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- Lidman Olsson, Emil O., et al.
(author)
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An exploratory study of phosphorus release from biomass by carbothermic reduction reactions
- 2023
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In: Proceedings of the Combustion Institute. - : Elsevier BV. - 1540-7489. ; 39:3, s. 3271-3281
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Journal article (peer-reviewed)abstract
- Phosphorus (P) from biomass can cause operational problems in thermal conversion processes. In order to explore the release mechanism of P to the gas phase, carbothermic reduction of meta-, pyro-, and orthophosphates of ash elements commonly found in biomass; sodium, potassium, magnesium, and calcium was investigated. Mixtures of each phosphate and activated carbon were heated to 1135 °C in a laboratory-scale reactor, with the CO and CO2 evolving from the sample monitored, and the chemical composition of selected residues analyzed to quantify the release of P. Thermal gravimetric analysis was also performed on selected samples. The alkaline earth phosphates were reduced in steps, following the sequence meta → pyro → ortho → alkaline earth oxide. However, the alkali metaphosphates appear to be reduced in one step, in which both alkali and P are released. Alkali pyro- and orthophosphate appear to undergo a two-step process. In the first step, mainly alkali is released and in the second step both alkali and P. An intermediate is produced in the first step, which has a K:P:O atomic ratio of about 2:1:2.7, indicating it might be a phosphite with the overall stoichiometry; K4P2O5. The reduction of alkaline earth phosphates could be interpreted using available thermodynamic data, whereas thermodynamic equilibrium calculations for the alkali phosphates did not correspond well to the experimental observations. Kinetics were derived for the different reduction reactions, and can be used to compare the reactivity of the phosphates. The work suggests that carbothermic reduction reactions are important for the release of P in the temperature range 850-1135 °C and relevant for biomass combustion, pyrolysis and gasification.
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| 2. |
- Lidman Olsson, Emil O., et al.
(author)
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Release of P from Pyrolysis, Combustion, and Gasification of Biomass - A Model Compound Study
- 2021
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In: Energy & Fuels. - : American Chemical Society (ACS). - 1520-5029 .- 0887-0624. ; 35:19, s. 15817-15830
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Journal article (peer-reviewed)abstract
- Phosphorus-rich biomass can cause operational problems in combustion units. Na-phytate, a model compound used to simulate phosphorus in biomass, was studied in a laboratory-scale reactor under temperature and gas atmosphere conditions relevant for pyrolysis, combustion, and gasification in fixed bed or fluidized bed reactors to understand the P and Na release behavior. Solid residues from Na-phytate thermal conversion were analyzed using ICP with optical emission spectrometry in order to quantify the P and Na release. The release mechanism was evaluated based on FTIR spectroscopy analysis of the residues, measurement of the flue gas CO/CO2 concentration, characterization of flue gas particles using SEM with EDS, and thermodynamic equilibrium calculations. Na-phytate decomposed in several steps under a nitrogen atmosphere, starting with condensation of the phosphate OH groups, followed by carbonization in the temperature range 300-420 °C. In the carbonization process, the phosphate units detached from the carbon structure and formed cyclic NaPO3. Above 800 °C, the C in the char reacted with the melted NaPO3 to form CO and gaseous elemental P. When the char produced from flash pyrolysis of Na-phytate at 800 °C for 10 min was exposed to 1% O2, 10% CO2, or 10% H2O (in N2), the release of Na and P to the gas phase in the temperature range 800-1000 °C was around 0-7%. However, the release of P in an inert atmosphere, with a holding time of 2 h or until full char conversion had been achieved, increased from around 4% at 800 °C to almost 30% at 1000 °C. The results indicated that carbothermic reduction reaction is responsible for the release of P and that NaPO3 vaporization is not the dominating mechanism for P and Na release at temperatures below 1000 °C. A small amount of P was released in the O2, CO2, and H2O containing gases because these gas species consumed the char and thereby inhibited the release of P.
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| 3. |
- Lidman Olsson, Emil O., et al.
(author)
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Release of phosphorus from thermal conversion of phosphorus-rich biomass chars – Evidence for carbothermic reduction of phosphates
- 2023
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In: Fuel. - : Elsevier BV. - 0016-2361. ; 341
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Journal article (peer-reviewed)abstract
- Biomass can be used to generate heat, power, or biofuels in thermal conversion processes such as combustion, gasification and pyrolysis. However, some types of biomass contain high levels of phosphorus, which can be released to the gas phase and cause operational or environmental problems. The mechanism(s) responsible for phosphorus release has not been convincingly established. Understanding the high-temperature phosphorus chemistry is also important in order to enable efficient recovery of phosphorus in residues from thermal conversion of biomass. In this work, the release of phosphorus from wheat bran char and sunflower seed char in different gas environments (100 % N2, 1–20 % O2, and 10 % CO2) and temperatures (900–1100 °C) was studied. The chars were converted in a horizontal tube reactor and characterized using ICP-OES, XRD, SEM-EDS, and 31P NMR. The release of ash-forming elements was determined using ICP-OES analysis of the char and sample residues, whereas the release of carbon was determined using CO and CO2 gas analysis. In both chars, phosphorus was present primarily together with potassium and magnesium, mainly as pyrophosphates in the wheat bran char, and largely as orthophosphates in the sunflower seed char. For wheat bran char, the release of phosphorus increased from 27 % at 900 °C to 71 % at 1100 °C in N2, whereas the release was at least 20 % lower in the oxidizing atmospheres (1–20 % O2, or 10 % CO2). The sunflower seed char reached a maximum release of 55 % at 1100 °C in N2. For wheat bran char, the molar ratio of released carbon/phosphorus was close to 2.5, which fits well with the theoretical value for carbothermic reduction of phosphates (P2O5(s, l) + 5C(s) → P2(g) + 5CO(g)). At 1100 °C, in N2, the release of phosphorus, potassium and sodium occurred mainly during the first 10 min. It was shown that KMgPO4, used as a model compound, could be reduced by carbon starting from 950 °C, but that some of the phosphorus remained in the condensed phase. The work provides a better understanding of phosphorus release and presents evidence showing that carbothermic reduction reactions can be an important phosphorus release mechanism for seed- and grain-based biomass char.
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| 4. |
- Olsson, Emil O. Lidman, et al.
(author)
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Thermal Conversion of Sodium Phytate Using the Oxygen Carrier Ilmenite Interaction with Na-Phosphate and Its Effect on Reactivity
- 2022
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In: Energy & Fuels. - : American Chemical Society (ACS). - 1520-5029 .- 0887-0624. ; 36:17, s. 9423-9436
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Journal article (peer-reviewed)abstract
- Chemical looping combustion (CLC) can be used to convert biomass for heat and/or power production while efficiently capturing the produced CO2. This is possible because the biomass is oxidized by an oxygen carrier instead of directly by air. However, the ash species in biomass can interact with the oxygen carrier causing agglomeration and/or reducing its reactivity. One of the ash elements previously reported to cause problems is phosphorus and especially in combination with alkali. In this work, the interaction between a benchmark oxygen carrier, ilmenite, and a phosphorus model compound, sodium phytate, was studied up to a temperature of 1100 degrees C in N-2 using a fixed bed setup. Activated carbon and NaH2PO4 (thermally decomposing to NaPO3) were also used to study the individual effect of carbon and inorganic Na-phosphate. The CO and CO2 concentration in the flue gas was measured to monitor the oxidation of the samples, which showed that ilmenite participated in the conversion of Naphytate starting from about 600 degrees C. Scanning electron microscopy coupled with energy dispersive X-ray spectroscopy analysis of cross sections of the ilmenite residues revealed that Na-phosphate (forming from Na-phytate) penetrates porous ilmenite particles to a greater extent compared to denser particles, which may reduce the agglomeration tendencies since a lower amount of sticky Naphosphate melt will coat the particle surface. The effect of Na-phytate on the reactivity of ilmenite was quantitatively determined in a fluidized bed using 50% syngas or CO in N-2. For a loading of 1.5 wt % Na-phytate, the reactivity toward CO decreased to only 20% of the reference sample. The reason was partly attributed to a decreased surface area but is likely also due to the formation of less reactive Na-Fe-phosphates. A compilation of thermodynamic data relevant for the NaPO3-FeOx (x = 1 or 1.5) system shows that NaPO3 can form a melt containing dissolved iron starting from around 600 degrees C and that sodium and phosphorus are present solely in this form above approximately 930 degrees C at equilibrium.
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