Autothermal pyrolysis is the Bioeconomy Institute’s latest development in thermal deconstruction of biomass into biofuels and biochemicals. BEI is developing the basic technology and is working to demonstrate it on a commercial scale.
In autothermal pyrolysis, only air is used as the fluidizing gas. As a result, the energy for pyrolysis is provided through partial oxidation of pyrolysis products within the reactor. By eliminating the heat transfer bottleneck of conventional pyrolysis, several-fold process intensification is achieved.
Autothermal pyrolysis has three advantages:
- Simplified reactor design. Air-blown operation removes the need for external thermal energy and gas recycle, allowing the design to be easily scaled up.
- Process intensification, which has the goal of increasing outputs of desired products with fewer inputs of chemicals, water, energy, labor, and/or capital and fewer outputs of wastes and pollutants. With autothermal pyrolysis, biomass feed rate can be several fold higher than for conventional pyrolysis. Process intensification of pyrolysis makes possible the construction of smaller, modular systems suitable for distributed processing of dispersed biomass feedstocks.
- Reduced capital costs of more than 25 percent, according to technoeconomic analysis.
We have modified our well-known pilot system to research autothermal pyrolysis. In this system, bio-oil is recovered in fractions, a patented BEI technology. The products include sugars for cellulosic ethanol and phenolic oils for production of bio-asphalt and diesel engine fuels. Autothermal pyrolysis also produces an excellent biochar, which can serve as a soil amendment and helps make the process carbon negative.
BEI is working with private and public partners on demonstration-scale projects of the technology. This effort is being accelerated by the DOE-funded RAPID Institute, the country’s tenth Manufacturing USA initiative. The project includes pilot-scale testing and design of components for the demonstration plant. We’re also analyzing life-cycle costs and the environmental impact.
Demonstrating Autothermal Pyrolysis at 50 Tons per Day Scale
Iowa project (Redfield, IA)
- Privately financed by Stine Seed Farms
- Engineering, Procurement and Construction provided by Frontline Bioenergy
- Conversion of corn stover into sugars, phenolic oil and biochar
Autothermal pyrolysis, by converting biogenic carbon into recalcitrant carbon that can be sequestered in croplands, has prospects for large-scale removal of carbon dioxide from the atmosphere. Our work in carbon removal via autothermal pyrolysis was acknowledged in 2022 by receipt of a Milestone Award from the XPRIZE Carbon Removal Program sponsored by the Musk Foundation.
See our Autothermal Pyrolysis flyer
Here are some BEI publications in autothermal processing:
Brown, R.C. (2021) Heterodoxy in Fast Pyrolysis of Biomass, Energy & Fuels 35, 987-1010
As the field of pyrolysis has matured, it has been accompanied by a kind of orthodoxy in its practice. Among these orthodoxies are: (1) oxygen should be excluded from the pyrolysis process; (2) little sugar is produced during pyrolysis; and (3) the major product of pyrolysis is a low-value emulsion in water. Adherence to these tenets is an impediment to the commercial development of fast pyrolysis. Over the past fifteen years, research at Iowa State University's Bioeconomy Institute has challenged these tenets with what might be called heterodoxy in the science and engineering of fast pyrolysis: adding oxygen, producing sugars and fractionating bio-oil into valorized products. This paper reviews these new approaches to pyrolysis and concludes with an outlook for further developing them.
Peterson, C.A., Brown, R.C. (2021) Global Gas-Phase Oxidation Rates of Select Products from the Fast Pyrolysis of Lignocellulose, Energy & Fuels 35, 17103-17113
The oxidation kinetics for products of fast pyrolysis at low temperatures (<600°C) are not well known. These will be important in effort to model autothermal pyrolysis, which has been recently developed to intensify the process, but which occurs at much lower temperatures than combustion. This study determines global oxidation rates at 400-600°C for three important products of fast pyrolysis: levoglucosan, xylose and acetic acid. Experiments were performed in a fluidized bed pyrolyzer with the reactor modeled as a series of CSTRs and PFRs to determine reaction rates. Oxidation rates at 500°C for the three model compounds varied by a factor of ten.
Peterson, C.A., Lindstrom J., Polin, J., Cady, S.D., Brown, R.C. (2020) Oxidation of Phenolic Compounds During Autothermal Pyrolysis of Lignocellulose, Journal of Analytical and Applied Pyrolysis 149, 104853
Fast pyrolysis is traditionally defined as the rapid decomposition of organic material in the absence of oxygen to produce primarily a liquid product known as bio-oil. However, the introduction of small amounts of oxygen to the process holds prospects of internally generating the energy needed for pyrolysis. The present study investigates the partial oxidation of lignin-derived compounds during pyrolysis, which generates both carbon oxides and aromatic carbonyl compounds. Analysis of lignin derived phenolic compounds was performed to determine if the composition had changed under oxidative conditions. NMR analyses indicates aromatic carbonyls increased under oxidative conditions, with a corresponding decrease in phenolic hydroxyl groups. Model phenolic compounds were pyrolyzed to help understand the role of partial oxidation during autothermal pyrolysis of lignocellulosic biomass.
Peterson, C.A., Brown, R.C. (2020) Oxidation Kinetics of Biochar from Woody and Herbaceous Biomass, Chemical Engineering Journal 401, 126043
The goal of this study was to determine oxidation kinetics for biochar produced from fast pyrolysis of various biomass feedstocks. In particular, the role of inherent ash content on the oxidation rate was evaluated. Thermogravimetric analysis (TGA) and fluidized bed combustion experiments were used to explore oxidation kinetics of six fast pyrolysis produced biochars with diverse ash content. Reaction rates varied by a factor of three under chemical kinetic-limited conditions, demonstrating inorganic content impacts oxidation rate. Chemical kinetic rate coefficients were proposed as a function of compositional parameters to determine overall fit and impact. Potassium content was found to have a positive correlation, best describing the differences in the oxidation kinetic rate coefficients. Additionally, feedstocks were subjected to a 1M HCL acid wash mitigating the catalytic activity of the metals. Acid washed biochars had lower oxidation kinetic rates compared to their unwashed counterparts, indicating the removal of catalytically active metals reduced oxidation rate. Gas composition (CO/CO2) was measured during fluidized bed experiments for both acid-washed and unwashed biochar, which varied between the six biochars. Formation of CO2 was greatly affected by catalytic metals, finding potassium content to correlate well with a higher percentage of CO2 formation as compared to CO. Comparison of oxidation rates were made between the two experimental apparatuses to measure the effect of attrition on biochar oxidation.
Brown, R.C. (2020) Process Intensification Through Directly Coupled Autothermal Operation of Chemical Reactors, Joule 4, 1-22
Autothermal operation of a chemical reactor involves coupling exothermic and endothermic chemical reactions for the purpose of thermal management without resorting to external energy sinks or sources. Often this is accomplished through regenerative or recuperative heat exchange between spatially or temporally separated exothermic and endothermic reactions. However, it is also possible to directly couple these reactions simultaneously within the same reactor volume, eliminating the heat transfer bottleneck that characterizes much of chemical manufacture. It is not widely recognized that directly coupled autothermal operation allows dramatic process intensification. This perspective defines autothermal operation and contrasts it with conventional heat transfer for thermal management of chemical processes. The intensification and cost savings that can be achieved are quantified and the implications to modular chemical process intensification are presented. Guidelines are proposed for designing directly coupled autothermal processes. Diverse examples are presented. The several challenges to expanding the field are discussed.
Polin, J.P., Carr, H. D., Whitmer, L. E., Smith, R. G., Brown, R. C. (2019) Conventional and Autothermal Pyrolysis of Corn Stover: Overcoming the Processing Challenges of High-Ash Agricultural Residues, Journal of Analytical and Applied Pyrolysis 143, 104679.
The high ash content of agricultural residues and other kinds of herbaceous biomass makes it a challenging feedstock for fast pyrolysis to bio-oil. Using corn stover as a representative feedstock, this study investigates fast pyrolysis of high ash, herbaceous biomass in a pilot-scale fluidized bed reactor using both conventional, nitrogen-blown and autothermal, air-blown operation. Initial efforts to pyrolyze corn stover were challenged by bed fouling, which prevented steady reactor operation. Substitution of coarser bed material allowed operation at higher superficial velocities, which promoted attrition and elutriation of recalcitrant biochar particles from the reactor. This resulted in dramatic improvement in stable reactor operation for both conventional and autothermal pyrolysis with bio-oil yields among the highest reported for pyrolysis of corn stover. The oxygen-to-biomass equivalence ratio required for autothermal operation was 6.8%. Autothermal operation also resulted in significant process intensification, increasing corn stover throughput from 7.8 kg hr-1 to 21.9 kg hr-1 for this 8.9 cm diameter reactor. Air-blown, autothermal operation did not significantly reduce bio-oil yield despite the presence of partial oxidation reactions. Carbon balances indicate carbon yields of biochar and aqueous, bio-oil light ends decreased by 18.5% and 4.7%, respectively, during autothermal pyrolysis compared to conventional pyrolysis while the more valuable, organic-rich heavy ends of the bio-oil were essentially preserved.
Polin, J. P., Peterson, C. A., Whitmer, L. E., Smith, R. G., Brown, R. C. (2019) Process Intensification of Biomass Fast Pyrolysis through Autothermal Operation of a Fluidized Bed Reactor, Applied Energy 249, 276-285.
Heat transfer is the bottleneck to fast pyrolysis of biomass. Although the enthalpy for pyrolysis of biomass is relatively small operation at temperatures around 500°C constrains heat carrier selection to inert gases and granular media that can sustain only modest thermal fluxes in practical pyrolysis systems. With heat transfer controlling the rate of pyrolysis, reactor capacity only scales as the square of reactor diameter and does not benefit from economies of scale in building larger reactors. We have eliminated this heat transfer bottleneck by replacing it with partial oxidation of pyrolysis products to provide the enthalpy for pyrolysis in a fluidized bed reactor, a process that can be described as autothermal pyrolysis. The oxygen-to-biomass equivalence ratio depends upon the kind of biomass being pyrolyzed and the level of parasitic heat losses from the reactor, but under conditions that simulate adiabatic operation, equivalence ratios are around 0.10, compared to 0.20 or higher for autothermal gasifiers. At this low equivalence ratio, there was no significant loss in bio-oil yield when operating the reactor autothermally (64.8 wt.%) as compared to conventional pyrolysis (64.4 wt.%). Carbon balances indicate that less valuable pyrolysis products (char and aqueous, bio-oil light ends) are consumed via partial oxidative reactions to provide the enthalpy for pyrolysis. While the carbon yields of char and bio-oil light ends decreased by 25.0% and 21.3%, respectively, the most valuable pyrolysis product (bio-oil heavy ends) only decreased 8.0%.
Kim, K. H., Brown, R., Bai, X. (2014) Partial Oxidative Pyrolysis of Acid Infused Red Oak Using a Fluidized Bed Reactor to Produce Sugar-Rich Bio-oil, Fuel 130, 135-141.
Acid infusion of lignocellulosic biomass as a pretreatment prior to fast pyrolysis has been shown to significantly increase the yield of sugar in the products. However, under these conditions char formation increases forming large agglomerates that clog the reactor and eventually interrupt operation of the system. In the present study, partial oxidative pyrolysis of acid infused red oak was performed in a fluidized bed reactor at 500 °C with the concentration of oxygen in the sweep gas ranged from 0 – 8.4 vol% in an effort to mitigate char agglomeration. The addition of oxygen reduced char agglomeration by up to 88.9 % compared to the control run during pyrolysis ensuring continuous run of the reactor. Moreover, the addition of oxygen increased the total sugar content in the bio-oil to as high as 20.6 g/100 g biomass. Stage fraction 1 (heavy fraction) of bio-oil obtained from oxidative condition contained up to 67 % of hydrolysable sugar and it was less acidic compared to standard pyrolysis.