SAF from Waste and Biomass: Process Simulation and Design of the Second Gasifier and Syngas Cooler for Low Pressure Gasifiers

The world's first commercial plant to produce sustainable aviation fuel (SAF) from municipal solid waste (MSW) began operations in 2022 , in the US. The process consists of the main steps. Gasification, 2^nd conversion step, syngas cooling, syngas purification and fuel synthesis.

The paper focusses on the 2^nd conversion step and the cooling section designed as full heat recovery.

As the raw syngas from the gasifier still contains some tars and heavy hydrocarbons, a chemical reactor is installed between upstream gasifier and downstream syngas cooler. The purpose of the reactor is to crack the tars and heavy hydrocarbons. The main point in the design is to limit the oxygen available to the synthesis gas. This is why it is usually referred to as a partial oxidiser (POx). Applying the POx significantly increases the quality of the syngas. The radiant syngas cooler is located downstream of the POx reactor. The syngas cooler plays a key role in meeting the syngas requirements of the entire process. By cooling the syngas, heat is recovered to produce high pressure steam which improves the overall thermal process efficiency. Downstream of the syngas cooler is the water sump and slag handling unit, where the cooling, collection and transport of ash and slag takes place.

This paper presents the chemical process simulation of this process step as well as an insight into the equipment design. Aspen Plus^® is used to model the POx reactor, syngas cooler and cooler sump, including all major chemical reactions in the syngas as well as between syngas, ash/slag and water. Additional focus was on the reaction of ash and syngas in the POx reactor to get a better understanding of contaminants for the material selection.

With respect to the full heat recovery, the same design was applied at biomass gasification plants, which went into operation in 2023 in Europe.