Transformation of Organic Household Leftovers into a Peat Substitute

A protocol for hydrothermal carbonization of vegetable food waste in an autoclave is presented, with subsequent dry thermal treatment at 275 °C in a continuous flow reactor desorbing volatile organic substances. The aim is to produce a carbon material suitable as soil amendment product or substrate component.

Transcript

We produce a peat substitute from residues. This procedure has some potential for organization of organic waste. Exact composition doesn't matter and seasonal variations are tolerated.

It provides an alternative to the use of hydrochar as solid fuel and therewith it helps to mitigate climate change by carbon capture and storage and saving fossil fertilizers. It is very important to close the outer clamps tight or the wires composition may change during heating because of water loss and the reaction must be repeated with a fresh mixture. Don't care too much about the residual pressure after hydrothermal carbonization.

Working with biomass, it is unpredictable. We observe both very low and very high residual pressures. The well for the thermal treatment has to be thin.

If it is too thick, preferential pathways can occur with good cause and heterogeneous treatment and produce unsuitable materials. To begin, select the biomass from kitchen leftovers such as fruit peels or inedible vegetable parts. Dry the sample of the biomass at 100-105 degrees celsius in an oven for two hours or overnight.

Weigh the dried sample. Calculate the suitable amounts of water and biomass for the reaction mixture. Have the reaction mixture volume half of the volume of the autoclave and the water content during hydrothermal carbonization as 85 weight percent.

Assume that the density of the mixture is approximately 1g/ml. Introduce the biomass and water into the autoclave, which is provided with the rupture disk and set at a burst pressure of 50 bar. Close the autoclave and open the nitrogen line to pressurize it up to 20 bar.

During the next 30 minutes, check the manometer to confirm that there is no pressure loss, indicating a properly closed vessel without leaks. Then, open the outlet valve to release the pressure and close the vessel again. Now, switch on the stirring.

Heat the autoclave to 215 degrees celsius within 30 minutes and maintain the temperature for at least four hours or overnight for the carbonization reaction. Monitor the pressure for the first two hours, which increases up to 21 bar, following the vapor pressure curve for water. When the autoclave has cooled down to room temperature by natural cooling, carefully release any residual pressure and open the autoclave.

To recover the hydrochar, on a Buchner funnel with vacuum, pour the mixture to separate the solid and liquid. Dispose the liquid phase as hazardous laboratory waste. Dry the solid at the temperature of 100-105 degrees celsius in an oven for two hours or overnight.

After that, weigh the dried sample. Calculate the mass balance of the hydrothermal carbonization. Take into account the dry weight of the biomass and the dry weight of the hydrochar product.

On a glass frit of a tubular quartz batch reactor, weigh 2-3g of dry, raw hydrochar for homogenous treatment. For larger amounts, such as 10-20g, use pelletized material with a particle size of 0.2-6mm. In a fume hood, insert a thermal coupler through the thermal well into the tubular quartz reactor with the end reaching the bed placed on the frit.

Place the reactor into the heating mantle and connect to the reactor a nitrogen flow of 20ml per minute. Place a small glass bottle below the reactor outlet to collect condensed liquids. Heat the reactor to 275 degrees celsius with a ramp of 10 degrees per minute.

Maintain this temperature for one hour. When cooled down to room temperature, disconnect the gas flow. Discard the liquid collected in the beaker to the non-halogenated organic waste.

Turn over the reactor to recover the carbon material into a crucible and weigh it. Calculate the mass balance for the thermal treatment. From the mass obtained in the thermal treatment and the dry biomass employed in the carbonization step.

First, crush the product in a mortar and weigh 10mg of the sample in a crucible of the TG apparatus. Place the crucible containing the sample in the auto-sampler of the TG apparatus and adjust the maximum temperature to 600 degrees celsius. Employ air as sweep gas and a temperature ramp of 10 degrees per minute.

Press the mouse button to start the analysis. Quantify the mass loss at 275 degrees celsius in the TG curve by calculating the difference between the initial weight and the weight observed at this temperature. This protocol transforms kitchen leftovers into hydrochar suitable for agricultural applications in two steps.

Hydrothermal carbonization followed by a thermal post-treatment. In the carbonization reaction, wet lignocellulosic biomass is transformed into a brown color carbonaceous material. The darker the brown color, the more advanced is the carbonization reaction.

The pressure during the carbonization reaction has to increase to at least 21 bar, which is the autogenous steam pressure at 250 degrees celsius. However, in general, the pressure is somehow unpredictable. It depends on the kind of biomass and it's state of degradation.

The mass yield of the carbonization involves a broad range from 30-90 weight percent. Mass yield is usually higher for woodier material with a high lignin content and lower for pure sugar polymers such as starch. Thermogravimetric analysis of the hydrochar samples shows the loss of volatiles between 200 and 300 degrees celsius thermal treatment is less than the loss from raw hydrochar.

At 275 degrees celsius, the mass loss was 34.6 weight percent for the untreated hydrochar sample. After the treatment at 200 degrees celsius, a reduction of volatile content of 17.5%was achieved. After treatments at 250, 275 and 300 degrees celsius, the corresponding mass loss was 6.01, 5.17 and 4.22 weight percentages of the total mass, respectively.

Also, the initial water content is not crucial for the outcome of the reaction. Take into account that a more concentrated solution provides a higher yield of organic solid material. Starting from more homogeneous feeding stocks at brewers pen grain or agro-industrial residues, higher value materials can be produced.

Applying higher temperatures during the second treatment increases the carbon content. Hydrochar has been proposed for diverse application. For instance, as a solvent, activated carbons or electrodes in batteries.

The preparation of this advanced material often involves a finishing thermal step. Hydrothermal carbonization technology has already been up-scaled to industrial scale. At this size, raw material is very heterogeneous and variable.

Here, you can see gardening residues being processed. At the present, the commercial product are pellets, you said, solid fuel, with a second treatment, a solid product is produced, increasing the contribution of the feeding stock to climate change mitigation.

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Journal (Chemistry)

Journal (Chemistry)

Transformation of Organic Household Leftovers into a Peat Substitute

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