Aachen team develops framework for model-based formulation of biofuel blends with tailored properties
A team at RWTH Aachen University has developed a framework for the model-based formulation of biofuel blends with tailored properties by considering the fuel’s molecular composition as the fundamental design degree of freedom. A paper on their work is published in the ACS journal Energy & Fuels.
The researchers envision that the model-based approach can (i) guide fundamental experimental investigations of the combustion behavior of blended biofuels toward the most favorable mixtures and (ii) identify promising conversion pathways for further elaboration by means of reaction engineering and conceptual process design. The latter is ultimately needed to bridge the gap from a mass- and energy-based molecular level analysis to a process level analysis addressing the economics of the involved conversion and separation steps.
Whereas biofuel production based on gasification or pyrolysis is thought to yield mixtures of hydrocarbons designed to match the properties of fossil fuels, the aqueous phase processing of carbohydrates derived from hydrolysis of cellulose and hemicellulose enables selective access to one or few oxygenated fuel components, e.g., ethanol, 2-methyltetrahydrofuran, γ-valerolactone, ethyl- levulinate or 2-methylfuran. The molecular structure of such oxygenated fuel can be tailored to exhibit desired physicochemical properties that unlock the full potential of advanced internal combustion engines.
In an attempt to optimize both production and quality of the fuel, an integrated product and pathway design problem is posed on the molecular level in the present contribution. The complexity of this design problem is driven (i) by the rich variety of oxygenated fuel components that can be obtained from refunctionalization of bioderived sugars in principle, (ii) by the numerous conversion pathways that connect these molecules with the biomass feedstock, and (iii) by the interactions between physicochemical fuel properties and the performance of an internal combustion engine.
… The focus of this contribution lies on the rational formulation of well-defined blends of few bioderived fuel components. As such we will not consider thermochemical biofuel production pathways which typically yield complex multicomponent mixtures of (oxygenated) hydrocarbons. Instead, we will focus on biofuels made through hydrolysis of cellulose and hemi-cellulose followed by selective synthesis of few oxygenated species via chemical and/or fermentation routes. The blends are formulated to maximize a process-related performance indicator, such as mass of fuel produced for a fixed input of biomass, while certain fuel quality requirements, including constraints on the derived cetane number, fuel density, or volatility, are met.—Dahmen and Marquardt
The method combines optimization-based reaction pathway screening with mathematical fuel property prediction by means of structural group contribution and quantitative structure–property relationship modeling in order to formulate and solve an integrated product and pathway design problem.
Case study. The paper describes a case study on the identification of promising blends and respective production routes for a spark-ignition (SI) engine. The goal of the exercise, said Manuel Dahmen and Wolfgang Marquard, both from the Aachen Process Systems Engineering group, was to produce a 100% bio-derived fuel with tailored properties from hexoses and pentoses from lignocellulosic biomass by utilizing renewable hydrogen from carbon-free energy sources such as wind or solar to boost the energy of fuel produced for a given quantity of biomass supplied.
The target engine was a boosted direct-injection spark-ignition engine. The objective was to maximize the energy of fuel produced (in terms of lower heating value) given a fixed feed of biomass; for the exercise, they decided not to constrain the supply of hydrogen, considering it instead as an enabler for directing as many carbon as possible from the carbohydrates toward the fuel, thereby maximizing the energy of fuel produced.
They considered 12 biobased intermediates which can be produced from hexoses and pentoses in large volumes. Reflecting the importance of knock-resistance and fuel volatility, two criteria were applied to select the palette compounds: (i) The normal boiling point of each blend component was below the final boiling point of a typical gasoline (∼225 °C). (ii) No blend component could exhibit a diesel-like autoignition tendency; the derived cetane number (DCN) of all blend components had to be smaller than ∼30.
In the case study, the maximization of the energy of fuel produced yielded a five-components blend rich in 1-butanol (44 mol %), cyclopentane (31 mol %), and ethyl acetate (15 mol %). This design features a high degree of hydrogenation (4.6 molH2/molfuel) and yields 0.75 MJfuel/MJbiomass (Blend A).
If DCN was constrained to ≤9, the result was a seven-components blend comprising 42 mol % ethanol, 26 mol % cyclopentanone, 15 mol % 2-butanone, 7 mol % 1-butene, 6 mol % 1-butanol, 3 mol % cyclopentane, and <1 mol % 1-propanol. (Blend B)
A simpler ternary blend was 1-butanol (45 mol %), ethanol (29 mol %), and cyclopentane (27 mol %). (Blend C). An optimal ternary blend consisted of ethanol (58 mol %), cyclopentane (27 mol %), and 2,5-dimethyltetrahydrofuran (15 mol %). (Blend D).
Manuel Dahmen and Wolfgang Marquardt (2017) “Model-Based Formulation of Biofuel Blends by Simultaneous Product and Pathway Design” Energy & Fuels doi: 10.1021/acs.energyfuels.7b00118