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

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.