### 3.2. Influence on methanol production

Once the RDF is gasified, the obtained syngas has to be properly conditioned, as already described in Section 2. The syngas composition variation will cause the alteration of some operating parameters of the conditioning section. Here, a controller is set to maintain MM equal to 2.1 at the inlet of the methanol synthesis reactor, by varying the percentage of syngas sent to the shift reactor. Consequently, the superheated stream to add to the shift reactor R-HTS, the methanol yield and CO2 produced will be affected by this variation, depicted in Figure 12. As reported in Section 3.1.1, syngas composition depends on the RDF LHV. In particular, methanol module exhibits a linked correlation with LHV. For this reason, three main syngas compositions corresponding to RDF with LHV equaling to 14–16–18 MJ/kg have been selected, with the aim of analyzing the influence of feedstock variation on methanol production. Figure 12 shows how the LHV values affect the main operating parameters of the conditioning section.

When waste LHV varies, also syngas flow rate is influenced and as a consequence methanol productivity. However, to better compare consumptions that occur for waste with different LHVs, it has been taken into account to obtain a fixed amount of methanol; in particular, when LHV decreases, a higher quantity of RDF to gasify is required. As represented in Figure 12, the values of operating parameters increase with the decrease in LHV. Then, we refer as design case to the process converting waste with lowest LHV of 14 MJ/kg.

#### 3.3. Optimized process: mass and energy balance

mentioned factors explain the maximum hydrogen content, at steam to RDF ratio near to 0.35 and the increase in CO2 content in syngas; on the whole, steam injection results in a reduction of methanol module. As results show, the increment of steam temperature is not enough relevant. On the other hand, steam might be able to destroy tar at a lower temperature and it could decrease the burner's outlet temperature which causes corrosion of refractory reactor covering. These possible benefits cannot be quantified with the support of the illustrated simulation that assumes thermodynamic equilibrium hypothesis. Obviously, a kinetic and fluid-dynamic model of the gasifier should be developed in the future to better analyze and optimize the process.

Figure 11. Methanol module, hydrogen % in syngas and O2 consumption to RDF (t/t) as a function of steam to RDF (t/t)

Figure 10. In the left panel, H2, CO and CO2% of syngas from gasifier as a function of its outlet temperature—third Gibbs reactor temperature (in Aspen Plus simulation). In the right panel, methanol module and carbon ratio as a function of

Once the RDF is gasified, the obtained syngas has to be properly conditioned, as already described in Section 2. The syngas composition variation will cause the alteration of some

3.2. Influence on methanol production

syngas outlet temperature.

156 Gasification for Low-grade Feedstock

value, for different steam inlet temperature '+' 130�C, '◊' 210�C and 'o' 300�C.

The Italian municipal solid waste generated, expressed in kg per person per year, is equal to 529 [2]. With this number in mind, and considering that RDF represents a third of MSW quantity, a defined RDF quantity to gasify has been considered. A gasification line has a period of planned and unplanned maintenance. To avoid a plant arrest, three gasification lines working in parallel were adopted, so that when one of them stops, the other two work at their maximum capacity, providing only an 80% of turndown. For the design case, a gasification line with a normal capacity of 7.5 t/h is adopted, so that the available RDF (with 14 MJ/kg) is equal to 540 t/d.

Referring to Figure 13, a utility consumption has been performed for a further detailed economical evaluation. As shown in Figure 13, a 540 t/d feed is required to produce 225 t/d of

Figure 12. The variation of main operating parameters with a lower heating value.

bio-methanol. The CO2 obtained comes from the CO2 removal system and the flue gases of a boiler used to supply steam for the hydrolysis reactor, the HTS reactor, the CO2 reboiler and the distillation reboilers. The cooling water (CW) reported in Figure 13 is low because it just represents the reintroduction of water in the cooling tower system.

kg MeOH). In this way, a reduction of 30% of GHG emission is obtained by comparing a WtC

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Moreover, to evaluate CO2 emission saved in a waste to chemical conversion, it is important to compare how much CO2 is emitted when RDF is burnt and methanol is produced in a conventional way and with the waste-to-methanol process. Therefore, the following ratio is

> CO<sup>2</sup> from a waste to methanol process ðCO<sup>2</sup> from combustion þ CO<sup>2</sup> from conventional methanol production

According to the value reported in Table 3, a saving of 54% is reached. Other than from an environmental point of view, even the consumption (in terms of methane usage) has been

The process scheme reported in Figures 1–6 provides an idea of the units required in a wasteto-methanol process. To evaluate the techno-economical feasibility of this process and decide whether or not this technology has a chance to be applied, a deep economical evaluation is required. Economic parameters used to estimate the cost of production (COP) are summarized

First of all, an analysis of the equipment employed is necessary to evaluate the CAPEX of the

As depicted from Table 4, the most relevant cost is related to the gasification lines, including the first purification treatment unit. Moreover, an Air Separation Unit is required when a gasification with pure oxygen is used. The additional costs for oxygen production appear justified on the basis that a higher syngas heating value is obtained [5] and less inert compounds circulate on the overall conditioning and synthesis unit. Once the total equipment cost is defined, an estimate of the total investment cost is predictable, as shown in Table 5. To define the CAPEX, an analysis of the total direct and indirect costs is necessary, including also

M €

reported, to emphasize the importance of a WtC process.

process with a WtE process.

4. Economical analysis

the contract profit and the contingency.

Table 4. Total equipment cost.

HT converter reactor (3 lines) 25 ASU, gasometer and compressors 12 Syngas purification and conditioning 7 Methanol synthesis and purification 10 Total 54

considered:

in Table 4.

process.

A comparison between a WtE and a WtC process with a typical incinerator, in terms of CO2 emission and methane consumption, is necessary to understand the relevance of a waste to chemical conversion. To evaluate those parameters, the same gasified quantity has been assumed. In Table 3, CO2 emissions of each process are reported in terms of CO2 kg per kg of methanol.

A WtE process could be seen as a waste disposal method and as an energy production system; likewise, a WtC process could be seen as a waste disposal method and as a methanol production process. Therefore, in order to correctly compare them, CO2 emission of WtE has to be added with the emission of a conventional methanol process, per unit of methanol produced. For that, waste combustion emission is equal to 2.96 kg CO2/kg MeOH, considering that 2.4 kg RDF, which would be converted for 1 kg of methanol produced and that the direct emission of process is 1.23 kg CO2/kg RDF. According to the same rules, WtC emissions are equal to the sum of the direct process emission (1.7 kgCO2/kg MeOH) and the emission connected to the conventional energy production related to the same MW amount which would be produced by converting, through WtE, the RDF quantity, used for 1 kg of methanol synthesis (0.96 kg CO2/

Figure 13. Overall process analysis consumption.


Table 3. Value considered to compare waste to energy and waste to chemical in terms of CO2 emission [8] and CH4 consumption.

kg MeOH). In this way, a reduction of 30% of GHG emission is obtained by comparing a WtC process with a WtE process.

Moreover, to evaluate CO2 emission saved in a waste to chemical conversion, it is important to compare how much CO2 is emitted when RDF is burnt and methanol is produced in a conventional way and with the waste-to-methanol process. Therefore, the following ratio is considered:

> CO<sup>2</sup> from a waste to methanol process ðCO<sup>2</sup> from combustion þ CO<sup>2</sup> from conventional methanol production

According to the value reported in Table 3, a saving of 54% is reached. Other than from an environmental point of view, even the consumption (in terms of methane usage) has been reported, to emphasize the importance of a WtC process.
