Coal gasifier is simulated using GASDS [37]. As schematically shown in Figure 3, this program includes a multi-scale, multi-phase and multi-component model which describe coal gasification system by means of detailed kinetic mechanisms for coal pyrolysis, char heterogeneous reactions and for successive gas-phase reactions [38]. It also includes the catalytic effect of ashes [39]. These kinetic mechanisms are then coupled with transport resistances resulting in first-principles dynamic modeling of non-ideal reactors of different types (e.g., downdraft, updraft, traveling grate).

Figure 3.

Multiscale nature and structure of a countercurrent coal gasifier [40]

The aim of the amine wash unit is to purify the syngas, that contains acid gases (H₂S and CO₂), coming from coal gasifier. Methyl Diethanolamine (MDEA) was chosen for its industrial application and its specific selectivity to hydrogen sulfide [41]. As already highlighted by Bassani et al. [30], it is necessary to apply at least three amine washing units. The first one allows both to sequestrate H₂S from the acid gas stream, and at the same time, to control the absorption of CO₂. Indeed, the inlet ratio between H₂S and CO₂ is a first crucial operative parameter for AG2S process. For instance, working with an excess of CO₂ is not convenient because CO₂, the main source of CO, would be an “heat absorber” in the Regenerative Thermal Reactor (RTR) of the AG2S process section. In fact, if there is an excess of CO₂, more oxygen should be required to reach the desired temperature, leading to a greater oxidation of H₂S, limiting its pyrolysis with the net reduction of syngas yield. The second amine-washing unit aims to adjust the ratio between H₂S and CO₂. Indeed, AG2S process requires a ratio at least 2 [i.e. the stoichiometric ratio of reaction (1)]. This amine wash unit is not required in novel CTM process if the ratio H₂S/CO₂ coming from the first amine unit is already ~2. On the contrary, H₂S is almost completely separated from CO₂ in the traditional process in order to be sent to the Claus process [42]. Finally, the third amine-washing unit splits the extra syngas produced in RTR from the unreacted acid gases, which are recycled to the AG2S process. Moreover, an additional amine wash unit is required in CTM process both in case of traditional and novel process. This unit removes the CO₂ from the syngas stream after the Water-Gas-Shift (WGS) reactor. However, a minor percentage of CO₂ (1–4 %mol) remains in the stream and can be useful in methanol process [43].

The amine washing section is simulated entirely through HYSYS software, with a template already existing in the commercial package. The configuration of an amine treatment unit is composed of a single absorption column, one regeneration column and all related equipment, such as pumps, heat exchangers and filters, as schematically reported in Figure 4.

Figure 4.

MDEA wash with regenerator

The acid gas stream coming from sweetening section is sent to the Acid Gas to Syngas (AG2S™) process [30]. The core of the plant is the RTR, which has a different configuration compared with the typical Claus furnace [44]. RTR is mainly composed by a furnace, a Waste Heat Boiler (WHB) and a heat exchanger. This design allows to produce a greater amount of H₂. The key idea is to feed an optimal ratio of H₂S and CO₂ and to preheat the inlet acid gas before the combustion. In this way, H₂S pyrolysis produces hydrogen selectively. Indeed, it is convenient to feed the acid gases to the RTR at high temperatures (e.g. 700 °C) in order to reduce the oxygen flow rate required to reach the furnace temperatures (1,100–1,350 °C). So, the oxygen stream is much lower than the typical oxygen provided to the Claus processes and the H₂S potential for pyrolysis is completely exploited.

It is important to emphasize that the reactor is simulated using DSMOKE software with a detailed kinetic scheme [45]. The latter is coupled within Aspen HYSYS^® with the use of MATLAB. This allows to include the detailed kinetics, within non-ideal reactor models and in turn into commercial environments for the simulation of chemical plants. The detailed kinetic scheme selected is made up of three different subsets of reactions that describe the kinetic of carbon [46], sulfur [47] and nitrogen [48]. The validity of this kinetic scheme was also proved with the comparisons with the experimental data provided by El-Melih et al. [32] and reported Figure 5. Indeed, Melih et al. analyzed and discuss the effect of a plug flow reactor temperature on the syngas recovery from acid gases at experimental laboratory scale.

Figure 5.

Effect of reactor temperature on syngas production and ratio (3% H₂S/2% CO₂ diluted in 95% N₂)

The catalytic reactor configuration is the typical one of the Claus process, but the reactions involved are mainly the hydrolysis of Carbon disulphide (CS₂) and Carbon sulphide (COS). The simulation of the catalytic reactor is carried out using conversion reactor in Aspen HYSYS^®. The typical conversion of hydrolysis reaction is about 75% on alumina catalyst [49] and of about 100% for Claus reaction. Figure 6 schematically summarizes the process flow diagram of the AG2S technology and also indicates the simulation tools used for each unit. This process configuration takes advantage by the unreacted acid gases recycle.

Figure 6.

Process flow diagram of AG2S technology with related simulation tools

The overall syngas, produced from coal gasification and AG2S section, is sent to the Water Gas Shift (WGS) reactor to adjust the ratio between hydrogen and carbon monoxide in order to be suitable for methanol production. The principal reaction that occurs in WGS reactor is:

The industrial scale WGS reactor consists of a High Temperature Shift (HTS) adiabatic stage followed by a Low Temperature Shift (LTS) with an intermediate cooling [50]. The initial HTS takes advantage of the high reaction rates, but is thermodynamically limited, which results in incomplete conversion of carbon monoxide and exit composition of ~2–4 %mol. To improve the equilibrium toward hydrogen production, a subsequent LTS reactor is employed to reduce to less than 1% the CO exit composition. The transition from the HTS to the LTS reactors requires an intermediate cooling unit [51]. Depending on reaction conditions, different catalysts must be employed in the two reactors to ensure optimal activity. The commercial HTS catalyst is an iron oxide-chromium oxide catalyst, whereas a copper-based catalyst is applied in the LTS reactor. The operation of HTS and LTS catalysts occurs within the temperature range 250–450 °C. The temperature increases along the reactor, due to the exothermic nature of the reaction. In HTS the inlet temperature is maintained at 350 °C to prevent exit temperature exceeding 550 °C, with the subsequent catalyst sintering. Industrial reactors operate from atmospheric pressure to up to 83 bar. The equilibrium reactors in Aspen Hysys are chosen [52], in simulating these reactors.

As already deeply discussed by Bozzano and Manenti [13], nowadays different processes are available for methanol synthesis operating in a wide range of pressures of temperatures. For this work, the methanol production process is simulated using an Aspen HYSYS flowsheet provided by Pellegrini et al. [53] that operates at 150 bar. The gases leave the reactor at 270 °C. In addition, a compression section is needed because methanol process operates at higher pressure compared with the one of coal gasification and amine wash units. For this reason, a compressor and a flash unit, that allows to separate the condensed water, are included in the Aspen HYSYS simulation. The reactions involved in the methanol synthesis reactor are:

The overall process is exothermic and so a cooling system is needed on methanol reactor. The heat recovered from the reactor effluent and the heat released by the reaction are used to produce steam for the reboilers of the subsequent separation section [54]. The unreacted gases and the product mixture are separated by cooling and expansion to about 74 bar with a recycle split factor equal to 100% in order to maximize the methanol production. The distillation column, used to separate the unconverted syngas [55], is here considered as a splitter, in order to reduce the simulation efforts. Figure 7 shows the process flow diagram of the methanol synthesis loop.

Figure 7.

Process flow diagram of methanol reaction section

Sulcis coal [56] was chosen as a possible and interesting feedstock for the plant, due to its high sulfur content (about 6 %wt) which can provide a reduction of CO₂ by H₂S during the gasification process [30]. Table 1 reports the ultimate analysis of Sulcis coal, carried out at Sotacarbo laboratories according to the international standards, together with the coal characterization in terms of the three reference components (COAL1, COAL2 and COAL3), according to the coal devolatilization model proposed by Sommariva et al. [57]. Table 2 summarizes the stream property and composition of a gasifier operating at 30 bar [58]. The gasifier diameter and bed height are equal to 4.0 and 15.0 meter respectively. Model predictions are obtained by assuming 4 countercorrent reactor layers without particle discretization. It is also important to underline that the formation of H₂S is not predicted but, based on previous experience, it is simply assumed as the 80–90% of inlet sulfur [28]. Therefore, 85% of sulfur goes to H₂S and the remaining 15% exits the gasifier as ashes. This assumption will be validated as future development also in terms of kinetic gasification model. Indeed, an in-deep comprehension of the phenomena that lead to the formation of H₂S could be help full in order to optimise the operation condition for this component production. Moreover, organosulfur species like COS and CS₂ will be correctly predict.

The over prediction of outlet water is possibly due to an underestimation of the catalytic effect of ash, as already discussed and explained in Bassani et al. [30]. The peak temperature is about 1,097 °C.

Table 3 shows the simulation results and the main operative parameters of MDEA wash units for the traditional CTM process, whereas Table 4 reports the corresponding results of the novel CTM process with the AG2S technology.

As shown in Figure 2 and by the results of Table 3 and 4, first MDEA wash unit is the same for the two processes. The second MDEA wash unit shows some differences. In the traditional process, this wash unit aims to completely separate H₂S from CO₂ in order to send only H₂S, with a little amount of CO₂ to the Claus process. On the other side, the novel process requires an optimal ratio between H₂S and CO₂. According to previous experiences, the Claus process is not directly simulated and we simply assume that H₂S is completely converted into sulfur and water [59]. Finally, the fourth amine wash shows a higher treated mass flow rate in the novel process. This is simply due to the extra syngas produced by AG2S technology.

As already mentioned, AG2S^™ technology allows to convert CO₂ producing an additional amount of syngas. Table 5 shows the predicted simulation results. The RTF works at atmospheric pressure with an inlet oxygen mass flow rate of 3,650 kg/h. According to our previous work [30], a residence time between 1–1.5 s is required in the furnace, where the temperature reaches 1, 250 °C. In the same way, the WHB is designed to quench the thermal reactor effluent, in order to prevent any possible recombination effect (e.g. hydrogen and sulfur into H₂S), which have been proven to be significant during a relatively slow cooling process [60]. For this reason, the residence time is set to 0.3 s.

The inlet ratio H₂S/CO₂ to the regenerative thermal furnace, deriving from mixing stream 6 and 13, is equal to about 2.5. This value is higher than the stoichiometric one and is a suitable starting point. However, it is not the optimal one because there is unconverted CO₂ in the outlet stream (14). This lead to an outlet ratio between H₂ and CO that is equal to 0.5 instead of 1 derived by reaction (1). For sure, a future study on the inlet optimal ratio will be conducted in order to increase the yield of this process. Another key point is that the recycle mass flow rate (stream 13) is equal to a half of the inlet mass flow rate (stream 6). This means that the equipment not exceed in terms of design and dimensions. Finally, the outlet mass flow rate of H₂S is equal to zero (see the streams no. 10, 11, 14). The complete conversion of H₂S is reached, as in the traditional Claus process, with an extra production of syngas.

The results of the simulations of the WGS reactor are reported in Table 6 and Table 7. The inlet water is set in order to obtain a mole ratio between H₂ and CO equal to about 3.2 in order to approach the optimal ratio suggested by Pellegrini et al. [53]. Moreover, the operative temperature of the reactor at equilibrium condition is ~270 °C.

The Aspen Hysys simulations is performed in order to reach a methanol productivity equal to about 75 ton/h, as already mentioned before. The results of both process simulations are reported in Table 8 and Table 9. The predicted methanol productivities are 79.18 ton/h and 79.6 ton/h for traditional and novel process, respectively. It is important to notice that the simulated productivity is close to the target value and the aim to increase methanol production is reached.

The features of the distillation train have to be taken in consideration for the global process due to the different grades of the commercialization of methanol [13].