Yanjun Dai

Activated carbon derived from carbon residue from biomass gasification and its application for dye adsorption: Kinetics, isotherms and thermodynamic studies

In this work, activated carbon (AC) as an effective and low-cost adsorbent was successfully prepared from carbon residue (or char, one of the by-products from woody biomass gasification) via physical activation. The surface area of char was significantly increased from 172.24 to 776.46m(2)/g after steam activation at 900°C. The obtained activated carbons were then employed for the adsorption of dye (Rhodamine B) and it was found that activated carbon obtained from steam activation exhibited the highest adsorption capability, which is mainly attributed to the higher surface area and the abundance of hydroxyl (-OH) and carboxyl (-COOH) groups on the activated carbon surface. Moreover, it was also found that the adsorption capability significantly increased under the basic condition, which can be attributed to the increased electrostatic interaction between the deprotonated (negatively charged) activated carbon and dye molecules. Furthermore, the equilibrium data were fitted into different adsorption isotherms and found to fit well with Langmuir model (indicating that dye molecules form monolayer coverage on activated carbon) with a maximum monolayer adsorption capability of 189.83mg/g, whereas the adsorption kinetics followed the pseudo-second-order kinetics.

Comparison of the co-gasification of sewage sludge and food wastes and cost-benefit analysis of gasification- and incineration-based waste treatment schemes.

The compositions of food wastes and their co-gasification producer gas were compared with the existing data of sewage sludge. Results showed that food wastes are more favorable than sewage sludge for co-gasification based on residue generation and energy output. Two decentralized gasification-based schemes were proposed to dispose of the sewage sludge and food wastes in Singapore. Monte Carlo simulation-based cost-benefit analysis was conducted to compare the proposed schemes with the existing incineration-based scheme. It was found that the gasification-based schemes are financially superior to the incineration-based scheme based on the data of net present value (NPV), benefit-cost ratio (BCR), and internal rate of return (IRR). Sensitivity analysis was conducted to suggest effective measures to improve the economics of the schemes.

Three-stage anaerobic co-digestion of food waste and horse manure

A novel compact three-stage anaerobic digester (HM3) was developed to combine the advantages of high solids anaerobic digestion (AD) and wet AD for co-digestion of food waste and horse manure. By having three separate chambers in the three-stage anaerobic digester, three different functional zones were created for high-solids hydrolysis, acidogenesis and wet methanogenesis. The results showed that the functionalized partitioning in HM3 significantly accelerated the solubilization of solid organic matters and the formation of volatile fatty acids, resulting in an increase of 11~23% in methane yield. VS reduction in the HM3 presents the highest rate of 71% compared to the controls. Pyrosequencing analysis indicated that different microbial communities in terms of hydrolyzing bacteria, acidogenic bacteria and methanogenic archaea were selectively enriched in the three separate chambers of the HM3. Moreover, the abundance of the methanogenic archaea was increased by 0.8~1.28 times compared to controls.

Co-gasification of sewage sludge and woody biomass in a fixed-bed downdraft gasifier: Toxicity assessment of solid residues

As the demand for fossil fuels and biofuels increases, the volume of ash generated will correspondingly increase. Even though ash disposal is now strictly regulated in many countries, the increasing volume of ash puts pressure on landfill sites with regard to cost, capacity and maintenance. In addition, the probability of environmental pollution from leakage of bottom ash leachate also increases. The main aim of this research is to investigate the toxicity of bottom ash, which is an unavoidable solid residue arising from biomass gasification, on human cells in vitro. Two human cell lines i.e. HepG2 (liver cell) and MRC-5 (lung fibroblast) were used to study the toxicity of the bottom ash as the toxins in the bottom ash may enter blood circulation by drinking the contaminated water or eating the food grown in bottom ash-contaminated water/soil and the toxic compounds may be carried all over the human body including to important organs such as lung, liver, kidney, and heart. It was found that the bottom ash extract has a high basicity (pH = 9.8-12.2) and a high ionic strength, due to the presence of alkali and alkaline earth metals e.g. K, Na, Ca and Mg. Moreover, it also contains concentrations of heavy metals (e.g. Zn, Co, Cu, Fe, Mn, Ni and Mo) and non-toxic organic compounds. Although human beings require these trace elements, excessive levels can be damaging to the body. From the analyses of cell viability (using MTS assay) and morphology (using fluorescence microscope), the high toxicity of the gasification bottom ash extract could be related to effects of high ionic strength, heavy metals or a combination of these two effects. Therefore, our results suggest that the improper disposal of the bottom ash wastes arising from gasification can create potential risks to human health and, thus, it has become a matter of urgency to find alternative options for the disposal of bottom ash wastes.

Chemically treated carbon black waste and its potential applications.

In this work, carbon black waste - a hazardous solid residue generated from gasification of crude oil bottom in refineries - was successfully used for making an absorbent material. However, since the carbon black waste also contains significant amounts of heavy metals (especially nickel and vanadium), chemical leaching was first used to remove these hazardous impurities from the carbon black waste. Acid leaching with nitric acid was found to be a very effective method for removal of both nickel and vanadium from the carbon black waste (i.e. up to 95% nickel and 98% vanadium were removed via treatment with 2M nitric acid for 1h at 20°C), whereas alkali leaching by using NaOH under the same condition was not effective for removal of nickel (less than 10% nickel was removed). Human lung cells (MRC-5) were then used to investigate the toxicity of the carbon black waste before and after leaching. Cell viability analysis showed that the leachate from the original carbon black waste has very high toxicity, whereas the leachate from the treated samples has no significant toxicity. Finally, the efficacy of the carbon black waste treated with HNO3 as an absorbent for dye removal was investigated. This treated carbon black waste has high adsorption capacity (∼361.2mg dye/g carbonblack), which can be attributed to its high specific surface area (∼559m2/g). The treated carbon black waste with its high adsorption capacity and lack of cytotoxicity is a promising adsorbent material. Moreover, the carbon black waste was found to show high electrical conductivity (ca. 10S/cm), making it a potentially valuable source of conductive material.

Energy performance of an integrated bio-and-thermal hybrid system for lignocellulosic biomass waste treatment.

Lignocellulosic biomass waste, a heterogeneous complex of biodegradables and non-biodegradables, accounts for large proportion of municipal solid waste. Due to limitation of single-stage treatment, a two-stage hybrid AD-gasification system was proposed in this work, in which AD acted as pre-treatment to convert biodegradables into biogas followed by gasification converting solid residue into syngas. Energy performance of single and two-stage systems treating 3 typical lignocellulosic wastes was studied using both experimental and numerical methods. In comparison with conventional single-stage gasification treatment, this hybrid system could significantly improve the quality of produced gas for all selected biomass wastes and show its potential in enhancing total gas energy production by a maximum value of 27% for brewers spent grain treatment at an organic loading rate (OLR) of 3gVS/L/day. The maximum overall efficiency of the hybrid system for horticultural waste treatment was 75.2% at OLR of 11.3gVS/L/day, 5.5% higher than conventional single-stage system.

A comparison of PM exposure related to emission hotspots in a hot and humid urban environment: Concentrations, compositions, respiratory deposition, and potential health risks

Particle number concentration, particle size distribution, and size-dependent chemical compositions were measured at a bus stop, alongside a high way, and at an industrial site in a tropical city. It was found that the industry case had 4.93×107-7.23×107 and 3.44×104-3.69×104#/m3 higher concentration of particles than the bus stop and highway cases in the range of 0.25-0.65μm and 2.5-32μm, respectively, while the highway case had 6.01×105 and 1.86×103#/m3 higher concentration of particles than the bus stop case in the range of 0.5-1.0μm and 5.0-32μm, respectively. Al, Fe, Na, and Zn were the most abundant particulate inorganic elements for the traffic-related cases, while Zn, Mn, Fe, and Pb were abundant for the industry case. Existing respiratory deposition models were employed to analyze particle and element deposition distributions in the human respiratory system with respect to some potential exposure scenarios related to bus stop, highway, and industry, respectively. It was shown that particles of 0-0.25μm and 2.5-10.0μm accounted for around 74%, 74%, and 70% of the particles penetrating into the lung region, respectively. The respiratory deposition rates of Cr and Ni were 170 and 220 ng/day, and 55 and 140ng/day for the highway and industry scenarios, respectively. Health risk assessment was conducted following the US EPA supplemented guidance to estimate the risk of inhalation exposure to the selected elements (i.e. Cr, Mn, Ni, Pb, Se, and Zn) for the three scenarios. It was suggested that Cr poses a potential carcinogenic risk with the excess lifetime cancer risk (ELCR) of 2.1-98×10-5 for the scenarios. Mn poses a potential non-carcinogenic risk in the industry scenario with the hazard quotient (HQ) of 0.98. Both Ni and Mn may pose potential non-carcinogenic risk for people who are involved with all the three exposure scenarios.

Study on Trough Receiver for Linear Concentrating Solar Collector

In this paper, a trough black body cavity receiver for linear concentrating collector, which has potential application at moderate working temperatures (near 100°C) or higher temperature (above 180°C), has been studied. Four different shapes of the receiver have been proposed and tested. The optical efficiencies of the four receivers are simulated by using light tracking method. The thermal performances of the cavity receivers are tested under temperature levels, 90°C and 150°C, where the concentrated solar radiation is simulated by electric heating film. Both optical and thermal analysis illustrates that the triangle trough receiver is the best choice. To reduce the thermal losses, three kinds of glass cover are fixed on the cavity receiver. The thermal losses of the three different receivers with glass cover have been tested, when the inlet temperature varies from 35°C to 95°C. The experimental results indicate that the glass cover can reduce the thermal losses effectively. The semicircular cavity is tested in an linear concentrating solar collector, in which the sunlight is concentrated by a linear Fresnel lens(aperture width is 0.4m and optical efficiency is 0.7) and scatters on the semicircular cavity wall, absorbed by the copper tube and transmitted to working fluid. The highest average efficiency can reach 43% when the inlet temperature of the working fluid is 90°C. It is found from experimental results that the efficiency of this designed cavity receiver is a little lower than that of evacuated tube, but has good potential in application because it is cheap and can be manufactured easily.

Evaluating the effects of activated carbon on methane generation and the fate of antibiotic resistant genes and class I integrons during anaerobic digestion of solid organic wastes

The effects of activated carbon (AC) on methane production and the fate of antibiotic resistance genes (ARGs) were evaluated through comparing the anaerobic digestion performance and transformation of ARGs among anaerobic mono-digestion of food waste, co-digestion of food waste and chicken manure, and co-digestion of food waste and waste activated sludge. Results showed that adding AC in anaerobic digesters improved methane yield by at least double through the enrichment of bacteria and archaea. Conventional digestion process showed ability in removing certain types of ARGs, such as tetA, tetX, sul1, sul2, cmlA, floR, and intl1. Supplementing AC in anaerobic digester enhanced the removal of most of the ARGs in mono-digestion of food waste. The effects tended to be minimal in co-digestion of co-substrates such as chicken manure and waste activated sludge, both of which contain a certain amount of antibiotics.

Metagenomic insight into the microbial networks and metabolic mechanism in anaerobic digesters for food waste by incorporating activated carbon

Powdered activated carbon (AC) is commonly used as an effective additive to enhance anaerobic digestion (AD), but little is known about how the metabolic pathways resulting from adding AC change the microbial association network and enhance food waste treatment. In this work, the use of AC in an anaerobic digestion system for food waste was explored. Using bioinformatics analysis, taxonomic trees and the KEGG pathway analysis, changes in microbial network and biometabolic pathways were tracked. The overall effect of these changes were used to explain and validate improved digestion performance. The results showed that AC accelerated the decomposition of edible oil in food waste, enhancing the conversion of food waste to methane with the optimized dosage of 12 g AC per reactor. Specifically, when AC was added, the proponoate metabolic pathway that converts propanoic acid to acetic acid became more prominent, as measured by 16S rRNA in the microbial community. The other two metabolic pathways, Lipid Metabolism and Methane Metabolism, were also enhanced. Bioinformatics analysis revealed that AC promoted the proliferation of syntrophic microorganisms such as Methanosaeta and Geobacter, forming a highly intensive syntrophic microbial network.