production of hydrocarbon fuels from biomass by catalytic fast pyrolysis

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catalyst activity was evident after five regeneration cycles. ... prior to regeneration are planned for the future. ... http://www.nrel.gov/biomass/pdfs/47764.pdf.
PRODUCTION OF HYDROCARBON FUELS FROM BIOMASS BY CATALYTIC FAST PYROLYSIS Kristiina Iisa Alexander R. Stanton Stefan Czernik National Renewable Energy Laboratory 1067 Cole Blvd. Golden, CO 80401 [email protected] [email protected] [email protected]

ABSTRACT Catalytic fast pyrolysis is a promising technology for producing transportation liquids that are compatible with the current petroleum products and infrastructure. Catalytic fast pyrolysis involves the rapid heating of biomass at intermediate temperatures (400-600°C) and short residence times (1-2 s) in the presence of catalysts. Fast pyrolysis in the absence of catalysts can produce a liquid oil in high yield but the product is highly oxygenated (contains typically 40% O), acidic, and chemically unstable and cannot be used for transportation fuels without further upgrading. Catalysts can promote the deoxygenation of the nascent pyrolysis vapors and convert them to highly deoxygenated oil consisting mainly of hydrocarbons. Zeolite catalysts and in particular HZSM-5 have been found to give the highest yields of hydrocarbons and lowest yields of unwanted by-products such as coke. A major advantage of catalytic pyrolysis is that it can convert all of the main components of biomass – cellulose, hemicelluloses, and lignin - into 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. 1. INTRODUCTION Transportation accounted for approximately 28% of the US primary energy consumption in 2010. (1) While there are a variety of renewable energy sources, biomass offers the only currently available option for producing renewable carbon-based fuels for transportation. Ethanol production from corn and sugar cane is an established technology, and there has been significant progress in the development of processes for the production of ethanol from cellulosic materials. (2,3) However, there is an increasing demand to develop drop-in fuels or intermediates that are compatible with the current refinery and transportation infrastructure. The production of such fuels needs to be done in an environmentally sustainable manner without adversely affecting food production or biodiversity. Liquid transportation fuels can be produced from biomass by biochemical or thermochemical methods. Thermochemical methods in general can convert all fractions of biomass while biochemical conversions are limited to the carbohydrate fraction. This gives thermochemical conversion the potential for higher utilization of biomass resources. The ability to utilize lignin, which constitutes up to 25% of biomass and is difficult to convert, is important.

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A promising method for producing liquid fuels from biomass is fast pyrolysis, which involves the heating of biomass at high heating rates in the absence of oxygen for short times of the order of one second at intermediate temperatures of 450-550°C. The process can produce a liquid product in high yields of up to 70% while retaining a similar fraction of the biomass energy content. (3-5) However, the product (pyrolysis oil, also called bio-oil) is highly oxygenated (contains typically 40% oxygen), acidic, chemically unstable, not miscible with hydrocarbons, not completely distillable, and has a low heating value, and cannot be used for transportation fuels without further upgrading. (4,5). The use of shape-selective zeolite catalysts for deoxygenating pyrolysis oil into hydrocarbons has been studied since the early 1980s. (6-8). Medium-pore size zeolites such as ZSM-5 have proven to give the highest hydrocarbon yields. (8) Aromatic compounds are the main products from zeolite upgrading with high proportions of benzene, toluene, and xylene (BTX). (9-10) The catalysts could be added directly in the pyrolysis reactor for in-situ vapor upgrading, and this process is often called catalytic pyrolysis. Diebold and coworkers (11-13) developed segregated vapor phase upgrading in which vapors from pyrolysis are passed into a separate reactor for upgrading. Recently, vapor phase upgrading whether in the form of an integrated process (catalytic pyrolysis) or segregated in a separate reactor has received intensified attention. The process has been studied in laboratory analytical reactors (14-16) and in bench-scale fluidized bed reactors (17-20). So far, the highest total hydrocarbon yields achieved have been in the range of 12-18%, and the oils from fluidized bed reactors have contained 13-22% oxygen. Czernik (21) reported oil with 10% oxygen. Carlsson has reported in laboratory scale the production of 30% from the carbohydrate fraction. (16) The goal of the research presented here was to establish the feasibility of higher hydrocarbon yields and lower oxygen contents by fast pyrolysis combined with vapor phase upgrading. Of particular interest was the ability to convert lignin and not only the carbohydrate (cellulose and hemicellulose) fraction of biomass. 2. EXPERIMENTAL Catalytic pyrolysis was studied in micro scale in a CDS Pyroprobe 5200 reactor connected to a Hewlett Packard 6890/5973 GC/MS. The Pyroprobe uses a wire coil to

rapidly heat a sample that is placed in a quartz sample tube that was in this case closed in one end. Approximately 0.51.0 mg of biomass was placed in the center of the tube on top of quartz wool followed by a layer of the catalyst and additional quartz wool as shown in Figure 1. The coil was heated at a nominal rate of 1000°C/s to the reaction temperature. The vapors were passed along a heated line to the GC/MS for identification and quantification.

Quartz Wool

Catalyst Biomass

Fig. 1: Diagram of the sample-tube loading for pyrolysis in the Pyroprobe reactor. For product quantification, the GC/MS was calibrated for benzene, toluene, and naphthalene. Quantification for aromatic hydrocarbons beyond these three compounds was done under the assumption that all single-ring aromatics had response factors equal to that of toluene, and all tworing aromatics had response factors equal to that of naphthalene. Each experiment was run in triplicate, and the average result together with the standard error is reported. Catalytic pyrolysis was also tested in a 2-inch bubbling fluidized bed reactor. Approximately 250 g of the catalyst was placed in the reactor to which biomass was fed at a rate of 120 g/h. The product vapors were collected in a train consisting of an electrostatic precipitator and two condensers kept at temperatures. A detailed description of the reactor is given in Czernik (21). The catalyst in the experiments was a ZSM-5-based zeolite from Albemarle (UPV-2). This catalyst was chosen because it had given the highest liquid hydrocarbon yields in a recent study. (22) The catalyst particles were ground, sieved and the particle size 200-350 µm was used in the tests. In the Pyroprobe tests, pine, lignin, and cellulose were pyrolyzed in the presence of the catalyst. The pine was slash pine provided by Idaho National Laboratory and the lignin was from soda pulping of annual plants provided by Granit SA. The cellulose was Avicel® PH-105. Ground mixed wood (predominantly hardwood) with particle size of