The authors like to thank Christoph Warth, Michael Russ, Carola Lepski, Julian Tejada and Dr. Rainer Kirchhof for their technical support. The study was funded by the BMBF (Project BiCoLim-Bio-Combustibles Limpios) under the grant number 01DN16036.

1. Preparation of sample material
<ol>
	<li>Harvest five year old willow, clone type &#8220;Tordes&#8221; ([Salix schwerinii x S. viminalis] x S. vim.), with a height of 12&#8722;14 m and a breast diameter of approximately 15 cm.</li>
	<li>Chip the wood and dry the chips in a kiln dryer for 24 h at 105 &#176;C.</li>
	<li>Cut the wood chips with a cutting mill and grind with a centrifugal mill to a particle size of 0.12 mm.</li>
</ol>
2. Microwave assisted hydrothermal carbonizati...

The MAHC allows the differentiation of the steps of the chemical degradation by applying different intensities of thermal treatment. Therefore, it is possible to assess the interactions between the mass loss, O/C-H/C ratio, heating value, ash component reduction, pH increase of the process water and accumulation of furans in the process water. The advantage of the MAHC method over the conventional HTC reactor method is based on the thermal conduction via microwaves that penetrate the whole reactor volume and conduct the ...

The application of microwaves for hydrothermal carbonization (MAHC) was used for the thermochemical transformation of biomass model compounds like fructose, glucose1,2 or cellulose3, and for organic substrates, preferably waste material4,5,6,7,8,9,10. The utilization of microwaves is advantageous as it allows an even heating of the treated biomass2,10 mainly through thermal losses of a dielectric solvent11,12, though the microwaves do not transfer enough energy to directly break chemical bonds and induce reactions13. The microwaves penetrate the whole reaction volume of the HTC reactor vessel and transfer the energy directly to the material, which is not possible with a conventional reactor that shows a slower heating rate due to the high heating capacity of the steel mantle and the sample itself14. The even excitation of the sample&#8217;s water molecules by microwaves allows an improved process control, as the temperature in the microwave reactor is evenly distributed11,14,15 and the cooldown after the reaction is much faster. Furthermore, conventional reactors heat up much slower and the chemical reactions occurring during the heating can bias the results that are usually assigned to the final temperature. The improved process control in an MAHC reactor enables a precise elaboration of the temperature dependency of selected HTC reactions (e.g., dehydration or decarboxylation). Another advantage of the even temperature distribution in the HTC-reactor volume is the lower adhesion of immobilized and completely carbonized particles on the inner reactor wall2. However, water is only an average microwave absorbing solvent that even shows decreasing microwave absorbance at higher temperatures, which limits the achievable maximum temperature. This negative effect is compensated when acids are produced during the HTC process or catalyzers (ionic or polar species) are added before the treatment. Microwave induced reactions show higher product yields in general11,15 and specifically of 5-hydroxymethylfurfural (5-HMF) from fructose in comparison to sand-bed catalyzed reactions12. They also have a much better energy balance then conventional heating methods15,16.
The fundamental chemical concept of hydrothermal carbonization is the degradation and successive polymerization of the biomass. In the course of these complex interacting reactions the tissue is depleted of oxygen, which increases the heating value. At first, the polymers hemicellulose and cellulose are hydrolyzed to sugar monomers17, though low temperatures mainly affect the hemicellulose18,19,20,21. In this early stage of the HTC reactions, organic acids are formed from the transformation of the sugar aldehydes and the deacetylation of hemicellulose. These acids can be acetic, lactic, levulinic,&#160; acrylic or formic acid20,21,22 and they decrease the pH of the reaction water in the reactor. Due to dissociation, they form free negative ions that increase the ion product in the process water. The increasing ion product allows the solving of cations, which are the major constituents of the ash in the biomass. By this mechanism, the tissue is depleted from emission precursors and slag formers (e.g., potassium, sodium, calcium, chlorine and heavy metals)23,24.
The formed organic acids can support the dehydration of sugar monomers to furans. A common sugar dehydration product is furfural and 5-hydroxymethylfurfural, which are feasible products for the chemical industry, as they serve as platform products (e.g., for the synthesis of biopolymers). 5-Methylfurfural can be formed by catalyzed reactions from cellulose25,26 or 5-hydroxymethylfurfural27. While the biopolymer synthesis is an artificial repolymerization under controlled conditions, the furans can also condense, polymerize and form high molecular weight aromatic structures in the complex chemical environment of the MAHC reactor. The interaction of the solubilized organic and inorganic compounds with the modified wood cell matrix add to the complexity of the reaction system20. The furan polymerization reaction pathways employ aldol condensation or/and intermolecular dehydration18,20 and yield hydrochar particles with a hydrophobic shell and a more hydrophilic core28. It is not yet revealed whether biomass particles are completely decomposed and then repolymerized or if the biomass particles serve as a template for the carbonization. However, the degradation and repolymerization reactions comprise dehydration and decarboxylation reactions, as well29,30, which induces the drop in the van Krevelen diagram towards the O/C and H/C ratios of black carbon.
While other studies proved the mineral reducing effect of conventional reactor based hydrothermal treatment31, of a water washing with combined mechanical leaching32 or water/ammonium acetate/hydrochloric acid washing33, our studies investigate the mineral leaching during low temperature carbonization with microwaves for the first time. As this study focuses on emission precursor leaching for fuel upgrading, it investigates the fate of potassium, sodium, magnesium, calcium, chlorine, sulfur, nitrogen and heavy metals. Fine dust precursors form volatile salts (e.g., KCl or K2SO4) at elevated temperatures in the gaseous phase. When these salts accumulate in the flue gas, heavy metals like zinc can scavenge them as nucleation particles, which leads to a particle growth chain reaction. At lower flue gas temperatures, salt condensation further triggers the particle growth and results in cancerogenous fine dust emission from the chimney. These emissions are at present the main factor that compromises the sustainability of biomass fuels. A sustainable energy supply relies on their reduction by expensive filters or their reduction in the fuels (e.g., by MAHC). As this study follows a practical approach, short rotation coppice (SRC) willow wood was chosen as a potential bioenergy feedstock with high growth rates. It can be grown by farmers on their fields for a self-sustainable power supply by gasification, but also for heat generation by direct combustion. A disadvantage of willow SRC is its high bark content due to a low stem:bark ratio at mature stage. The bark contains a lot of minerals in comparison to wood34,35,36,37 and yields higher quantities of gaseous or particle emissions38. Low temperature HTC can improve the combustion properties of SRC willow wood and, thereby, contribute to a sustainable heat and power supply. Another important parameter of the HTC biocoal investigated in this study is its energy density, its higher initial combustion temperature and its higher final combustion temperature39.

<table>
	<tbody>
		<tr>
			<td>5MS non-polar cloumn</td>
			<td>Thermo Fisher Scientific,Waltham, USA</td>
			<td>TraceGOLD SQC</td>
			<td>GCMS</td>
		</tr>
		<tr>
			<td>9&#181;m polyvinylalcohol particle column</td>
			<td>Methrom AG, Filderstadt, Germany</td>
			<td>Metrosep A Supp 4 -250/4.0</td>
			<td>Ion chromatography</td>
		</tr>
		<tr>
			<td>argon</td>
			<td>Westfalen AG, M&#252;nster, Germany</td>
			<td>UN 1006</td>
			<td>ICP-OES</td>
		</tr>
		<tr>
			<td>calorimeter</td>
			<td>IKA-Werke GmbH &#38; Co.KG, Stauffen, Germany</td>
			<td>C6000</td>
			<td>higher and lower heating value</td>
		</tr>
		<tr>
			<td>centrifuge</td>
			<td>Andreas Hettich GmbH &#38; Co.KG, Germany</td>
			<td>Rotofix 32 A</td>
			<td></td>
		</tr>
		<tr>
			<td>centrifuge mill</td>
			<td>Retsch Technology GmbH, Haan, 
			Germany</td>
			<td>ZM 200</td>
			<td></td>
		</tr>
		<tr>
			<td>ceramic dishes</td>
			<td>Carl Roth GmbH&#38;Co.KG, Karlsruhe, Germany</td>
			<td>XX83.1</td>
			<td>Ash content</td>
		</tr>
		<tr>
			<td>cutting mill</td>
			<td>Fritsch GmbH, Markt Einersheim, Germany</td>
			<td>pulverisette 19</td>
			<td></td>
		</tr>
		<tr>
			<td>D(+) Glucose</td>
			<td>Carl Roth GmbH&#38;Co.KG, Karlsruhe, Germany</td>
			<td>X997.1</td>
			<td>higher and lower heating value</td>
		</tr>
		<tr>
			<td>elemental analyzer</td>
			<td>elementar Analysesysteme GmbH, Langenselbold, Germany</td>
			<td>varioMACRO cube</td>
			<td>elemental analysis</td>
		</tr>
		<tr>
			<td>exicator</td>
			<td>DWK Life Sciences GmbH, Wertheim, Germany</td>
			<td>DURAN DN300</td>
			<td>Ash content</td>
		</tr>
		<tr>
			<td>GC-MS system</td>
			<td>Thermo Fisher Scientific,Waltham, USA</td>
			<td>Trace 1300</td>
			<td>GCMS</td>
		</tr>
		<tr>
			<td>hydrochloric acid</td>
			<td>Carl Roth GmbH&#38;Co.KG, Karlsruhe, Germany</td>
			<td>HN53.3</td>
			<td>ICP-OES</td>
		</tr>
		<tr>
			<td>ICP OES</td>
			<td>Spectro Analytical Instruments GmbH, Kleve, Germany</td>
			<td>Spectro Blue-EOP- TI</td>
			<td>ICP-OES</td>
		</tr>
		<tr>
			<td>Ion chromatograph</td>
			<td>Methrom GmbH&#38;Co.KG, Filderstadt, Germany</td>
			<td>833 Basic IC plus</td>
			<td>Ion chromatography</td>
		</tr>
		<tr>
			<td>kiln dryer</td>
			<td>Schellinger KG, Weingarten, Germany</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>kiln dryer</td>
			<td>Schellinger KG, Weingarten, Germany</td>
			<td></td>
			<td>Ash content</td>
		</tr>
		<tr>
			<td>mesh filter paper</td>
			<td>Carl Roth GmbH&#38;Co.KG, Karlsruhe, Germany</td>
			<td>L874.1</td>
			<td>ICP-OES</td>
		</tr>
		<tr>
			<td>microwave oven</td>
			<td>Anton Paar GmbH, Graz, Austria</td>
			<td>Multiwave Go</td>
			<td></td>
		</tr>
		<tr>
			<td>muffel furnance</td>
			<td>Carbolite Gero GmbH &#38;Co.KG, Neuhausen, Germany</td>
			<td>AAF 1100</td>
			<td>Ash content</td>
		</tr>
		<tr>
			<td>nitric acid</td>
			<td>Carl Roth GmbH&#38;Co.KG, Karlsruhe, Germany</td>
			<td>4989.1</td>
			<td>ICP-OES</td>
		</tr>
		<tr>
			<td>oxygen</td>
			<td>Westfalen AG, M&#252;nster, Germany</td>
			<td>UN 1072</td>
			<td>higher and lower heating value</td>
		</tr>
		<tr>
			<td>pH-meter</td>
			<td>ylem Analytics Germany Sales GmbH &#38; Co. KG, Weilheim,Germany</td>
			<td>pH 3310</td>
			<td>pH</td>
		</tr>
		<tr>
			<td>sample bag</td>
			<td>IKA-Werke GmbH &#38; Co.KG, Stauffen, Germany</td>
			<td>C12a</td>
			<td>higher and lower heating value</td>
		</tr>
		<tr>
			<td>Standard Laboratory Vessels and Instruments</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>standard samples</td>
			<td>Bernd Kraft GmbH, Duisburg, Germany</td>
			<td></td>
			<td>ICP-OES</td>
		</tr>
		<tr>
			<td>sulfonamite</td>
			<td>elementar Analysesysteme GmbH, Langenselbold, Germany</td>
			<td>SLBS4782</td>
			<td>elemental analysis</td>
		</tr>
		<tr>
			<td>teflon reaction vessels</td>
			<td>Anton Paar, Austria</td>
			<td>HVT50</td>
			<td></td>
		</tr>
		<tr>
			<td>teflon reaction vessels</td>
			<td>Anton Paar, Austria</td>
			<td>HVT50</td>
			<td>ICP-OES</td>
		</tr>
		<tr>
			<td>tin foil</td>
			<td>elementar Analysesysteme GmbH, Langenselbold, Germany</td>
			<td>S12.01-0032</td>
			<td>elemental analysis</td>
		</tr>
		<tr>
			<td>tungstenVIoxide</td>
			<td>elementar Analysesysteme GmbH, Langenselbold, Germany</td>
			<td>11.02-0024</td>
			<td>elemental analysis</td>
		</tr>
		<tr>
			<td>twice deionized water</td>
			<td>Carl Roth GmbH&#38;Co.KG, Karlsruhe, Germany</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>twice deionized water</td>
			<td>Carl Roth GmbH&#38;Co.KG, Karlsruhe, Germany</td>
			<td></td>
			<td>higher and lower heating value</td>
		</tr>
		<tr>
			<td>twice deionized water</td>
			<td>Carl Roth GmbH&#38;Co.KG, Karlsruhe, Germany</td>
			<td></td>
			<td>ICP-OES</td>
		</tr>
	</tbody>
</table>

reducing willow wood fuel emission by low temperature microwave assisted hydrothermal carbonization

Biomass is a sustainable fuel, as its CO2 emissions are reintegrated in biomass growth. However, the inorganic precursors in the biomass cause a negative environmental impact and slag formation. The selected short rotation coppice (SRC) willow wood has a high ash content (<img alt="Equation 1" src="/files/ftp_upload/58970/58970eq1.jpg" /> = 1.96%) and, therefore, a high content of emission and slag precursors. Therefore, the reduction of minerals from SRC willow wood by low temperature microwave assisted hydrothermal carbonization (MAHC) at 150 &#176;C, 170 &#176;C, and 185 &#176;C is investigated. An advantage of MAHC over conventional reactors is an even temperature conductance in the reaction medium, as microwaves penetrate the whole reactor volume. This allows a better temperature control and a faster cooldown. Therefore, a succession of depolymerization, transformation and repolymerization reactions can be analyzed effectively. In this study, the analysis of the mass loss, ash content and composition, heating values and molar O/C and H/C ratios of the treated and untreated SCR willow wood showed that the mineral content of the MAHC coal was reduced and the heating value increased. The process water showed a decreasing pH and contained furfural and 5-methylfurfural. A process temperature of 170 &#176;C showed the best combination of energy input and ash component reduction. The MAHC allows a better understanding of the hydrothermal carbonization process, while a large-scale industrial application is unlikely because of the high investment costs.

The results of the elemental analysis revealed differences between the O/C-H/C ratio of the willow wood and the MAHC biocoals (Figure 1). The raw material shows higher O/C-H/C ratios and a higher variation of the values. The MAHC treatment reduced the value variation due to homogenization in the microwave reactor. The precision of the microwave reactor allowed the differentiation of three stages of degradation. The H/C ratio was reduced at 150 &#176;C and the...

Watch this Scientific Journal Video about Reducing Willow Wood Fuel Emission by Low Temperature Microwave Assisted Hydrothermal Carbonization at JoVE.com

Reducing Willow Wood Fuel Emission by Low Temperature Microwave Assisted Hydrothermal Carbonization

A protocol for the emission precursor depletion from low quality biomass by low temperature microwave assisted hydrothermal carbonization treatment is presented. This protocol includes the microwave parameters and the analysis of the biocoal product and process water.

Biomass is a sustainable fuel, as its CO2 emissions are reintegrated in biomass growth. However, the inorganic precursors in the biomass cause a negative ...

In our group, we applied low temperature microwave treatment in order to deplete willow biomass of fine dust precursors. This can help in upgrading biomass for clean combustion and thereby increase the biomass fuel potential of a country. The microwaves allow an even and fast biomass heating because they excite the water molecules in the whole reactor volume.Water is an intermediate microwave-absolving solvent and in order to reach higher reactor temperatures, catalyzers like organic acids have to be added. After drying, use a cutting mill to cut the willow wood chips. Place the wood chips in a centrifugal mill and grind to a particle size of 0.12 millimeters.Transfer 500 milligrams of the ground raw material in a 50-milliliter PTFE reaction vessel with a spatula. Add 10 milliliters of demineralized water. Screw down the reaction vessel cap, so that the pressure valve on the cap is on the same level as the cap brim.Select a microwave oven with 850 watts and a magnetron frequency of 2, 455 megahertz. Put 12 reaction vessels with raw materials in the microwave oven and close the oven. Set up a temperature program for 150, 170, or 185 degrees Celsius.Start the microwave oven with new raw materials for each single program. After the program is completed, remove the reaction vessels to allow them to cool and reactivate. Then, transfer them under a fume cupboard and slowly unscrew the lid to release the pressure inside.Open the vessels. Add 35 milliliters of twice-distilled water to each reaction vessel and shake to mix. Pour the solution from each vessel to a centrifuge cylinder and centrifuge at 1, 714 times g for 10 minutes.The process water is drained into another tube and stored frozen at minus five degrees Celsius for pH and GC-MS analysis. Freeze the centrifuge cylinder with the remaining biocoal pellet at minus five degrees Celsius for several hours. Then, take out the biocoal pellet and dry it at 105 degrees Celsius for 24 hours.After that, weigh the biocoal pellet and calculate the weight loss induced by the MAHC. First, weigh 20 empty ceramic dishes individually. Add one gram of sample into each dish.From each temperature treatment, prepare five dishes with the dried raw material and five dishes with the dried MAHC biocoal. Place the open ceramic dishes into a muffle furnace and close the furnace. Program a temperature program for the muffle furnace and start the program.After the program is completed, let the muffle furnace cool down to 105 degrees Celsius. Then, open the furnace and transfer the ceramic dishes to a desiccator filled with a drying agent consisting of silica gel. Close the desiccator and vacuum dry with the help of a vacuum pump.Take out the ceramic dishes after 24 hours of cooling. Weigh the ceramic dish containing the ash and calculate the ash weight by subtracting the weight of the empty ceramic dish. In a plastic sample bag having a defined calorific value of 46, 479 joules per gram, fill with one gram of glucose.Put the sample bag into the combustion crucible of a calorimeter bomb. Use a pipette to add five milliliters of twice-deionized water in the bottom of the bomb and screw down the bomb. Put the bomb into the calorimeter and close the calorimeter.On the calorimeter, enter the weight of the sample, one gram, and change settings to sample bag method. Then, start the calorimeter. After the measurement is completed, take out the bomb, turn it upside down, and shake it slowly for one minute.Unscrew the bomb and use twice-demineralized water to flush out the sample into a volumetric flask. Rinse the bomb several times till the volume reaches 50 milliliters. Repeat the calorimeter measurement five times with each MAHC biocoal and the raw material.After treatment in the calorimeter bomb, transfer the five milliliters of solution to a 50-milliliter volumetric flask and add 45 milliliters of twice-demineralized water to mix. After calibration of the ion chromatograph, insert the sample suction tube into the flask and draw approximately three milliliters of the sample with the syringe into the precolumn. Start the analysis run.To perform induced coupled-plasma optical emission spectroscopy, first transfer 400 milligrams of the dried raw material or the MAHC biocoal into a 50-milliliter PTFE reaction vessel with a spatula. Add three milliliters of 69%nitric acid and nine milliliters of 35%hydrochloric acid. Screw down the reaction vessel cap, so that the pressure valve in the cap is on the same level as the cap brim.Put the reaction vessels of the samples to be heated in the microwave oven and close the oven. Program the temperature program for complete degradation of the organic material and start the microwave oven. After the program is completed, remove the reaction vessels.Allow them to cool and reactivate. Under a fume cupboard, release the pressure inside the vessels and open them. Pour the samples into a 50-milliliter bulb cylinder, then rinse the reaction vessel thoroughly with twice-deionized water and transfer into the bulb cylinder.Top up the cylinder to the 50 milliliter mark with twice-deionized water to ensure even dilution of all samples. In a funnel with a piece of 150-micrometer mesh filter paper on top, filter the sample. Transfer the filtrate into 50-milliliter conical centrifuge tubes.Load vials containing standard samples into the auto-injector of the ICP-OES, and run the calibration. Then load the raw material or biocoal samples in the auto-injector of the ICP-OES and run the ICP-OES analysis with the same parameters. After the ICP-OES analysis, obtain the elemental concentrations from the software, which is based on the calibration curves obtained from standard samples.The results of the elemental analysis revealed higher oxygen-carbon versus hydrogen-carbon ratios and a higher variation of the values for the raw material. The MAHC treatment reduced the value variation due to homogenization in the microwave reactor. The hydrogen-carbon ratio was reduced at 150 degrees Celsius.The oxygen-carbon ratio was reduced at 170 degrees Celsius and further reduced at 185 degrees Celsius. The temperature induced an increasing brown color, while the process water showed the same tendency, caused by increase in aromatic rings. Different elements show a different temperature-dependent leaching into the processed water.Chlorine and potassium were intensively transferred to the processed water at 150 degrees Celsius, while sulfur, magnesium, barium, calcium, sodium, zinc, manganese, and strontium showed their highest depletion rate at 170 degrees Celsius. Only the silver and lithium concentrations in the biocoal showed an even decrease rate. Nitrogen was not affected by the MAHC treatment at all.This method allows a depletion of fine test precursors from the treated biomass and upgrades otherwise unfeasible biomass for combustion. By applying this matter to different biomass, the fuel upgrading technique can be transferred to many potential fuel feedstocks that are not used for combustion yet. As the temperature in the microwave is easy to control, further research that utilizes microwave-assisted hydrothermal carbonization can reveal reaction steps in the early stages of biomass hydrothermal carbonization.

Reducing Willow Wood Fuel Emission by Low Temperature Microwave Assisted Hydrothermal Carbonization

Abstract