Sascha R.A. Kersten

Hydrodeoxygenation of pyrolysis oil fractions: process understanding and quality assessment through co-processing in refinery units

Hydrodeoxygenation (HDO) of pyrolysis oil fractions was studied to better understand the HDO of whole pyrolysis oil and to assess the possibility to use individual upgrading routes for these fractions. By mixing pyrolysis oil and water in a 2:1 weight ratio, two fractions were obtained: an oil fraction (OFWA) containing 32 wt% of the organics from the whole oil and an aqueous fraction water addition (AFWA) with the remaining organics. These fractions (and also the whole pyrolysis oil as the reference) were treated under HDO conditions at different temperatures (220, 270 and 310 °C), a constant total pressure of 190 bar, and using 5 wt% Ru/C catalyst. An oil product phase was obtained from all the feedstocks; even from AFWA, 29 wt% oil yield was obtained. Quality parameters (such as coking tendency and H/C) for the resulting HDO oils differed considerably, with the quality of the oil from AFWA being the highest. These HDO oils were evaluated by co-processing with an excess of fossil feeds in catalytic cracking and hydrodesulfurisation (HDS) lab-scale units. All co-processing experiments were successfully conducted without operational problems. Despite the quality differences of the (pure) HDO oils, the product yields upon catalytic cracking of their blends with Long Residue were similar. During co-processing of HDO oils and straight run gas oil in a HDS unit, competition between HDS and HDO reactions was observed without permanent catalyst deactivation. The resulting molecular weight distribution of the co-processed HDO/fossil oil was similar to when hydrotreating only fossil oil and independent of the origin of the HDO oil.

Recent developments in fast pyrolysis of ligno-cellulosic materials

Pyrolysis is a thermochemical process to convert ligno-cellulosic materials into bio-char and pyrolysis oil. This oil can be further upgraded or refined for electricity, transportation fuels and chemicals production. At the time of writing, several demonstration factories are considered worldwide aiming at maturing the technology. Research is focusing on understanding the underlying processes at all relevant scales, ranging from the chemistry of cell wall deconstruction to optimization of pyrolysis factories, in order to produce better quality oils for targeted uses. Among the several bio-oil applications that are currently investigated the production and fermentation of pyrolytic sugars explores the promising interface between thermochemistry and biotechnology.

Principles of a novel multistage circulating fluidized bed reactor for biomass gasification

In this paper a novel multistage circulating fluidized bed reactor has been introduced. The riser of this multistage circulating fluidized bed consists of several segments (seven in the base-case design) in series each built-up out of two opposite cones. Due to the specific shape, a fluidized bed arises in the bottom cone of each riser segment. Back-mixing of gas and solids between the segments is prevented effectively. The absence of back-mixing combined with the enlarged solids residence time in each segment (each segment is a fluidized bed) creates the opportunity to operate, spatially divided, separate process steps in a single reactor. The benefit of a concept in which different processes are carried out in separate segments of the same reactor has been demonstrated for the specific case of biomass gasification. In the novel reactor it is possible to create oxidation segments in which O2 reacts exclusively with char (carbon). This results in an increased carbon conversion and consequently improved gasification efficiency. Creating an exclusive char combustion zone, aimed at improving both the carbon conversion and the thermal efficiency, has also been applied successfully in a conventional CFB biomass gasifier (ECNs CFB 100 kg wood/h) by building a flow restriction in the riser between the primary air nozzles and the biomass feed point.

Pyrolysis based bio-refinery for the production of bioethanol from demineralized ligno-cellulosic biomass

This paper evaluates a novel biorefinery approach for the conversion of lignocellulosic biomass from pinewood. A combination of thermochemical and biochemical conversion was chosen with the main product being ethanol. Fast pyrolysis of lignocellulosic biomasss with fractional condensation of the products was used as the thermochemical process to obtain a pyrolysis-oil rich in anhydro-sugars (levoglucosan) and low in inhibitors. After hydrolysis of these anhydro-sugars, glucose was obtained which was successfully fermented, after detoxification, to obtain bioethanol. Ethanol yields comparable to traditional biochemical processing were achieved (41.3% of theoretical yield based on cellulose fraction). Additional benefits of the proposed biorefinery concept comprise valuable by-products of the thermochemical conversion like bio-char, mono-phenols (production of BTX) and pyrolytic lignin as a source of aromatic rich fuel additive. The inhibitory effect of thermochemically derived fermentation substrates was quantified numerically to compare the effects of different process configurations and upgrading steps within the biorefinery approach.

Liquefaction of Lignocellulosic Biomass: Solvent, Process Parameter, and Recycle Oil Screening

The liquefaction of lignocellulosic biomass is studied for the production of liquid (transportation) fuels. The process concept uses a product recycle as a liquefaction medium and produces a bio-oil that can be co-processed in a conventional oil refinery. This all is done at medium temperature (≈ 300 °C) and pressure (≈ 60 bar). Solvent-screening experiments showed that oxygenated solvents are preferred as they allow high oil (up to 93% on carbon basis) and low solid yields (≈ 1-2% on carbon basis) and thereby outperform the liquefaction of biomass in compressed water and biomass pyrolysis. The following solvent ranking was obtained: guaiacol>hexanoic acid ≫ n-undecane. The use of wet biomass results in higher oil yields than dry biomass. However, it also results in a higher operating pressure, which would make the process more expensive. Refill experiments were also performed to evaluate the possibility to recycle the oil as the liquefaction medium. The recycled oil appeared to be very effective to liquefy the biomass and even surpassed the start-up solvent guaiacol, but became increasingly heavy and more viscous after each refill and eventually showed a molecular weight distribution that resembles that of refinery vacuum residue.

Supercritical water gasification of sewage sludge: gas production and phosphorus recovery

In this study, the feasibility of the gasification of dewatered sewage sludge in supercritical water (SCW) for energy recovery combined with P-recovery from the solid residue generated in this process was investigated. SCWG temperature (400°C, 500°C, 600°C) and residence time (15min, 30min, 60min) were varied to investigate their effects on gas production and the P recovery by acid leaching. The results show that the dry gas composition for this uncatalyzed gasification of sewage sludge in SCW mainly comprised of CO2, CO, CH4, H2, and some C2-C3 compounds. Higher temperatures and longer residence times favored the production of H2 and CH4. After SCWG, more than 95% of the P could be recovered from the solid residue by leaching with acids. SCWG combined with acid leaching seems an effective method for both energy recovery and high P recovery from sewage sludge.

Recycling nutrients in algae biorefinery

Algal fuel cells: Repeated nutrient recycling is demonstrated by reusing the aqueous phase obtained from the hydrothermal liquefaction (HTL) of microalgae. This is achieved, for the first time, by performing a complete set of four continuous growth–HTL cycles. Results show similar growth rates in each cycle, the potential of nutrient reduction, as well as cell morphology changes. This study demonstrates progress towards the standalone operation of algae biorefineries

CO2-enhanced extraction of acetic acid from fermented wastewater

The industrial process of recovering fermentation-based volatile fatty acids (VFAs) utilizes H2SO4 to acidify the fermentation broth containing VFA-salts [e.g. Ca(CH3COO)2] to enable formation of molecular VFAs. Molecular VFAs are then recovered by liquid–liquid extraction. However, acidification with H2SO4 results in production of large quantities of salts (e.g. CaSO4). Using CO2 rather than mineral acids for acidification of fermentation broth is an environmentally benign alternative which eliminates salt formation. In this study, CO2 was applied in pressures up to 40 bar to enhance the efficiency of extraction of acetic acid (HAc) from fermented wastewater. HAc extraction under atmospheric conditions was also investigated to obtain benchmarks. The ionic liquid [P666,14][Phos] and trioctylamine (TOA) dissolved in n-octanol were applied as solvents to extract HAc from fermented wastewater model solutions containing HAc (1 wt%) and various salts resulting in pH ranging from 2.8 to 6. A more pronounced increase in extractability of HAc, expressed as HAc distribution (D = [HAc]solvent/[HAc]aqueous), was observed for [P666,14][Phos] with increasing CO2 pressure. A mathematical model taking into account carbonic acid equilibria and dissociation of HAc and salts showed that the measured influence of CO2 cannot be explained by the effect of CO2 on aqueous phase pH. Thus, it may be concluded that the pressurized CO2 has altered the fluid properties of the solvents and made them more accessible for HAc. This suggests that applying pressurized CO2 may enhance extraction efficiency of processes other than those involving extraction of volatile fatty acids.

Aromatics extraction from pyrolytic sugars using ionic liquid to enhance sugar fermentability

Fermentative bioethanol production from pyrolytic sugars was improved via aromatics removal by liquid-liquid extraction. As solvents, the ionic liquid (IL) trihexyltetradecylphosphonium dicyanamide (P666,14[N(CN)2]) and ethyl acetate (EA) were compared. Two pyrolytic sugar solutions were created from acid-leached and untreated pinewood, with levoglucosan contents (most abundant sugar) of 29.0% and 8.3% (w/w), respectively. In a single stage extraction, 70% of the aromatics were effectively removed by P666,14[N(CN)2] and 50% by EA, while no levoglucosan was extracted. The IL was regenerated by vacuum evaporation (100mbar) at 220°C, followed by extraction of aromatics from fresh pyrolytic sugar solutions. Regenerated IL extracted aromatics with similar extraction efficiency as the fresh IL, and the purified sugar fraction from pretreated pinewood was hydrolyzed to glucose and fermented to ethanol, yielding 0.46g ethanol/(g glucose), close to the theoretical maximum yield.

The interplay between chemistry and heat/mass transfer during the fast pyrolysis of cellulose

Biomass derived sugars are expected to play an important role as platform chemicals. Herein, we have shown that in the temperature range of 370 °C to 765 °C of the heat source a constant high sugar yield of ∼70% (C-basis) can be obtained from the fast pyrolysis of Avicel cellulose while producing hardly any gas (<1%) and solid residue (<1% above 450 °C). This opens the opportunity to combine the advantages of thermochemical processes, such as high conversion rates and products not being heavily diluted with water, with an increased value of the product slate. In this paper, firstly the screen-heater used to study the very early stages of cellulose pyrolysis is introduced and characterized. Secondly, yield data as a function of process and pyrolysis conditions are presented and interpreted, also using mathematical models, with respect to chemistry, heat transfer, mass transfer and their interplay. It has been shown that next to heat transfer and the residence time in the vapor phase also the escape rate of products from the reacting particle (mass transfer) is a key process determining the overall mass loss rate and/or the product distribution.