The two most established processes for obtaining gasoline from biomass are Fischer-Tropsch (F-T) synthesis or methanol-to-gasoline (MTG) process involving the two steps of syngas-to-methanol and then methanol-to- gasoline.

F-T synthesis yields a mixture of hydrocarbon molecules ranging from C₁ up to C₃₀₊, following the Anderson-Schulz-Flory distribution. Selectivity for gasoline-range hydrocarbons (C₅–C₁₂) can reach only around 45–48%. Upgrading the F-T product is necessary prior to use in engines.

Products from the MTG process are dominated by aromatic and branched aliphatic hydrocarbons belonging to the gasoline fraction, and the gasoline selectivity is about 80%. MTG gasoline has a high octane number of 90–95 and needs no enhancement. The MTG process thus has advantages in terms of product selectivity and lower plant investment cost, as compared to gasoline production from the F-T process, the Qingdao team explained.

The conventional MTG process first converts syngas (from gasification) to methanol over a catalyst, then partly dehydrates the methanol to form an equilibrium mixture of methanol, DME and water. This mixture is converted to gasoline over a ZSM-5 catalyst.

To reduce investment and operation costs, engineers have made many refinements and adjustments to the basic MTG technology, including catalyst development and optimal integration of technological process. Too, a one-step process for the direct synthesis of DME from syngas was also developed.

In the conversion of syngas to product, the ratio of hydrogen to CO in the feed is influential. As it happens, thermodynamic and experimental studies have confirmed that the optimum H₂/CO ratio for DME synthesis is 1. Syngas derived from biomass gasification thus has an advantage for one-step DME synthesis due to its natural composition with H₂/CO ratio of 1.

… the BTL process including biomass gasification, direct synthesis of DME and DME to gasoline (DTG: Dimethyl ether to gasoline) is a promising route. This route provides higher efficiency and lower investment and operation cost than that involving methanol synthesis. Moreover, compared to the MTG process, DTG process exhibits some other advantages, such as lower heat duty, adiabatic temperature rise, and higher yield and selectivity of hydrocarbon product.

—Wang et al.

Schematic of biomass to gasoline via one-step DME synthesis. 1. Hopper; 2. Pyrolyzer; 3. Down draft gasifier; 4. Cyclone; 5,7. Heat exchanger; 6. Bag filter; 8. Scrubber; 9,11. Roots blower; 10. Wet gas tank; 12,13. Sulfur removal; 14. Oxygen removal; 15. Compressor; 16. CO2/Water-removal; 17,18. Dehydrator; 19. Multitubular reactor; 20. Preheater; 21. Adiabatic reactor; 22. Water-cooled condenser; 23. Gas-liquid separator; 24. Burner. Wang et al. Click to enlarge.

The system begins with biomass gasification with oxygen. The crude syngas is cleaned, then transported by a roots blower into a wet gas holder. The clean syngas is pressurized and then scrubbed with deionized water to remove most of CO₂, and after that the humid syngas flows into a dryer to dehydration. The dried syngas is directly used to synthesize DME in a fixed reactor, and then the stream mixture consisting of DME, CO₂ and unreacted syngas is introduced straightway into a preheater to further elevate the temperature.

Subsequently the high temperature mixture is fed into the second fixed bed reactor to synthesize gasoline at the same pressure level. The stream mixture of the products and unreacted syngas is cooled to ambient temperature by passing through water cooler, the condensable hydrocarbons and water, and non-condensable gas are separated in a gas-liquid separator. The off-gas from the gas-liquid separator is partially recycled or combusted to produce heat.

In addition to featuring a high octane number, the DTG-produced gasoline has high aromatics and paraffins content, and relatively low content of naphthenes and olefins. The content of aromatics in the gasoline is about 41 wt%, which is similar to the conventional MTG gasoline, but much higher than that in commercial gasoline.

Many countries currently set limits on aromatics and olefins contents in gasoline; the DTG gasoline would need to be refined to meet the new regulations in the most countries, and it also can be used as a blending agent in gasoline pool.

The team noted that among the aromatics, durene, which has a high melting point (79 ˚C) due to its high molecular symmetry, must be minimized in the gasoline. The content of durene in the DTG gasoline is about 4%, and is acceptable under most conditions, the researchers said.

Resources

• Zhiqi Wang, Tao He, Jianqing Li, Jingli Wu, Jianguang Qin, Guangbo Liu, Dezhi Han, Zhongyue Zi, Zhuo Li, Jinhu Wu (2016) “Design and operation of a pilot plant for biomass to liquid fuels by integrating gasification, DME synthesis and DME to gasoline” Fuel 186, 587–596 doi: 10.1016/j.fuel.2016.08.108