(133c) Biomass Pyrolysis Under Reactive Gas Environments

In light of worldwide concern over CO2 emissions and climate change, biomass has received attention as a future fuel source due to its renewable, carbon-neutral nature. Though many methods exist for utilizing biomass as an energy resource, a liquid product produced from biomass known as bio-oil has economic and energetic advantages due to its ease in transport. Bio-oil is typically produced by the fast pyrolysis of biomass, a process in which biomass is thermally decomposed quickly in the absence of oxygen to produce a predominantly liquid product (Czernik and Bridgwater 2004); typically, an inert species such as N2 is used for the reaction atmosphere. Though bio-oil produced by pyrolysis offers promise as an alternative to petroleum fuels, several properties of the substance, such as high oxygen content, make it undesirable as an intermediate for fuels or chemical production without additional upgrading or refining. The presence of oxygen in bio-oil negatively impacts its heating value and increases its hydrophilicity to the point that it does not blend readily with conventional petroleum products; further, the oxygenated component has proven rather difficult to remove in an efficient manner (Czernik and Bridgwater 2004; Oasmaa and Czernik 1999). Measures for downstream bio-oil upgrading such as hydrotreating or hydrocracking have the capability to produce gasoline-range hydrocarbons, but offer low liquid fuel yields per ton of biomass. Thus, an in-situ modification of bio-oil seems to be a logical strategy to improve the properties of bio-oil in the interest of increasing its value for use as fuel or a chemical feedstock.

In an effort to reduce the amount of oxygen contained in bio-oil, researchers have investigated pyrolysis in the presence of H2 (hydropyrolysis), a process which has previously received extensive attention in the field of coal pyrolysis (Braekman-Danheux et al 1995; Liao et al 1998). Hydropyrolysis of biomass has been shown to produce bio-oil with a reduced oxygen content compared to that produced by pyrolysis under N2 (Onay et al 2006; Rocha et al 1999). Other advantages in the conversion of biomass via hydropyrolysis include increased overall biomass conversion, higher bio-oil yields, decreased production of char and the improved generation of value-added chemicals (Liao et al 1998). Pyrolysis of coal has been performed under reactive gases other than H2, such as CH4, and has shown interesting results. Coal pyrolysis in a CH4 environment produces enhanced yields of valuable chemicals such as ethylene and benzene in the product, reaching up to five times the amount produced by coal pyrolysis under N2 at similar conditions (Steinberg 1987).

Given the possibilities of improved bio-oil quality and yield as well as using biomass as a feedstock for value-added chemicals, an investigation of biomass pyrolysis under environments such as CH4, coal- and/or bio-syngas seems warranted. With this in mind, the objective of this study is to evaluate biomass pyrolysis under reactive gas environments as a method to favorably alter the chemistry and yield of the pyrolysis bio-oil product.

All experimental trials in this study are performed in RTI's bench-scale pyrolysis unit. The unit has a maximum biomass feedrate of 200 g/hr and includes a cyclone to remove char from the gas stream exiting the pyrolysis reactor. A jacketed condenser and quench vessel are used to partition bio-oil exiting the reactor from pyrolysis off-gas. Fast biomass pyrolysis studies are conducted using various reactive gas environments (described below) at experimental conditions previously found to give high bio-oil yields from the pyrolysis unit; these operating conditions include a biomass residence time of 1-2 seconds and a pyrolysis temperature of 400 - 550°C at atmospheric pressure. Fully characterized woody biomass is used as the feedstock for these pyrolysis experiments.

Reactive gases considered in this study include H2, CO and CH4, and each of these species react at pyrolysis conditions with oxygen contained in biomass to form H2O and/or CO2. Considering that woody biomass is generally 40% oxygen by weight, the concentration of each reactive gas is adjusted to theoretically remove or convert 100% of the oxygen contained in the biomass. The total gas flow rate is set to achieve the desired residence time in the reactor system with the balance of the carrier gas being N2. Product gas is analyzed by online gas chromatography, while collected bio-oil is characterized by elemental analysis and GC/MS. Bio-oil, gas and char yields from each pyrolysis trial are calculated to determine material balances over the system.

References

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