2.2.2. Purification unit

The purification section is modeled using component separator, equilibrium and Gibbs reactor blocks of Aspen Plus simulator. The syngas obtained after the precleaning step is sent to a compression section to raise the pressure till 15 barg. To avoid deactivation of the catalyst used in the methanol synthesis reactor, a purification unit is necessary to decrease sulfur to ppb level (Figure 4). This purification unit is composed of the following:


With all these units, a sulfur ppb level is obtained, so the syngas could be properly conditioned, avoiding any catalyst deactivation.

Figure 4. Syngas purification scheme.

#### 2.2.3. Conditioning unit

• Melting zone RG1, where mainly heterogeneous combustion reactions and melting process occur at almost 2000�C; in this region, a fixed methane flow rate (as auxiliary fuel) is added, while the temperature is regulated with the introduction of a controlled amount of oxygen.

• Gasification zone RG2, where the oxygen amount decreases and a reduction atmosphere

• Stabilization zone RG3, where a fresh amount of oxygen and methane, with a specified mass ratio, is introduced to raise the temperature near to 1100�C in order to enhance

The heat produced in the RYIELD reactor is proportionally added to RG1 and RG2 (as depicted in Figure 3 with dashed red lines) in order to take into account all contributions for the right

From the third Gibbs reactor, a hot raw syngas stream is obtained, which is suddenly cooled in quench equipment, modeled by a flash block. In a simplified way, also the acid and alkaline scrubbers have been simulated with flash blocks, neglecting the pH and composition variation,

The purification section is modeled using component separator, equilibrium and Gibbs reactor blocks of Aspen Plus simulator. The syngas obtained after the precleaning step is sent to a compression section to raise the pressure till 15 barg. To avoid deactivation of the catalyst used in the methanol synthesis reactor, a purification unit is necessary to decrease sulfur to ppb

1. Hydrolysis Gibbs reactor R-HYDRO is an adiabatic reactor where the hydrolysis reaction IV occurs, to convert COS to H2S. Syngas and medium pressure steam are fed to this reactor, with a flow rate ratio depending on the COS content and set by a controller; a control is also placed to set the steam pressure in order to obtain an inlet gas temperature

2. Sulfur removal LO-CAT, modeled with a component separator, where 99.9% of H2S is removed, according to the technical specifications provided by the vendor [17].

3. Sulfur-polishing step ZNO-BED, operating at 200�C, where 99.7% of residual H2S and

With all these units, a sulfur ppb level is obtained, so the syngas could be properly condi-

but mainly taking into account the decrease in water content in the syngas mixture.

has been established; thus principally gasification reactions occur.

methane and tar decomposition and avoid dioxin formation.

level (Figure 4). This purification unit is composed of the following:

of 180�C, thanks to heat recovery.

80% of residual COS are removed.

tioned, avoiding any catalyst deactivation.

Figure 4. Syngas purification scheme.

formulation of energy balance.

150 Gasification for Low-grade Feedstock

2.2.2. Purification unit

As previously reported, in order to achieve a methanol module suitable for methanol synthesis, a portion of the purified syngas is sent to a shift reactor and a subsequent unit to remove carbon dioxide (Figure 5).

Simulation of this block in Aspen Plus is carried out with a split unit that separate the syngas stream sent to the shift reactor; a controller is used to define the flow rate of this stream, depending on the composition of the syngas, which in turn depends on the fluctuation of RDF composition. Superheated medium pressure steam is added to the split syngas; another controller is placed to define the right amount of steam that has to be added in order to respect a steam/dry-syngas molar ratio equal to 1.5 [24]. The inlet R-HTS temperature is 320�C, in order to activate the Fe/Cr/Al2O3 catalyst. The R-HTS is an equilibrium reactor where the shift reaction is imposed. A CO2 removal unit follows R-HTS to separate the carbon dioxide formed by reaction VI and thus increasing the MM value. The CO2 REMOVAL is a component separator where a 95% of CO2 removal is set. In detail, this separation consists of an amine (MDEA) unit absorber and a regenerator, with heat process recovery for the regeneration section, providing a pure CO2 gas stream [25]. After this treatment, the conditioned syngas is reconnected to the split ones, reaching an MM value of 1.7.

#### 2.2.4. Synthesis and recovery unit

The syngas is then compressed to be suitable for methanol synthesis till 56 barg. When methanol synthesis occurs, low methanol conversion is obtained, so to increase this, a recycle loop is required to circulate the unreacted reagents at the reactor entrance. In order to reach an MM equal to 2.1, part of the recycled stream is sent to the Pressure Swing Absorption system, modeled with a component separator where the 85% of H2 recovery is set, while CO and CO2 are removed with the off-gas. The conditioned syngas, added with the hydrogen stream, is now suitable for methanol synthesis, and after being preheated at 158�C, it is sent to the adiabatic Gibbs reactor R-MeOH. The referring reactor is a Davy-Johnson-Matthey isothermal reactor, where the reagents are heated in the reactor till 240�C by using the heat of synthesis reaction. The effluent of R-MeOH is cooled till 30�C and separated, using the block flash separator, to recover raw methanol as much as possible. To achieve a methanol fuel grade, the stream is purified using two distillation towers: the first one is just an exhaustion distillation, used to remove as much as possible the off-gas, while the second one is a typical methanolwater distillation column. The off-gas, depicted in Figure 6, coming from the purge of the PSA and the distillation column, is collected and sent to a boiler where steam is produced.

Figure 5. Syngas-conditioning scheme.

Figure 6. Methanol synthesis and purification scheme.
