NREL researchers report on two approaches to upgrading biomass pyrolysis oil for hydrocarbon fuels

19 June 2012

At last month’s World Renewable Energy Forum 2012 in Denver, Colorado, researchers from the US National Renewable Energy Laboratory presented papers on two different approaches to upgrade pyrolysis oils to hydrocarbon fuels or fuel intermediates.

Fast pyrolysis—the rapid heating of biomass to intermediate temperatures (400-600 °C) in the absence of oxygen and the rapid cooling of the resulting vapors to bio-oil—is an efficient method to convert all fractions of biomass, including lignin, into a liquid product: pyrolysis oil. Fast pyrolysis can convert about 70% of the mass and energy into the liquid product. However, bio-oil, or pyrolysis oil, comprises many oxygenated organic chemicals with water-miscible and oil-miscible fractions.

While combustible, it is not 100% miscible with hydrocarbons. It has only a modest modest heating value of ~17 MJ/kg; a high density ~1.2 kg/l; it is acidic (pH ~2.5); and it ages badly, with viscosity increasing over time. In short, bio-oil needs to be upgraded to produce a fungible liquid transportation fuel.

The NREL team of Kristiina Iisa, Alexander Stanton, Stefan Czernik found in their study that catalytic fast pyrolysis with vapor phase upgrading can produce hydrocarbons compatible with current transportation fuels. NREL researcher Richard French found that partial hydrogenation of bio-oil to produce a liquid with acceptable physical and chemical properties and then completing the upgrading in a petroleum refinery where economies of scale and existing infrastructure can be used to advantage can reduce cost of upgrading.

Catalytic fast pyrolysis with vapor phase upgrading. While fast pyrolysis in the absence of catalysts can produce high-yields of bio-oil, the product is highly oxygenated (contains typically 40% O), acidic, and chemically unstable. Catalysts, noted Iisa et al. can promote the deoxygenation of the nascent pyrolysis vapors and convert them to highly deoxygenated oil consisting mainly of hydrocarbons.

We studied the liquid hydrocarbon formation from woody biomass and biomass fractions over zeolite catalysts. Similar hydrocarbon mass yields were obtained both from cellulose and lignin in micro-scale experiments. Woody biomass gave up to 36% carbon conversion to pure hydrocarbons at optimum conditions. Alkylated one-and two-ring aromatic compounds were the main products though some phenolic compounds were present at lower catalyst-to-biomass loadings as well. Oil with oxygen content of less than 3% was successfully produced in a bench-scale fluidized bed reactor.

—Iisa et al.

The use of shape-selective zeolite catalysts for deoxygenating pyrolysis oil into hydrocarbons has been studied since the early 1980s, the researchers noted. Medium-pore size zeolites such as ZSM-5 have proven to give the highest hydrocarbon yields. The team’s goal was to establish the feasibility of higher hydrocarbon yields and lower oxygen contents by fast pyrolysis combined with vapor phase upgrading.

Among their findings were that for pine, the average carbon conversion to liquid-range hydrocarbons was 36% with a catalyst-to-biomass ratio of 10:1 at 600°C. This conversion presents more than 50% of the theoretical yield. If biomass could be converted to hydrocarbons with similar conversion in large scale, yields of 50-60 gallons of hydrocarbons per metric dry ton biomass could be obtained.

The results suggest that if vapor phase upgrading is successfully developed into a commercial process, significant quantities of renewable transportation fuels can be produced. The laboratory-scale conversions in this work if applicable in industrial scale translate to the production of 50-60 gallons of hydrocarbon liquids per dry metric ton of biomass. Future work is required in developing more efficient and robust catalysts and developing integrated processes. Minimizing coke formation and overcoming the hydrogen deficiency of biomass remain challenges.

—Iisa et al.

Partial hydrogenation of bio-oil to intermediates. Bio-oil liquids can be upgraded to a gasoline- or diesel-like liquid via hydrogen and a catalyst in a process similar to petroleum hydrotreatment. However, notes French, both hydrogen and transportation costs are high.

The Global Energy Management Institute (GEMI) at the University of Houston suggested an approach to reduce costs could be the mild hydrotreating of the bio-oil and then co-processing the partially deoxygenated products with petroleum-derived material in a refinery, thus taking advantage of the economies of scale and existing, highly efficient infrastructure of refineries. GEMI recommended reducing the severity of hydrotreating to leave about 7% oxygen in the bio-oil (instead of the original 35–40%), thus avoiding hydrogenating aromatics while reducing hydrogen consumption, catalyst costs, and hydrotreater capital costs. The residual acidity of the oil could then be accommodated by diluting with crude oil or an internal refinery stream (naphtha, gas oil, etc.).

For this approach to be acceptable for refineries, however, the products from the mild or partial hydrotreating would have to meet a number of criteria, including:

• the acidity of the bio-oil must be reduced from the typical TAN (total acid number) value of more than 100 to about 15, assuming that hydrotreated bio-oil would be blended in a 1:8 ratio;

• the hydrotreated bio-oil must be miscible with hydrocarbons; and

• the hydrotreated bio-oil must be highly volatile so that it is amenable to fractional distillation.

French’s objective was to evaluate the impact of catalyst type (precious metals versus traditional NiMo catalyst) on the hydrotreating process. Bio-oil was hydrotreated in the presence four precious metal catalysts at varying temperatures and pressures and the results were compared to those of a more standard NiMo catalyst. The aim was to produce oil suitable as refinery-ready intermediate at a carbon conversion of 55% or above. The oil quality criteria were good volatility (>90% volatile matter); low oxygen content (<10%); low TAN (<15, to give an acid number after blending < 2); and a miscibility of at least 1:10 in representative hydrocarbons.

These results show that the volatility and miscibility criteria and the desired oxygen content can be achieved with several of the catalysts. The severe condition with the nickel catalyst also met the acidity target. Hydrogen consumption was considerably less (1.2-2.0% vs. 5%) than that used in the design case. Thus, it is reasonably expected that a product satisfactory for blending into a refinery can be achieved at a yield (based on carbon) of 55%. The precious metal catalysts may also produce a satisfactory product by water washing of the product to remove acid.

Thus two possibilities are suggested for an improved process—the use of nickel on a stable support or the use of a precious-metal catalyst followed by water washing of the light product. Platinum has added promise because of its reduced hydrogen consumption.

—French 2012

Resources

• Kristiina Iisa, Alexander Stanton, Stefan Czernik (2012) Production of Hydrocarbon Fuels from Biomass by Catalytic Fast Pyrolysis (WREF 2012, 0736)

• Richard French (2012) Partial Hydrogenation of Biomass Pyrolysis Oils to Liquid-Fuel Intermediates (WREF 2012, 0739)