WP4: Blending and Upgrading of BtL fuels
The primary objectives of WP4 were to design and blend fuels containing BtL products for evaluation in single-cylinder engines (WP5), in multi-cylinder engines and vehicles (WP6) and to develop upgrading concepts for BtL naphtha. Two main BtL products, a higher-boiling diesel blend component and a lighter-boiling naphtha, should have been produced in WP2 and optimized for testing through research conducted in WP4 and WP5.
The objectives of WP4 were to twofold:
- Design and blend fuels containing BtL or BtL-like products for evaluation in single-cylinder engines and in multi-cylinder engines and vehicles
- To develop upgrading concepts for BtL naphtha.
The primary output were blended fuels in sufficient volumes and qualities to enable performance testing in engines and vehicles and conclusion on naphtha upgrading options
A literature study by IFPEN has pointed out the most relevant fuel properties that impact conventional Diesel combustion and exhaust emissions (cetane number, boiling range, chemical composition, lower heating value...). Because of their higher cetane number, lower density and very low aromatics content compared to a standard Diesel fuel, Fischer-Tropsch blends show interesting performance in conventional Diesel combustion in terms of emissions reduction. Nevertheless, due to a slight decrease in the energy density FT blends can lead to a slight increase of volumetric fuel consumption. Moreover, if the engine settings are optimised further emissions reduction and/or engine efficiency can be achieved.
Based on the results of the literature study a list of base components potentially interesting to improve the fuel formulation and the related physical and chemical properties have been identified : n-alkanes, iso-alkanes and naphthenes. These components have been selected in order to comply with two different boiling ranges: a low boiling range (160-240°C) and a high boiling range (240°C-320°C). All these products have been fully characterised by chemical-physical analyses have been
used for the "laboratory scale" and the larger blending actions.
Analyzed fuel properties
LHV: significant increase only with N-alcanes (0.9 to 1.2%)
This can be explained by the slight H/C ratio increase, as expected by adding such molecules
LHV (H) 120 MJ/kg >> LHV (C) 32.5 MJ/kg
The BtL-Naphtha upgrading was investigated in CERTH by the following technologies: i) catalytic cracking followed by an olefins saturation step ii) hydroisomerization iii) separation iv) catalytic cracking for high added chemicals production. For the 3 first technologies the objective was to produce an upgrading naphtha fuel with a different Cetane Number (lower or higher) compared to the original BtL naphtha. The 4th technology was an alternative option for BtL naphtha usage.
The main conlusions:
- The catalytic cracking of BtL-naphtha can reduce CI by about 3-5 units with however an unavoidable presence of aromatics (up to about 5%) and a decrease in naphtha yield (up to about 30%). Therefore, it was concluded that this upgrading process is not satisfactory for the BtL naphtha.
- It was demonstrated that hydroisomerization is an attractive option for upgrading the BtL-naphtha, as CI reduction of about 13 units can be achieved without losses in naphtha yield.
- Separation and extraction of individual components from BtL-naphtha as an option to increase its CN proved to be complicated and its feasibility to depend strongly on the CN of the raw fuel. Aromatics separation could increase CI by about 3 units, CI change by isoparaffins separation depends on the type of naphtha, while the removal of light components can increase CI up to about 5 units
- Catalytic cracking of BtL naphtha at high temperature and using selective catalysts could be a viable option for
upgrading BtL-naphtha towards the production of high added value chemicals (olefins). Two selective catalysts tested in this process showed that we can produce 12%wt propylene at temperatures around 600°C. Cat-A is more selective catalyst in this process for propylene and ethylene while CAT-B for butelenes.