From Black Liquor To Second Generation Transportation Fuels
Résumé
The conversion of the huge EU potential of unrecycled organic wastes and residues to gasoline, diesel or kerosene could contribute up to 16% of the present demand in transportation fuels. Among these various industrial wastes, black liquors (BL), generated from paper and pulp Kraft process might represent a relevant option with a stable production of around 30 Mt/y (dry organic).
A number of scenarios can be designed to concatenate the elemental steps of waste-to-fuel processes. The value chain selected here describes a stepwise upgrading of raw BLs: i) hydrothermal liquefaction (HTL), ii) removal of troublesome alkaline and sulfur residues, iii) hydrogenation (HDO) for oxygen removal and decrease of molar weight and finally iv) co-processing with crude oil distillates (vacuum gas oil, VGO) by catalytic cracking (FCC) to produce second generation transportation biofuels.
As for the HTL of black liquor to produce a readily separable oil-like product, i.e. biocrude, it is shown that up to 75% of the BL’s energy content can be extracted as an oil-like product under suitable process conditions. Parameters like reaction temperature and time, catalyst type (e.g., NaOH, NaCO3, Ca(OH)2), and effect of additives (e.g., guaiacol, glycerol) on key physical properties like viscosity have been tested and optimized.
For the HDO step, experiments were carried out in water as slurry medium, testing various catalysts selected on HDO model compounds and catalyst-to-biocrude ratios as well as biocrude concentrations. It was found possible to liquefy up to 55 wt.% based on dry organic matter of biocrude. Despite a still high viscosity due to the high averaged molecular weight (ca 2074 g/mol), an oxygen content of about 12 % has been reached as a target for further co-processing.
Lab-scale Micro Activity Test (MAT) experiments simulating FCC process were carried out to co-process the upgraded BL with VGO over equilibrated FCC catalysts, targeting at gasoline hybrid fuels of standard yields and quality. For a 1/9 fossil/bio feedstock ratio, it was observed a slight increase in naphtha yields with more olefins and aromatics due to hydrogen transfer from hydrocarbons to oxygenates during cracking, with a slight increase in coke formation due to depleted medium in hydrogen.
A global assessment of all these trends confirm the interest of such a biowaste-to-biofuels scenario, which remains to be verified at demonstration scale. This is planned to be done within a new EU contract which targets demonstration of co-processing from various upgraded biogenic wastes, including HTL treated black liquors3.