Fast Pyrolysis of Wheat Straw in the Bioliq Pilot Plant - American

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FAST PYROLYSIS OF WHEAT STRAW IN THE BIOLIQ® PILOT PLANT Cornelius Pfitzer, Nicolaus Dahmen, Nicole Troeger, Friedhelm Weirich, Jörg Sauer, Armin Günther, and Matthias Müller-Hagedorn Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01412 • Publication Date (Web): 29 Aug 2016 Downloaded from http://pubs.acs.org on September 1, 2016

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FAST PYROLYSIS OF WHEAT STRAW IN THE BIOLIQ® PILOT PLANT Cornelius Pfitzer1, Nicolaus Dahmen*, Nicole Tröger, Friedhelm Weirich, Jörg Sauer Karlsruhe Institute of Technology (KIT), Institute of Catalysis Research and Technology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany Armin Günther Air Liquide Global E&C Solutions Germany GmbH, Olof-Palme-Str. 35, 60439 Frankfurt am Main, Germany Matthias Müller-Hagedorn Air Liquide Forschung und Entwicklung GmbH, Gwinnerstraße 27-33, 60388 Frankfurt am Main, Germany

KEYWORDS Fast pyrolysis, twin screw mixer reactor, wheat straw, bioliq process, biosyncrude, bio-oil, ageing

ABSTRACT. Fast pyrolysis is the first step of the bioliq® concept, which is developed at the Karlsruhe Institute of Technology (KIT) together with Air Liquide (Lurgi Technologies) for synthetic fuels production from lignocellulosic biomass via gasification. In the 2 MW bioliq® fast pyrolysis pilot plant shredded wheat straw is mixed with a hot heat carrier (sand) in a twin

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screw mixer reactor. At a temperature of 500 °C and in the absence of oxygen the biomass particles are rapidly decomposed within seconds and pyrolysis gas, solids, as well as organic and aqueous condensate are produced. Representative results of the product yields and properties obtained from selected pyrolysis test campaigns from 2013 to 2015 are presented. It is shown that the mass ratio between the two liquid condensates can be adjusted by appropriate process design and operating conditions. Product stability is discussed, giving evidence that by process internal recycling of the organic condensate a controlled thermal maturing can be performed. It could be demonstrated in pilot scale, that stable pyrolysis products can be produced from ash rich biomass feedstocks like wheat straw.

INTRODUCTION Fast pyrolysis processes convert solid, organic feedstock in the absence of oxygen into liquid, gaseous and solid products at temperatures typically around 500 °C. They are characterized by a high rate of heating the particulate feedstock, gas residence times of a few seconds and an instant cooling of the produced vapors. As a consequence, a maximum of liquid condensate, also referred to as bio-oil, is obtained. Bio-oil appears as a dark-brown viscous liquid with a higher heating value (HHV) of around 22 MJ/kg being roughly half that of fossil crude oil. It consists of several hundred organic species with a relatively high oxygen content compared to fossil oil. A comprehensive overview on the current state-of-the art regarding process fundamentals, technologies, feedstocks as well as product yields and properties is given in the comprehensive reviews by R. Venderbosch and W. Prins as well as by A. Bridgwater, both containing a valuable list of references.1,2 The technical development of fast pyrolysis processes is observed and


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frequently reported by the IEA Bioenergy Task 34 “Direct thermal liquefaction”.3 After approx. 30 years of intensive research and development work fast pyrolysis plants are in commercial operation. Envergent in Canada and USA, Fortum in Finland4, and Empyro in the Netherlands5 produce bio-oil as heating fuel in a scale of several 10.000 t/a each. As feedstock, mainly wood is in use, which is easy available from the market and leads to good-natured bio-oils. Due to its chemical and physical properties and combustion behavior crude bio-oil is not directly applicable as motor fuel. Also it is practically not miscible with diesel, gasoline or other bio-fuels.6,7 Quality improvements for bio-oil applications others than heating such as the use as refinery feed can be achieved by upgrading as reviewed e. g. by S. Xiu.8 Upgrading aims at stabilization, deoxygenation, and eventually reduction of molecular mass of bio-oil components by cracking. According to the desired application, a multistage hydrogenation process becomes necessary, which at the example of upgrading bio-oil to a refinery feed is reported by Venderbosch.9 The aim of the fast pyrolysis process developed at KIT is not bio-oil production for direct use as heating or transportation fuel, but to produce a gasification fuel for pressurized entrained flow gasifier in the bioliq® BtL (biomass to liquids) process for synthetic bio-fuels production.10 For this purpose, the solid pyrolysis products are suspended in the liquid bio-oil forming a bio-slurry or biosyncrude as an intermediate fuel with high energy density, allowing for more convenient handling, storage, and transportation compared to the original biomass. Liquid-like slurries appear to be suitable gasifier fuels which can easily be pumped and atomized into a pressurized entrained flow gasification reactor.11 Compared to the feedstock requirements of combustion engines, gasification processes can be specially designed in such a way that they are capable to utilise much lower feedstock qualities, e. g. containing also solids and ash. In an entrained flow gasifier equipped with a cooling screen


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ash is even needed to generate a molten slag layer at the inner gasifier wall to protect it from corrosion and erosion.12 The molten slag is then flowing out of the reactor and recovered after solidification in a water quench. The lower requirements on bio-oil quality as gasifier feed allow for simplifications of the fast pyrolysis process itself. As an example, no complete separation of char from the condensate is required. Bio-oils contain approx. 60±10 % of the initial biomass energy, but combined with the pyrolysis char, the energy content into biosyncrude can be raised up to 85 %.13 Therefore, the pyrolysis products are conditioned in a way that stable gasification fuels can be obtained by forming suspensions from the liquid condensates and solids containing both, char and ash.14 In an application scenario, this pre-treatment may occur in de-centralized facilities, while the gasification and subsequently following gas cleaning and synfuel synthesis require large, industrial facilities due to economy-of-scale reasons. To conduct fast pyrolysis of biomass, different reactor types are available. 3 Heat carrier is contacted with the finely chopped biomass feedstock in fluid bed reactors, but also by assistance of mechanical mixing in an auger or the rotating cone reactor. Also biomass in larger pieces and logs can be converted by ablative flash-pyrolysis. The so called twin screw mixer reactor as used in the bioliq® fast pyrolysis has originally been developed in Germany by Lurgi more than half a century ago for the fast pyrolysis of fossil materials like coal, oil shale or vacuum residues of an oil refinery and was commercially applied as Lurgi-Ruhrgas process.15,16 Twin screw mixer reactors operated at 600 °C with up to 1.1 m outer screw diameter and 600 m³/h circulating heat carrier were already built. The essential design characteristic is the circulation of a hot, grainy heat carrier in a closed loop via the mixer reactor and a heat exchanger. The heat exchanger and the reactor are the source and the sink of heat in the closed loop, respectively. Heat carrier


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circulation is maintained either by a pneumatic or a mechanical heat carrier lift followed by gravity flow. In this paper, the lay-out, operation procedure and test run results of the fast bioliq® fast pyrolysis pilot plant are presented. Detailed results on pilot plant operation in this stage of process development are not frequently reported in literature, commonly restricted by to Intellectual Property Rights related issues.

PILOT PLANT DESCRIPTION The fast pyrolysis pilot plant as well as the other process steps of the bioliq® process were designed to make use of lignocellulosic biomass with high ash content. The conceptual design and engineering was based on bench scale tests with different types of wood and straw in 5-10 kg/h fast pyrolysis process development units available at KIT and Lurgi. Lurgi, today Air Liquide, was contracted for plant design, construction and commissioning. Operation of the pilot plant was jointly carried out within a cooperation and license agreement with Air Liquide for further process development and optimization. Technical key feature of the fast pyrolysis pilot plant design is the reactor system consisting of a twin screw mixer reactor combined with a heat carrier loop and, as a consequence of using ash rich feedstocks, a two-stage condensation system allowing the formation of two stable condensates: A tar condensate rich in organic material and an aqueous condensate containing up to 85 wt.% of water.


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The main equipment of the pyrolysis plant is installed in a weather protected steel racket of 8 x 8 m with a height of 27 m. Control room, utilities, biomass pretreatment and product handling are situated in a separate building close by (Fig. 1). Operational levels above the 11 m reactor level outside of the weatherproof cladding are accessible by ladders with safety cage.

Figure 1. Images of the operations building with fast pyrolysis pilot plant (left) and the twin screw reactor (right) Figure 2 shows a simplified process flow chart of the pilot plant designed for 500 kg/h dry biomass feed capacity, equivalent to ca. 2 MW thermal fuel capacity. The facility can be separated into the biomass pre-treatment section, the feeding section, reactor system with heat carrier loop, cooling section for product recovery and the final section for biosyncrude preparation and storage of biosyncrude. These sections are described in the following in more detail. Feedstock preparation: The feedstock preparation section was delivered by company Neue Herbold (Sinsheim, Germany) as package unit. The air dry wheat straw supplied from local farmers with an average humidity of ca. 10 wt.% is delivered in square bales with a typical


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weight of about 300 kg each. These are cut to particles with a size equal or smaller than 10-20 mm by a two stage cutting mill. Impurities such as small stones and metal pieces are removed after the first debaling and milling stage by a magnet separator combined connected to a vibrating feeder. The chopped straw is pneumatically transported and fed to a 60 m3 silo by a cyclone via a rotary valve; the blower air is cleaned by a filter system.

Figure 2. Simplified flow chart of the bioliq fast pyrolysis pilot plant. Feeding system: The straw particles are removed from the silo by a rotating discharge arm and a screw conveyor. A belt-weigher allows determining the mass flow rate, before the straw is further delivered to a bucket conveyor, which elevates the feed material to the 11 m reactor level.


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Here it is dropped into the feeding vessel equipped with mixing paddles, from which transfer to the reactor top via a metering screw is facilitated. In order to substitute losses of heat carrier material, sand can be added by that way from a 2 m3 storage vessel to the bucket elevator where it is transported together with the straw to the feeding vessel. Twin screw mixer reactor and heat carrier loop: In the mixer reactor biomass feed material and hot heat carrier are contacted and intensively mixed. The purpose of the heat carrier is to supply the heat for rapid warming-up and pyrolysis of the biomass by quickly mixing the cold bio-material with an excess of hot sand as carrier material with a flow rate of 2500 to 5000 kg/h. River washed quartz sand with a medium particle size of slightly above 1 mm is used with an inventory of 2 t. During operation, particle size reduction due to abrasion takes place. The fines from sand are discharged with the solid pyrolysis products. Typically, the loss of sand abrasives is in the range of 1 wt.% related to the feed throughput. The principal design characteristics of the twin screw mixer reactor and its working principle for fast pyrolysis was introduced already by Kornmayer et. al..13 Due to the short reaction time the reactor is of compact design (Fig. 1, right). The intertwining screws of 1.6 m feeding length and 190 mm outer diameter rotate in the same direction permanently cleaning each other as well as the inner reactor walls. With a rotation speed of ca. 95 s-1 a sufficient balance between radial mixing and axial transport is obtained. The reactor temperature of 500 °C is controlled by the ca. 600 °C hot heat carrier influx; temperature is measured as control signal at the reactor outlet. Here, heat carrier containing some solid pyrolysis products drops down into a vessel for residual degassing and is further directed and injected to the base point of a 20 m long vertical lift pipe, in which the heat carrier is re-heated by hot flue gas and, at the same time, is pneumatically transported back into the storage vessel on top of the pyrolysis plant construction. Heating-up the sand to 600 °C is facilitated by up to 1000


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m3/h hot flue gas, generated in a combustion chamber by natural gas incineration. Heat is also provided by combustion of the pyrolysis char contained in the heat carrier by adjusting an appropriate oxygen excess in the combustion chamber. In the sand storage vessel the carrier gas is separated from the solid heat carrier particles by multiple redirection of the gas flow. Still remaining fines are separated by a hot gas cyclone. After gas cooling by water and/or steam injection an air-pre-heater and a hot gas filter (< 200 °C) are passed, before the CO containing flue gas is combusted in a flare. Product recovery: Hot pyrolysis vapours, condensable and non-condensable ones, as well as most of the solid pyrolysis products leave the reactor over-head. The solids are separated by a hot gas cyclone, removed from the collecting vessel via ball valves into a screw cooler, discharging the solid products into buffer vessels under inert nitrogen atmosphere. Nitrogen is used as fluidizing agent in the collection vessels, when the fine solid material stops flowing. The still solids containing 500 °C hot pyrolysis vapours and gases enter a quench cooler, in which a first condensation step is conducted by injecting an excess of already liquefied organic condensate (condenser I in Fig. 2). The now ca. 90 °C warm organic condensate is collected in the condenser, to the top of which an electrostatic precipitator is attached on way to the second condensation stage. The condensate is recycled with a flow rate of ca. 32 m3/h via a desintegrator keeping particulates below 300 microns size. After passing a spiral heat exchanger the recycle stream of now ca. 85 °C is split into the quenching stream (ca. 10 m3/h) and two re-cycle streams to the condenser top for countercurrent aerosol removal by washing and to the condenser bottom phase. The viscosity of the organic condensate at the condensation temperature selected (and the water content adjusted thereby) is usually below 80 mPa s-1. However, the robust system can also tolerate viscosity peaks up to 200 mPa s-1.


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At critical locations, the process installations are equipped with poking and rapping devices. To facilitate heating-up and to compensate heat losses which can lead to undesired condensation of volatiles, the piping in the heat carrier and hot vapour system are electrically trace heated to 400 °C and 500 °C, respectively. Also, after stand-by periods or process disturbances, the plant can be re-started faster then. Plugging in the quench cooler and the feed inlet into the reactor have been found as most frequently occurring disturbances. In the quench cooler entrance an automated poking system controlled by pressure difference has been successfully employed. Aerosol particulates are separated from the non-condensed vapour and gas stream by an electrostatic precipitator and retained in the condenser. For operation of the electric filter a minimum oxygen limit in the product vapor line of < 0.5 vol.% is maintained, when necessary by addition of nitrogen. By impactor measurements, aerosol particle concentrations between 70 and 95 g/Nm3 were found with average particle D50-values of 1.5-1.8 µm and D90 values below 5 µm. The non-condensed vapors exit the electrostatic precipitator over-head and enter the second condensation step (condenser II in Fig. 2). At around 30 °C an aqueous condensate is obtained in a wash column. The recycled condensate is cooled down by tube bundle heat exchangers. Entrained or later formed aerosols are separated in a droplet separator, after which a blower directs the non-condensable pyrolysis gases to the flare. For pressure control, recycle of the gas stream is established. In a commercial application, the pyrolysis gas can be used to cover the energy demand for the pyrolysis process, which has been validated based on process development unit test runs as reported earlier.10 When starting-up the process, ethylene glycol is


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used to fill up the first condensation loop, the piping of which is also warmed up by electrical trace heating to prevent temperatures below 80 °C. Product handling: Both condensates are discharged intermittently into buffer tanks from the condenser and wash column. From the buffer tanks, the condensates can be transferred to the tank farm directly or via the colloidal mixer system, where solid pyrolysis products, which are stored and made inert in conical steel containers, can be admixed in 1 m3 batches,. Also, pyrolysis oils and char of other origin can be handled and treated in this part of the plant which was delivered as package unit by MAT Mischanlagentechnik (Immenstadt, Germany). Before being filled into the 30 m3 storage tanks, wet grinding is possible to adjust the particle size and thus, rheological behavior of suspensions. Biosyncrude produced from pyrolysis condensates and solids contained in storage vessels are continuously stirred and circulated in closed loops by appropriate pumps. Operation practice and experience: The pilot plant is currently operated on a campaign basis in three (one week operation) or four shifts for longer operation. A shift team of seven members consists of the shift leader, four operators (mechanics and electricians) for plant control, sampling, plant inspection and technical service, and two laboratory workers. Heat carrier composition and particle size distribution as well as water, ethylene glycol, and solids content, and finally viscosity of the organic condensate are analyzed frequently during operation. After a test campaign, the products are fully analyzed. First results on test runs of the pilot plant carried out from 2011 to 2013 have been reported earlier, where also the analytical methods applied are described in more detail.18 However, in 2013 some modifications in order to optimize the pyrolysis process have been conducted in the


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pilot plant. A number of improvements were made concerning the heat exchangers in the organic condensate cycle, cleaning devices in the product vapor pipeline reactor-cyclone-quench, biomass feed-in to the reactor, product handling, and the hot gas filter. Therefore in this paper, the results of the more recent test campaigns run in 2013-2015 are considered in the following.

PRODUCT YIELDS AND PROPERTIES The yields of char (including the ash content), liquid condensates and gas produced are the essential basis for the selection of suitable fast pyrolysis process conditions and equipment design for an envisaged application. The total product yields “as received” are important in regard to product utilization. In regard to the liquid condensates these data are not easy to compare, because they are obscured by the water content, consisting of feedstock humidity and reaction water formed during pyrolysis, and in some cases also by a certain solids content formed by char and ash. For better comparison total liquid organic yields are therefore most commonly reported on a water and ash free basis. In the bioliq® process the liquid and solid fractions are used, as they are, for mixing of gasification fuels. For this practical reason the product distribution obtained in this work is reported as received. Product yields of pilot plant runs: For three selected pilot plant test runs, Fig. 3 displays the product yields of pyrolysis gas, organic and aqueous condensate together with the solid fraction recovered from both the product gas and the flue gas cyclone. The relatively small amounts of fines recovered from the hot gas filter are also taken into account, but which are not used for biosyncrude production. In the organic condensate, a solids content of usually < 10 wt.% is


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contained. The water content is adjusted between 12 and 18 wt.% by varying the condensation temperature between 85-90 °C. In the aqueous condensate, practically no particles are found; its water content is usually between 80-85 wt.%. The values in Fig. 3 are integral values over the whole period of the campaigns converting 15.0, 17.6 and 36.6 tons of wheat straw within 100, 120, and 220 hours of plant operation time in the test campaigns 49-2013, 19-2014 and 5/6-2015, respectively. Wheat straw feed rated ranged between 300 and 400 kg/h. Feedstock humidity was typically around 9-10 wt.%, the ash content ranged 4-5.5 wt.%, and the HHV of the straw used was around 17 MJ/kg. While the bio-oil temperature in condenser I was 83-89 °C in the 2013 and 2014 campaigns, it was adjusted to 81-84 °C in the 2015 test run. As a result a trend to higher bio-oil yield can be observed in the latter test campaign.

100

Product yields / wt.%

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80 Organic condensate Aqueous condensate Solids Pyrolysis gas

60 40 20 0

5/6-2015

19-2014

KW 49 2013

Test campaigns

Figure 3. Pyrolysis product yield distribution for the selected pilot plant test runs. Product properties: Exemplary, product characteristics are given in Table 1 for one selected test run, that of 49-2013. A detailed description of the results and methods applied can be found elsewhere.18


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The viscosity of the organic condensate is strongly temperature dependent. At operation conditions in condenser I between 80-90 °C, the organic condensate shows viscosities fairly below 100 mPa·s and is free flowing and well pumpable with no problems. After cooling down, at room temperature, the viscosity can rise up to ca. 500 mPa⋅s and even higher values. To assure proper operation during handling and storage the organic condensate is kept warm at ca. 40 °C, where the dynamic viscosity is less than half of that value. Table 1. Product properties of pyrolysis condensates and solids received in the 49-2013 campaign. Property

Value

Dimension

Water content

14

wt.%

Solid content

5

wt.%

Inorganic material

1

wt.%

Density

1205

kg m-3

Viscosity (80 °C)

60

mPa·s

Homogeneity

single phase

Higher heating value

23

Organic condensate

MJ kg-1


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Aqueous condensate Water content

75

wt.%

pH value

3

Density

1000

Homogeneity

one phase


7

MJ·kg-1

Mineral content

40

wt.%

d95

94

µm


23

MJ·kg-1

kg·m-3

Solid fraction

Regarding the energy content of the product fractions, the organic and the aqueous condensate contain 50 % and 15 % of the total energy, respectively. The solid fraction contributes to 28 % and the pyrolysis gas to 8 % of the energy.19 From this consideration it becomes clear that the aqueous condensate by far is not a waste stream but a product, containing a significant share of the bioenergy. Therefore, in regard to the bioliq process with biosyncrude gasification as its central element, the pyrolysis process has to be optimized in regard to the production of fuels with sufficient heating value and properties suitable for the subsequently following gasification.


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Not the yield of the two individual condensates, but their composition and properties are decisive in this context. For biosyncrude production, they are mixed with the solid product in different proportions; the larger portion of char (up to 40 wt.%) can be mixed to the aqueous condensate, while the organic condensate may contain up to 20 wt.% of solids maintaining acceptable rheological properties.14 Around 1/3 of the char is presently used for process energy supply by internal combustion in the lift pipe. Comparison of product yield distribution of straw pyrolysis: Compared to wood, fast pyrolysis of wheat straw as feedstock with its usually higher ash content of 4-7 wt.% leads to a significant decrease in organic condensate yield.20-22 This is also found in the condensate yields obtained in the pilot plant runs as well as in other own studies in lab and bench scale. The product yield distribution, particularly in fractionating condensation, depends on the technical set-up, operation mode, and in particular on the temperature of the first bio-oil condensation step. This is illustrated in Fig. 4. Solids (char and ash) and pyrolysis gas yields vary between 25-30 and 20-25 wt.%, respectively. Depending on the condensation conditions, the ratio between organic and aqueous condensate varies significantly. By intensive pre-drying and/or ash removal prior to the pyrolysis process, even a homogeneous condensate could be produced from wheat straw. The first option requires additional efforts in both technical equipment and energy demand. The second option was studied and reported in detail by Oudenhoven et al..22 However, the bioliq concept has been designed to handle a multitude of ash rich materials. Phase separation may occur using one or the other feedstock used; therefore, staged condensation appears to be a reasonable process design. Ash components are separated in the second step of the bioliq® process by operating the gasification in slagging mode, where the molten ashes are recovered as slag from the process.


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100

80

Product yields / wt.%

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Organic condensate Aqueous condensate Solids (char+ash) Pyrolysis gas

60

40

20

0

Lurgi

BioBoost

Kornmayer

Figure 4. Product yield distribution of fast pyrolysis test with different types of equipment, operation mode and condensation conditions. The first column shown in Fig. 4 is a result of the Lurgi experiments which have been performed in advance to the pilot plant design. They were carried out at the 3-5 kg/h PDU using the same process. In the test runs, wheat straw with 6 wt.% of ash and 2.8 % humidity was used as feedstock. Bio-oil temperature after first condensation step was 120 °C in average fairly above the boiling point of water, which is the reason for a larger aqueous condensate yield obtained in the second condensation stage compared to the other results given in the figure. In earlier work with KIT´s process development unit, char and organic condensate were recovered in a single condensation step.13 The mixture was collected as a sludgy product, covering the heat exchanger surface in a thick but soft layer, which was frequently removed in periods of several seconds by a scraper. As feedstock, wheat straw with 9.7 wt.% humidity and 6.0 wt.% ash content was used, condensation temperature was ranged from 60-70 °C. The yield of organic condensate in Fig. 4 is given on a solid free basis.


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The BioBoost results (obtained within the EU BioBoost project) have been generated in KIT´s 10 kg/h process development unit; after solids separation by two cyclones, organic and aqueous condensate is recovered in a two-stage condensation, the first of which was operated at around 90 °C. Wheat straw of 6.1 wt.% ash content and 9.1wt.% humidity was used. A detailed comparison of the joint and the separate char condensation is given by Funke.24 Due to the different types of equipment, process conditions, and specific feedstock properties (even when using one type of feedstock) an interpretation and comparison of the results is not simple and only trends can be concluded. In any case it makes clear, how sensitive product yields depend on the processing details. In this context it should be considered, that not only the product yield, but also the product properties in regard to their planned application are decisive for selection of one or the other process design. PRODUCT STABILITY Stability of pyrolysis oils is an important issue in bio-oil production, handling and usage. Due to the nature of the fast pyrolysis process, intermediate products are quenched after short reaction time far away from thermodynamic equilibrium, exhibiting some potential for further reactions when heated up again. Particularly the use of ash rich materials like straw in combination with thermal/chemical ageing affects relevant fuel properties such as an increase of viscosity, water content, pour point, flash point, and molecular weight along with a decrease in volatility as well as an increasing tendency to phase separation. This is principally known from a number of studies, and there are some options to maintain or achieve stable biooils: co-solvents may be added up to a few percentages reducing the rates of reaction by dilution or by chemical reactions (esterification, acetal formation) with appropriate


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solvents such like C1-C4 alcohols or ethylene glycol. However, even then bio-oils remain a complex multi-component mixtures as a kind of emulsion sensitive to chemical and physical changes. Influence of char on ageing: In the bioliq fast pyrolysis a certain amount of solids is allowed to remain in the organic condensate. It may be expected, that a longer contact to the ash components or char may favor ageing reactions. This was investigated by applying a similar method as reported by Oasmaa for wood derived bio-oil recovered from fluid bed pyrolysis. Ageing phenomena were evaluated according to a solvent fractionation scheme developed to characterize bio-oil composition.25-27 By this method, pyrolysis liquids are divided into a waterinsoluble (WIS) and a water-soluble (WS) fraction. While the first one is split up again by dichloromethane treatment into soluble and insoluble fractions (DS and DIS, respectively), the second one is extracted by dimethyl ether, accordingly into DDS and DDIS fractions. Oasmaa reported the composition of practically particle free, wood derived bio-oil freshly recovered from the fluid bed fast pyrolysis plant at VTT compared to bio-oil being exposed to 80 °C for 24 hours of storage time. 24 As shown in Fig. 5, an increase in higher molecular weight components (HMM lignin) can be observed on cost of monomolecular substances like sugars, lignin derivatives, and carbonylic substances.


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DIS DS DDIS DDS Water

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Aged bio-oil

Figure 5. Ageing of wood based bio-oil according to Oasmaa.25 The abbreviation in the legend denote: DIS: water insoluble, high molecular weight lignin derived compounds (HMM), solids; DS: water insoluble, low molecular weight lignin, extractives; DDIS: water insoluble, sugars, hydroxyl acids, water; DDS: water soluble, carbonylic compounds, volatile organic acids, lignin derived monomers). By comparing the WS and WIS fractions obtained from a sample of bio-oil produced in the bioliq® pilot plant it was found that the insoluble fraction increased for a particle free bio-oil from 27.7 to 31.3 wt.% when aged at 80 °C for 24 hours. In case of the solids containing bio-oil sample the WIS content was higher in the fresh bio-oil sample (due to the char content), but the increase during ageing from 31.9 to 35.4 wt.% was found to be nearly exactly the same and is also in order of magnitude as observed in the experiments of Oasmaa mentioned above. Organic condensate residence time: In the bioliq® fast pyrolysis plant, organic condensate is recycled in a system comprising of the quench cooler, condensation vessel, electrostatic precipitator, heat exchangers, recycle pipelines and connection tubing, combined with the discontinuous product discharge. As a consequence, the bio-oil is exposed to elevated


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temperatures for some time. To determine the residence time distribution of the complete condensation system, a displacement method was used in the starting phase of a pyrolysis campaign, when the condensation system was filled up with ethylene glycol. Its displacement by the organic condensate formed when starting the pyrolysis process was measured by NMR and fitted to a residence time model. The average residence time was calculated for each component of the condensation system, either assuming ideal plug flow or continuous stirred tank behavior, which turned out to be sufficient. From that the residence time sum function of the overall system was calculated which resulted in an average residence time of 5.6 hours at a condensate temperature of around 80 °C and the flow rates given in the different sub-systems in the condensation system.

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DIS DS Extractives DDIS DDS Water

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Figure 6. Controlled thermal ageing of straw based bio-oil Controlled thermal ageing: Already in 2011, bio-oil sampled from condenser I was compared to the same material stored for 3 months at room temperature. Within the experimental error, no significant deviations could be found for the different fractions obtained from the solvent extraction scheme.28 It should be noted, that the extraction scheme applied here was somewhat


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different from that used by Oasmaa,25 so that a direct comparison is not advisable but only the consideration of relative changes. The main difference in the extraction method is the determination of the sugar derivatives in the DDIS fraction. Here, the water soluble fraction is extracted by a solvent mixture of dichloromethane-diethyl ether (1:1). The DDS (dichloromethane/diethyl ether soluble) was calculated by difference. In Fig. 6 DDS and DDIS represent the diethyl ether/dichloromethane soluble and insoluble fractions, respectively, obtained from the water soluble bio-oil fraction. Even though more detailed studies are necessary it becomes obvious that the bioliq® bio-oil recovered shows a kind of artificial ageing during recirculation in the condensation system, meaning that an improved stability for subsequent storage can be attained. Effect of staged condensation: In recent investigations, the properties of the organic condensate were observed during stand-by operation of the pilot plant. In a period of ca. 48 hours no feedstock was fed to the pyrolysis reactor; heat carrier and condensate loops were maintained in operation. The organic condensate loop is an open system; at the ca. 80 °C of temperature volatile substances escape via the electrostatic precipitator to condenser II. In parallel, ageing of the condensate is expected to take place. It is not possible to clearly attribute the observed changes to the one or the other effect. However, the change of the bio-oil properties during this stand-by period was characterized by solvent fractionation, size exclusion chromatography and viscosity measurements. There was clear evidence that substances of higher volatility like acetic acid, propionic acid, hydroxyl acetone with boiling points of 118 °C, 140 °C, and 145 °C, respectively, evaporate from the organic condensate and appear accordingly in the aqueous condensate. Therefore, the water soluble fractions, DDS and DDIS, show a decrease in Fig. 7, accompanied by reduction of water and, accordingly, an increase of solids content, altogether


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leading to an increase of the HHV by around 8 %. The phenols content is reduced in the organic condensate during stand-by operation, but no corresponding rise was found in the aqueous condensate, which is a clear hint towards ageing reactions. Accordingly the water insoluble fractions, DS and DIS, show a significant increase indicating the formation of higher molecular weight components. However, also the evaporation of volatile substances leads to a relative increase of the less volatile components of the organic condensate. Size exclusion chromatography showed a significant increase of the molecular weight within the LMM lignin fraction, while that of HMM lignin showed even a reduction. Another proof for seasoning reactions occurring is the formation of esters, which could be observed by Infrared spectroscopy of the aged sample.

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DIS DS Extractives DDIS DDS Water Solids

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Figure 7. Change in bio-oil composition during stand-by operation.


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The stand-by period was finished, when viscosity became too high and approached the operation limits of the condensate recycle pump. At this point ethylene glycol was added to keep the process running. CONCLUSION

The Lurgi-Ruhrgas concept with twin-screw reactor and heat carrier loop was successfully transferred to a new type of feedstock, namely biomass. Some changes to earlier applications with fossil fuels had to be made e. g. in the feeding section due to the unpleasant properties of straw particle tending to aggregation and bridging. Due to the different feedstock and thus product characteristics, also the product recovery system had to be adapted. The flow properties of the fine solid product gave rise to some subsequent improvements and changes of equipment in the solids recovery system. The combustion of the char contained in the lift pipe turned out to be a well adjustable tool for accurate control of the process heat demand. The condensation system became relatively complex. After improvement by installation of a desintegrator to limit the size of particulates contained in the organic condensate and by installing spiral heat exchangers stable operation was achieved, allowing to smoothly controlling the condensation temperature and thus, condensate properties. It can be concluded, that by the state-of-the-art achieved stable condensates can be produced from ash rich material such as wheat straw. Next step in KIT´s fast pyrolysis process development is to verify the multi-feed ability of the process. Wood will be used, also to allow for comparison of product yields and properties to other fast pyrolysis and thermal liquefaction processes. Then, Miscanthus as purpose grown energy plant and bagasse from sugar cane processing are planned for use. In parallel, the process


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will further be optimized in order to allow for setting-up a design package in regard to commercialization. Of particular interest is the further conditioning of the products in regard to the production of gasification fuels as a main subject in the development of the bioliq® process. Parallel to pilot plant operation, research and development work is conducted with regard to the process fundamentals . Currently, the optimization of the twin-screw reactor by combining of particle with fluid dynamic models as well as the description of the mass and energy transfer during hot vapor condensation by thermodynamic models are investigated within PhD studies. From that, new findings and knowledge for further improvement and optimization are expected.

AUTHOR INFORMATION Corresponding Author Nicolaus Dahmen, [email protected]

ACKNOWLEDGMENT Pilot plant operation and related R&D investigations are team work. The pilot plant workforce, the analytical team as well as the R&D groups involved are gratefully acknowledged. Thanks are owed to Dr. H.-R.Paur and his team for their aerosol measurements. Prof. Dr. Heike Steinert, Hochschule Mannheim, is acknowledged for supervising the BSc thesis of C. K. Feutse on ageing phenomena during stand-by operation. Financial support has been provided by the Federal Ministry of Food and Agriculture and FNR (Agency for Renewable Resources) for erection of the bioliq® fast pyrolysis pilot plant. Air Liquide is thanked for the fruitful and intensive cooperation.


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10: Dahmen, N.; Henrich, E.; Dinjus, D.; Weirich F. The bioliq bioslurry gasification process for the production of biosynfuels, organic chemicals, and energy. Energy, Sustainability and Society 2:3 (2012) SpringerOpen Journal (www.energysustainsoc.com/content2/1/3), 44 pages 11: Raffelt, K.; Henrich, E.; Steinhardt, J.; Dinjus, E. Preparation and characterization of biomass slurries: a new feed for entrained flow gasification, in: Science in Thermal and Chemical Biomass Conversion, Volume 2, A.V. Bridgwater, D.G.B. Boocock (Eds.), CPL press, Newbury, 2006, pp. 1547-1558. 12: Schingnitz, M. Flugstromvergasung, Gaskombinat Schwarze Pumpe Verfahren (GSP). In Die Veredelung und Umwandlung von Kohle; Schmalfeld, J., Ed; DGMK: Hamburg, 2008; pp. 537552. 13: Henrich, E. ; Dahmen, N.; Weirich, F.; Reimert, R.; Kornmayer, C. Fast pyrolysis of lignocellulosics in a twin screw mixer reactor. Fuel Processing Technology 2016, 143, 151–161 14: Nicoleit, T.; Dahmen, N.; Sauer, J. Production and storage of gasifiable slurries based on flash-pyrolyzed straw. Energy Technology 2016, 4, 221-229. 15: Schmalfeld, P. The use of the Lurgi-Ruhrgas process for the distillation of oil shale. In Quarterly of the Colorado School of Mines. pp 129-145. 16: W.R. Rammler, et al. Gewinnnung von Öl durch Schwelen von Öl und Teersand als Beitrag zur Energieversorgung, Chem. Ing. Tech. 1981, 53, 96-104. 17: Schmalfeld, J. Lurgi-Ruhrgas-Verfahren (LR). In Die Veredelung und Umwandlung von Kohle; Schmalfeld, J., Ed; DGMK: Hamburg, 2008; pp 157-173. 18: Pfitzer, C.; Dahmen, N.; Weirich, F.; Tröger, N.; Sauer, J. First results of the bioliq fast pyrolysis pilot plant. Proceedings 22nd European Biomass Conference and Exhibition, Hamburg, Germany, June 23-26, 2014.


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19: Müller-Hagedorn, M.; Demir, G., Sivasubramaniam, V., Bachmann, H., Günther, A., Schaarschmidt, I, Sauer, J, Dahmen, N., Weirich, F., Tröger, N. Pyrolysis products from largescale flash pyrolysis of straw. Pyro2014 Conference, May 2014, 19.-23. May, 2014 Birmingham, UK. 20: Fahmi, R.; Bridgwater, A.V.; Donnison, I.; Yates, N.; Jones, J.M. The effect of lignin and inorganic species in biomass on pyrolysis oil yields, quality and stability. Fuel 2008, 87, 1230– 1240. 21: Oasmaa, A.; Solantausta, Y.; Arpiainen, V.; Kuoppala, E.; Sipilä, K. Fast Pyrolysis Bio-Oils from Wood and Agricultural Residues. Energy Fuels 2010, 24, 1380–1388. 22: Tröger, N.; Richter, D.; Stahl, R. Effect of feedstock composition on product yields and energy recovery rates of fast pyrolysis products from different straw types. Journal of Analytical and Applied Pyrolysis 2013, 100, 158–165. 23: Oudenhoven, S.R.G.; Westerhof, R.J.M.; Kersten, S.R.A. Fast pyrolysis of organic acid leached wood, straw, hay and bagasse: Improved oil and sugar yields. Sustainable Journal of Analytical and Applied Pyrolysis 2015, 116, 253–262. 24: Funke, A.; Niebel, A.; Richter, D.; Abbas, M.; Müller, A.-K.; Radloff, S.; Paneru, M.; Maier, J.; Dahmen, N.; Sauer, J. Fast pyrolysis char - Assessment of alternative uses within the bioliq® concept. Bioresource Technology 2016, 200, 905-913. 25: Oasmaa, A.; Kuoppala, E. Solvent Fractionation Method with Brix for Rapid Characterization of Wood Fast Pyrolysis Liquids. Energy & Fuels 2008, 22, 425-4248. 26: Sipilä, K.; Kuoppala, E.; Fagernäs, L.; Oasmaa, A., Characterization of biomass-based flash pyrolysis oils. Biomass and Bioenergy 1998, 14, 103-113.


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27: Oasmaa, A.; Kuoppala, E.; Elliott, D.C. Development of the basis for an analytical protocol for feeds and products of bio-oil hydrotreatment, Energy & Fuels 2012, 26, 2454-2460. 28: Boscagli, C.; Raffelt, K.; Zevaco, T.; Olbrich, W.; Otto, T.; Sauer, J.; Grunwaldt, J.-D. Mild hydrotreatment of the light fraction of fast-pyrolysis oil produced from straw over nickel-based catalysts. Biomass and Bioenergy 2015, 83, 525-538.


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Fast Pyrolysis of Wheat Straw in the Bioliq Pilot Plant - American

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