Mid Sweden University

The excessive presence of duckweed has led to the deterioration of water quality, which requires an efficient method for its high-value conversion. Therefore, catalytic pyrolysis of duckweed was conducted to elucidate the effects of H3PO4 impregnation on the yield and composition of the gaseous, liquid and solid products. The results showed that after H3PO4 impregnation, the solid and gas yields increased by up to 43.67 % and 51.78 %, respectively, compared to the case without H3PO4. Both the yields and fraction of H2 increased remarkably, after H3PO4 impregnation, resulting in an increase of H2/CO ratio. At low temperature of 400–500 °C, H3PO4 impregnation effectively reduced the oxygenated compounds and facilitated the formation of N-containing compounds. With increasing temperature, the phosphoric acid promoted the aromatization of aliphatic hydrocarbons to form aromatic hydrocarbons and enhanced the production of phenols. Additionally, the introduction of H3PO4 increased the carbon retention in solid char by 9.9–19.3 % probably due to the formation of relatively stable phosphate esters via crosslink reactions with the saccharides in duckweed. The optimal temperature and H3PO4 impregnation ratio were recommended considering the yield and composition of the products, as well as the energy consumption during pyrolysis. 

Duckweed (DW) has a promising potential for wastewater treatment due to its outstanding performance in the fixation of nutrient elements and heavy metals. The conversion of harvested duckweed into value-added products through pyrolysis is an attractive method for duckweed utilization as fuels or chemicals. In this work, the duckweed was prepared by deashing treatment and subsequent impregnation with different phosphoric acid concentrations (ADW-P). The pyrolysis behavior and kinetics of raw and impregnated duckweeds were studied with respect to the ash contained in the duckweed and the phosphoric acid catalytic effect by thermogravimetric analysis-fourier transform infrared spectrometer (TG-FTIR) and pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS). The results show that the pyrolysis reaction zone is extended for the impregnated duckweed by reducing the initial pyrolysis temperature and increasing the final temperature. Specifically, a shoulder peak situated at the right side of the main peak is present in the DTG curves of ADW-P, suggesting the formation of relatively stable substance after phosphoric acid impregnation. Phosphoric acid promotes the release of H2O, CH4, CO2 and NH3 as well as light volatiles with C[dbnd]C and C[dbnd]O groups during pyrolysis. Meanwhile, decarboxylation and aromatization as well as deamination of organic compounds are strengthened, resulting in an increase of aromatic hydrocarbons and furans and a decrease of N-heterocyclic compounds. The activation energy of ADW-P is lower than that of DW and ADW at conversion rate less than 75% but increases remarkably at high conversion rate. This implies that phosphoric acid facilitates the thermal decomposition of DW at low temperatures but hinders its decomposition at high temperatures probably due to the formation of stable cross-linked structures such as phosphate and polyphosphate esters. 

Torrefaction is regarded as a promising way to improve the fuel properties of biomass. In this work, a typical agricultural biomass of cotton stalk with high supply availability was employed to reveal the correlation between torrefaction conditions and fuel quality as well as pelletizing property. Cotton stalk was torrefied at 220–300 °C with a wide oxygen concentration of 0%–21% using a fixed bed reactor. The fuel qualities of torrefied samples were analyzed and the pelletizing properties were investigated using a universal material testing machine. The results showed that both non-oxidative and oxidative torrefaction significantly improved the heating value at a maximum of 20.48%, while extreme conditions of 300 °C with 10%–21% concentration were avoided due to the excessive consumption of combustible substances. Four key pelletizing parameters, including pellet density, compressive strength, durability and hydrophobicity, were improved, while the energy consumption increased, mainly attributed to the reduction of hydrophilic functional groups and the increased friction force. Response surface methodology was introduced and it was indicated that the pelletizing properties were sensitive to the temperature, followed by oxygen. The operating conditions were optimized by central composite design and a torrefaction temperature of 260–270 °C with an oxygen concentration of 2%–3% were recommended to produce torrefied biomass pellet with good fuel and pelletizing properties. 

Decentralized composting is an emerging method for managing biowaste, engaging waste generators as active recyclers in the waste management cycle. Evaluating performance and identifying optimization opportunities within this composting framework is essential to maximize its benefits and address its challenges. In small-scale composters, fresh waste is continuously mixed with previously added materials, shifting the typical composting process. As with larger systems, the composition of the feedstock influences the temperature profile and the quality of the final product. The issue of whether to include animal-source waste remains controversial in the development of standards and program guidelines. On the other hand, evaluating a leachate recycling method could help prevent nutrient loss and mitigate environmental impacts when bulking agents are lacking. In this study, kitchen and garden wastes were composted in 500-L static composters under cold climate conditions. We examined obtained compost stability, maturity, and quality parameters to determine the effects of adding animal by-product waste and/or recycling leachate. Our findings indicate that including animal by-products allows reaching sanitation temperatures under cold weather conditions and that recycling leachates could reduce nutrient losses and alleviate environmental and other user concerns while improving temperature, stability, maturity, and product quality patterns in decentralized composting. 

The use of cheap product gas from biomass air gasification to produce methane via anaerobic digestion is a novel and potential pathway for the large-scale production of biomass-based substitute natural gas (BioSNG). In this experimental work, the product gas biomethanation (PGB) was studied with respect to the biosludge enrichment and inoculum partial grinding as well as the mesophilic and thermophilic conditions. The results show that the biosludge enrichment can effectively stop methanogenesis inhibition from the product gas, particularly CO, thus increase the biomethanation reaction rate and shorten the reaction start-up time. The inoculum partial grinding treatment can clearly change the microorganism composition and effectively reduce the diversity of microorganisms in the mixed bacterium system for the mesophilic biomethanation, thereby improving the product gas biomethanation efficiency, which is limited for the thermophilic biomethanation.

Pyrolysis is a thermo-chemical conversion method for harmless and resource utilization of sewage sludge, which gives carbon-containing products with high added value and benefits for GHG reduction towards “carbon peaking and carbon neutrality” goals. In this work, co-pyrolysis of sewage sludge and poplar wood was studied to investigate the effects of the wood blend ratio and the volatile-char interactions on the pyrolysis product characteristics. It was found that the synergistic effect during co-pyrolysis could enhance the production of aromatic hydrocarbons but inhibit the formation of nitrogen-containing and phenolic compounds. Meanwhile, the aromaticity of the char increased with increasing the wood blend ratio, resulting in an enhanced quality of the char. The volatile-char interactions could facilitate the cracking of large molecules in volatiles into small-molecule gases, leading to an increase in the gas yield of 0.6–14.6 %, and especially the H2 yield of 16.2–53.8 %, as compared to the case without interaction in the experiment. The char yields hold fairly constant but the physicochemical structure of the char changed significantly with the interactions. Specifically, the O-containing functional groups on the char surface decreased significantly with increasing aromaticity and stability. More importantly, the total phosphorus content of char was increased by 11.3–33.6 %, as compared to the case without interaction, with the enhanced conversion of non-hydroxyapatite phosphorus to hydroxyapatite phosphorus. The interaction can increase bio-availability of the phosphorus and make biochar to be a better organic fertilizer in application. 

The syngas produced from biomass gasification is a great potential energy resource, which can well be utilized to produce biomass-based substitute natural gas (BioSNG) via syngas biomethanation. CO biomethanation is one of the key issues in the biomethanation process and was studied experimentally in this work with respect to the effect of anaerobic granular sludge semi-disaggregation. The results show 1.07 times higher averaged CH4 production rate with the semi-disaggregated granular sludge than the whole granular sludge at 35 °C, and 1.69 times higher at 55 °C. The main mechanisms behind the enhanced CH4 production rate, especially under the thermophilic condition, are the improvement of microbial interspecific syntrophic association caused by the higher electron and substrate transfer rate, and more active cell growth and metabolism as reflected in higher abundance of functional genes and enzymes and less useless extracellular polymeric substances. The CO biomethanation enhancement occurs in the conversion of the substrate to the intermediate products. The semi-disaggregation of anaerobic granular sludge or similar way to strengthen interspecific association is an effective approach to improve the ability and tolerance of microbial cultures under the CO atmosphere. This technique can well be applied for the energy conversion from the CO-rich gas substrates into BioSNG via CO biomethanation under the thermophilic condition, or for the production of intermediates as fuels/chemicals under the mesophilic condition. 

Alkali-associated problems are key issues for the efficient use of straw that is available as a major renewable energy resource worldwide. The effects of six bed materials commonly used in fluidized bed reactors on straw pyrolysis and char gasification were evaluated using online monitoring of alkali release and thermogravimetric analysis. Scanning electron microscopy with energy dispersive spectroscopy was used to determine the elemental composition of the char surface. In the straw pyrolysis stage, alkali release is reduced by the addition of dolomite and silica due to alkali adsorption on the bed materials, and enhanced by the addition of alumina because of its high sodium content. In the char gasification stage, silica, sea sand, olivine, and ilmenite reduce the char reactivity and alkali release, which is attributed to transfer of Si and Ti from the bed materials to the char and reaction with alkali to form stable and catalytically inactive compounds. Alumina also reduces the char conversion rate by transfer of Al to the char and formation of K-Al-Si and Ca-Al-Si compounds, while alkali release from the straw and alumina blend remains high due to the high Na content in alumina. Dolomite initially appears to increase the char gasification reactivity, but the results are affected by conversion of volatile matter that deposited on the dolomite in the straw pyrolysis stage. Dolomite also significantly increases the alkali release, which is attributed to Ca reactions with aluminosilicate compounds that allow potassium to remain in volatile form. Fresh bed materials are concluded to have significant effects on straw conversion depending on their chemical composition, and the results can contribute to the understanding required for efficient use of straw in commercial applications of biomass thermochemical conversion. 

In order to realize the harmless treatment and resource utilization of municipal sludge, the effect of volatile-char interaction on the properties of sludge pyrolysis products was investigated using a two-stage pyrolysis reactor. The experimental results show that volatile-char interaction promotes the decomposition of organic compounds into small molecular gases, which increases the gas yield and reduces the liquid and oil yields. Under the experimental conditions, the volatile-char interaction increases the mass yields of H2, CO and CO2 to varying degrees, and the yield of H2 increases maximum by 55.7–114.5% compared to that without interaction. The effect of interaction on the composition of bio-oil occurs mainly at low temperature range of 400–600 ℃ by reducing the content of oxygen-containing compounds except for aldehydes/ketones. The physical and chemical structure of solid char changes dramatically with the interaction. The enhanced removal of O-containing groups occurs and the degree of aromaticity of the char is increased due to the strengthened dehydrogenation and polycondensation of hydrocarbons. Meanwhile, the transformation of non-hydroxyapatite phosphorus to hydroxyapatite phosphorus in solid phase products is greatly promoted at high temperature of 700–800 ℃, increasing the potential utilization of char as carbon-based phosphate fertilizer in agriculture. 

The nano-scaled zero valent iron (nZVI) particles were applied to strengthen the syngas biomethanation under the thermophilic condition in a continuous bubble column reactor with gas circulation. The CH4 productivity was increased by 6.80% from 71.20 mmol & BULL;Lr  1 & BULL;day  1 to the highest 76.04 mmol & BULL;Lr  1 & BULL;day  1 at the nZVI concen-tration of 2.5 g/L. The measurement of iron concentration and the observation of the iron nanoparticles dis-tribution indicate that nZVI can act as an electron conduit to enhance more efficient direct interspecies electron transfer by physically close contact between microorganisms, instead of biological corrosion. Further analysis of metabolic products shows that the nZVI addition can stimulate the EPS secretion, and the direct electron transfer relying on nZVI particles tends to replace other transfer modes. Microbial community analysis reveals that the Bacteria Bacteroidia and Firmicutes, and Archaea Methanothermobacter are the potential dominating enriched syntrophic partners. The expression of functional genes involved in methane production was also found to in-crease. On the other hand, the nZVI accumulation can lead to the albefaction and inactivation of partial sludge granules due to its toxicity. The negative effects of nZVI at high concentration are also more pronounced. This work shows the feasibility of improving continuous syngas biomethanation by strengthening interspecific as-sociation and accelerating electron transfer.