A simple and innovative prototype for biomass pyrolysis is presented, together with some experimental results. The setup uses only the thermal solar energy provided by a system of reflecting mirrors (Linear Mirror II) to heat a selected agro-waste biomass, such as wheat straw. At the end of the pyrolysis process, solar carbon with a high energy density (around 24 - 28 MJ/kg) is produced from a biomass with an energy density of 16.9 MJ/kg. The perspectives for a future industrial application of this setup are also discussed.
Worldwide, there have been many studies to identify biomass sources and their amount, showing that there are at least several billion tons of biomass potentially available for conversion. In particular, agricultural crop residues, besides being used for animal feeding, can have a great potential as a raw under-used energy resource [
Historically, biomass has played a relevant role as a renewable energy source at low scale. On average, biomass contributes to less than 10% of the total energy supplies in industrialized countries, but only 3.0% to 3.5% of the yearly produced biomass is used in applications not related to foodstuff [
Managing biomasses like herbaceous plants and grasses, which are bulky and moist (over 20% - 50% moisture depending on time left to dry) may imply high costs related to logistic aspects, such as drying, aggregating and transporting steps [
Unprocessed agricultural residues or woody wastes typically degrade over time due to natural deterioration since they are highly prone to decomposition and breakdown processes with exposure to moisture, pests, and other uncontrolled environmental conditions. One needs therefore a biomass pre-treatment, which can increase its energy density, slow down its biological degradation, and reduce its hydrophilicity. In recent years, several technologies of thermochemical biomass conversion have been developed, and gasification and pyrolysis have shown their capacity to recover the energy stored in plants by the photosynthetic process. These thermal processes provide an efficient, environmentally acceptable, and cost-effective method for the exploitation of a sustainable energy source [
This paper concerns the pyrolysis of biomasses using uniquely solar thermal energy. Pyrolysis is a thermochemical, endothermic process, taking place under inert atmospheric conditions or in a limited supply of air. Among influential process parameters, the maximum temperature reached during the pyrolysis process is the most critical one to influence charcoal yields and properties. This temperature refers to the highest treatment temperature (HTT) for the raw feedstock during the process. Increasing HTT results in a progressive loss of hydrogen and oxygen and a concomitant enrichment in carbon [
In this paper, we present tests with a particularly simple solar concentrating device that has become available very recently, the Linear Mirror II [
The Linear Mirror II has been developed by the academic spinoff Isomorph srl. As shown in
A more detailed discussion of the Linear Mirror II principle of function can be found in [
The tests were performed at the laboratory Eurofins, and the complete test report was made public at [
The device consists of a stainless steel retort (100 cm × 44 cm × 46 cm), which rotates around a horizontal axis at a speed of two rotations per minute. It can be considered a very simple version of a rotary kiln with manual material provision and extraction. The retort is heated by the Linear Mirror II. A secondary reflector is placed in the focal plane of the Linear Mirror II system and deflects the concentrated light (as shown in
Temperature measurements are presented for a run, which has been conducted in a sunny day.
Sensor A indicated that the operating temperature of 500˚C was reached in about 90 minutes. Sensor B remained at ambient temperature during the first ten minutes of the experiment. After this time the sensor reached a steady state temperature of 300˚C in about 100 minutes.
This heating process was accompanied by a strong production of fumes and gases, which stopped when sensor B had reached 300˚C. After 100 min the temperatures of sensors A and B were approximately constant indicating that the system had reached an operative thermal equilibrium. The slight decrease of about 50˚C of the temperature measured by sensor A and B after 100 minutes can be attributed to the diminished solar intensity in the afternoon. After about 150 minutes the experiment was stopped by bringing the reflecting mirrors of the Linear Mirror II system into their park position. In correspondence a strong decrease of the temperature of sensor A can be seen.
A direct measurement of the temperature of the material was not performed: in order to get some a posteriori information on the temperature within the material and consequently on the nature of the material, its HHV has been compared to reference samples as described in Section 3.2.
The pyrolysis retort was filled with 5.0 kg of straw, and exposed to the concentrated sunlight, and was rotating at a speed of two rotations per minute. At the end of the process, the material inserted in the pyrolysis setup was reduced from an initial mass of 5.0 kg to 1.7 kg (which corresponds to a yield of about 36%).
Since the pyrolysis process took 100 min of time, the device is able to process about 20 kg of straw in one day (8 hours), transforming it to 6.0 kg of solar carbon.
Chemical properties of straw and of the solar carbon produced were characterized through standard procedures.
The feedstock used in this study was wheat straw provided by a local wheat farm located in Gorizia (Italy). The herbaceous biomass was ground and sieved to the particle size range 0.5 - 2.0 mm. The particles were dried at 105˚C for 8 hours before the experiment.
Thermogravimetric analyses of straw and charcoal were performed in a TGA 500TA instruments. TG analysis of the straw and of charcoal (solar carbon) was carried out in N2 and subsequently in air at a flow rate of 100 cc/min. All the analyses are characterized by an isothermal step at 150˚C under N2 flow to quantify the initial moisture of specimens [
The first loss is associated to the amount of moisture in the straw, that is around 7 wt%. The decomposition of straw in N2 increases rapidly from 170˚C to 500˚C registering the maximum rate at 325˚C. In this range the weight loss is almost of 70%, then the process slows dramatically and at 900˚C the total mass has been reduced of the 80%. The 20% of mass left is constituted by a fraction of refractory carbon (~13 wt%) and by ash (~7 wt%).
The formation of refractory carbon at the end of the process is probably due to the partial condensation of lignin and cellulose to form graphitic structures [
The higher heating value (HHV) of the wheat straw and of the solar carbon were measured using a bomb calorimeter IKA C200 instrument with the DIN 51900-1 standard. The vessel was pressurized at 30 bar of oxygen. The uncertainty of the measurements is about 120 J/g. The high heating value (HHV) of solar charcoal ranges between 24.5 to 28.2 MJ/kg depending on the analyzed specimen, this variability could be due to an not homogeneous pyrolysis inside the retort, however the values measured are significant higher than that of straw 16.9 MJ/kg and close to the HHV of fossil fuel.
For the sake of comparison, several samples of the same wheat straw, used also in the pyrolysis setup described here, have been heated at different temperatures in controlled conditions, using an oven for tests with a maximum operating temperature of 1050˚C and isolation walls of high density ceramic fiber. The mass of the samples before and after heating, and therefore the residual mass in percentage have been measured, as shown in
Raising the temperature, the residual mass percentage decreases as one would expect, and the higher heating value (HHV) increases. These values can be compared to the corresponding values obtained for the solar carbon in the pyrolysis experiment: this confirms that the temperature reached in the retort were between 350˚C and 500˚C.
The chemical and physical properties of wheat straw and the solar charcoal are summarized in
Such analysis gives the weight percent of carbon, hydrogen, nitrogen, and sulfur in the samples simultaneously [
Values measured are in agreement with those reported for straw-based bio-chars produced by conventional pyrolysis at 500˚C [
Scanning electron microscopy (SEM) images were obtained with a EVO-40 XPP (Zeiss) microscope equipped with an energy-dispersive X-ray detector (EDX), at 5 - 10 KV.
SEM images of solar charcoal (
On the base of the above results and on the categorization proposed by Keiluweit, Nico, Johnson and Kleber [
Temperature programmed oxidation (TPO) was carried out over 4 mg of solar carbon in a home-made equipment under 10 v% O2 in He flow (500 cc/min) at a heating ramp of 10˚C/min up to 800˚C. The combustion products were monitored with a FT-IR multi gas analyser (Multi-Gas 2030, MKS Instruments).
The combustion of the solar charcoal was investigated with a temperature programmed oxidation (TPO)
Temperature (˚C) | % residual mass | HHV (MJ/kg) |
---|---|---|
300 ± 10 | 51.5 ± 0.5 | 22.6 ± 0.1 |
400 ± 10 | 39.6 ± 0.4 | 27.5 ± 0.1 |
500 ± 10 | 31.2 ± 0.3 | 28.5 ± 0.1 |
Parameters (wt%) | Wheat straw | Solar carbon |
---|---|---|
Carbon | 46 ± 1 | 70 ± 1 |
Hydrogen | 6.0 ± 0.1 | 3.5 ± 0.1 |
Nitrogen | 0.3 ± 0.1 | 0.8 ± 0.1 |
Oxygen | 41 ± 1 | 15 ± 1 |
Sulfur | 0.3 ± 0.1 | 0.4 ± 0.1 |
Ash | 7.0 ± 0.1 | 10.5 ± 0.1 |
O/C (molar ratio) | 0.7 | 0.2 |
H/C (molar ratio) | 1.6 | 0.6 |
experiment in airflow.
TG analysis of solar charcoal in N2 (
reached inside the pyrolyzer was on average lower than that measured at its outer walls. Such internal temperature was probably sufficiently high to decompose hemicellulose and cellulose, but only partially the lignin whose conversion to char requires temperature higher than 400˚C [
The Linear Mirror II system has been successfully used to drive the pyrolysis of an agriculture biomass such as wheat straw using only the sunlight as source of heating.
The Linear Mirror II has been already used to heat water harvesting up to 50 kWh. The Linear Mirror II combined with a prototype rotatory kiln was able to transform 5 kg of straw in charcoal in about two hours. Assuming eight lighted hours of operation per day means a conversion of 20 kg of straw a day equivalent to 94 kWh [
After the solar driven pyrolysis process most of this energy is found in the resulting carbon ?50 kWh―the rest in the pyrolysis gases, i.e. in products with a higher energy density than the initial biomass. Carbon and gases can be stored for a long period of time and used as source of thermal energy at the occurrence. Solar pyrolysis of cheap biomass offers therefore an interesting combination of traditional solar thermal energy with biomass energy, which can help to substitute fossil fuels.
This substitution would not require new infrastructure or technologies especially if based in simple and scalable system such the Linear Mirror II. Further work is in progress to optimize the system and to make it an industrial product.
Acknowledgements
This work was in part sponsored by a grant from Camera di Commercio di Gorizia (Fondo Gorizia) and was supported by Area Science Park Trieste.
HansGrassmann,MartaBoaro,MarcoCitossi,MarinaCobal,EnricoErsettis,ElvisKapllaj,AndreaPizzariello, (2015) Solar Biomass Pyrolysis with the Linear Mirror II. Smart Grid and Renewable Energy,06,179-186. doi: 10.4236/sgre.2015.67016