Decarbonisation Technology - May 2023 Issue

of steam and oxygen, into predominantly H 2 and CO, with significant amounts of tar. The second stage uses a catalytic chamber to break down the tars from the first stage into additional H 2 and CO using oxygen free radicals to catalyse reformation reactions. Tars are not washed away, as in conventional gasifier units, and instead converted in the process to significantly enhance the yield of valuable products. The produced syngas might contain nitrogenous and sulphur acidic and volatile species (such as ammonia and hydrogen sulphide). The syngas is cooled and treated with chemical wash prior to compression up to the necessary pressure for the GTL production unit. Gas-to-liquids FT process Compressed syngas is further polished to get rid of any remaining impurities that can poison the GTL reaction catalysts. The purified syngas is then water-shifted to increase the yield of H2 against CO. The CO2 formed in the water gas shift reaction is removed by solvent wash before entering the FT section. The FT process produces long-chain hydrocarbons, which can include waxes and byproducts such as off-gases, water, and oxygenated compounds by combining H2 with CO in a catalysed exothermic reaction. In certain licensed technologies, using a combination of novel catalyst and process conditions, the FT process can be modulated to produce very little wax. Conventional GTL processes, based on fossil fuels to produce liquid hydrocarbons, typically employ low-temperature Fischer–Tropsch (LTFT), producing maximum wax. Subsequently, a hydrocracker breaks down the wax into distillate range cuts, some lighter liquid hydrocarbons, and off-gas. This two-stage process of producing viable liquid fuels has traditionally been geared towards larger installations. The scale of waste to SAF production facilities is envisaged to be somewhat smaller (say around 5,000 barrels per day) and therefore to reduce Capex and Opex, a hybrid FT process was studied for this project. The selected FT technology claims to create very little wax by employing a unique catalyst that restricts the formation of the hydrocarbon chain.

The advantage of this system is that it produces a SAF range product without a hydrocracking unit. At most, mild hydrotreating may be required. The reactions for gasification, water gas shift, and FT are exothermic in nature, hence thermal energy can be recovered from the hot effluents. This high-quality heat can be used elsewhere in the facility, such as drying of RDF, CO2 stripping, and steam production, thereby reducing the net import of fuel to the facility and lowering Opex. By producing syncrude at lower emissions, the overall lifecycle emissions of SAF product can be reduced significantly. This aspect of energy integration between various licensed units and processes highlights the critical role the engineering contractor plays as an integrator in maximising the overall facility energy efficiency and in reducing emissions. Syncrude stabilisation The syncrude produced from the FT unit still has dissolved gases that need to be removed before it can be stored safely in atmospheric tanks. This is achieved by stripping in a steam-reboiler stabiliser, and the light ends vapour stream is utilised as a fuel blend. The stabilised syncrude can then be pumped to on-site buffer storage tanks for delivery and export to the offtaker(s). Emissions and effluent treatment A water balance around the Protos Biofuels process was performed to maximise the reuse of water produced within the facility (such as from the FT section) to satisfy the water demands by units such as syngas cleaning and water gas shift. Much of the water produced can be recycled but requires treatment due to the presence of acidic, alkaline, and oxygenated species and particulates brought in by a combination of RDF characteristics and reaction chemistry. However, the identified contaminants require specialised water treatment techniques that maximise water recovery and meet the strict water quality requirements, while remaining economically attractive. Similarly, CO2 produced from the water gas shift unit needs to be captured, treated, and compressed for exporting from the facility. The CO 2 capture technology selection depends on the concentration of CO2 , syngas operating