Comprehensive effect of increased calcium content in coal on the selenium emission from coal-fired power plants: Combined laboratory and field experiments.

Coal combustion is the major contributor to global toxic selenium (Se) emissions. Inorganic elements in coals significantly affect Se partitioning during combustion. This work confirmed that the calcium (Ca) in ash had a stronger relationship with Se retention at 1300 °C than other major elements. Ca oxide chemically reacted with gaseous Se, and its sintering densification slightly affected Se adsorption capacities (44.45 -1840.71→35.17 -1540.15 mg/kg) at 300 - 1300 °C. Therefore, Ca in coals was identified as having potential for hindering gaseous Se emissions, and coals with increased Ca contents (2.74→5.19 wt%) were used in a 350 MW unit. The decreased Se mass distribution (3.54%→2.63%) in flue gas at air preheater inlet (320 -362 °C) confirmed the effectiveness of increased Ca content on gaseous Se emission reduction. More gaseous Se further condensed and was chemically adsorbed by fly ash when passed through an electrostatic precipitator, resulting in a significant increase in the Se content of fly ash. Additionally, the corresponding Se leaching ratio decreased from 4.88 - 35.74% to 1.87 - 26.31%, indicating enhanced stability of Se enriched in fly ash. This research confirmed the feasibility and environmental safety of sequestration of gaseous Se from flue gas to fly ash by increasing the Ca content in coals.

Improving Fine Particle Removal Using a Single-Channel Slit Bubbling Device in Wet Flue Gas Desulfurization System.

To improve the cleanliness of coal-fired power plants' particulate matter emissions, a novel device (single-channel slit bubbling particle removal device (SCSB-PRD)) is proposed to improve the wet flue gas desulfurization system's (WFGDs) collaborative particle removal effect. Actual coal-fired flue gas was used to test the particle removal performance. The results showed that the flue gas temperature had no obvious effect on the scrubbing effect of the SCSB-PRD. The scrubbing space, scrubbing liquid volume, and flue gas flow rate effectively changed the gas-liquid flow state, and the bubbling state was the key factor in particle removal. The jet-bubbling contact state was more conducive to removing particles than the foam bubbling state. The jet-bubbling state improved the removal efficiency of fine particles by approximately 30% compared to the foam bubbling state. The device operated in a single stage, and the removal performance of the particulate matter reached more than 60%. Even the submicron particles had a satisfactory removal performance of greater than 50%. The particulate matter concentration at the outlet of the WFGDs was reduced to less than 10 mg/m3, which provides a feasible transformation path for ultraultra-low emissions of particulate matter from coal-fired power plants.

Modeling of arsenic migration and emission characteristics in coal-fired power plants.

After coal combustion, the minerals present in fly ash can adsorb arsenic (As) during flue gas cooling and reduce As emissions. However, a quantitative description of this adsorption behavior is lacking. Herein, the As adsorption characteristics of minerals (Al/Ca/Fe/K/Mg/Na/Si) were investigated, and a model was developed to predict As content in fly ash. Lab-scale experiments and density functional theory calculations were performed to obtain mineral As adsorption potential. Then, the model was established using lab-scale experimental data for 11 individual coals. The model was validated using lab-scale data from ten blended coals and demonstrated acceptable performance, with prediction errors of 2.83-11.45 %. The model was applied to a 350 MW coal-fired power plant (CFPP) with five blended coals, and As concentration in the flue gas was predicted from a mass balance perspective. The experimental and predicted As contents in fly ash were 11.92-16.15 and 9.61-12.55 µg/g, respectively, with a prediction error of 17.39-22.29 %, and those in flue gas were 11.52-16.58 and 5.37-34.04 µg/Nm3. Finally, As distribution in the CFPP was explored: 0.74-1.51 % in bottom ash, 74.05-82.70 % in electrostatic precipitator ash, 0.53-1.19 % in wet flue gas desulfurization liquid, and 0.13-0.73 % in flue gas at the stack inlet.

Gas-Pressurized Torrefaction of Lignocellulosic Solid Wastes: Deoxygenation and Aromatization Mechanisms of Cellulose.

A novel gas-pressurized (GP) torrefaction method at 250 °C has recently been developed that realizes the deep decomposition of cellulose in lignocellulosic solid wastes (LSW) to as high as 90% through deoxygenation and aromatization reactions. However, the deoxygenation and aromatization mechanisms are currently unclear. In this work, these mechanisms were studied through a developed molecular structure calculation method and the GP torrefaction of pure cellulose. The results demonstrate that GP torrefaction at 250 °C causes 47 wt.% of mass loss and 72 wt.% of O removal for cellulose, while traditional torrefaction at atmospheric pressure has almost no impact on cellulose decomposition. The GP-torrefied cellulose is determined to be composed of an aromatic furans nucleus with branch aliphatic C through conventional characterization. A molecular structure calculation method and its principles were developed for further investigation of molecular-level mechanisms. It was found 2-ring furans aromatic compound intermediate is formed by intra- and inter-molecular dehydroxylation reactions of amorphous cellulose, and the removal of O-containing function groups is mainly through the production of H2O. The three-ring furans aromatic compound intermediate and GP-torrefied cellulose are further formed through the polymerization reaction, which enhances the removal of ketones and aldehydes function groups in intermediate torrefied cellulose and form gaseous CO and O-containing organic molecules. A deoxygenation and aromatization mechanism model was developed based on the above investigation. This work provides theoretical guidance for the optimization of the gas-pressurized torrefaction method and a study method for the determination of molecular-level structure and the mechanism investigation of the thermal conversion processes of LSW.

Gas-pressurized torrefaction of lignocellulosic solid wastes: Low-temperature deoxygenation and chemical structure evolution mechanisms.

A novel gas-pressurized (GP) torrefaction realizes deeper deoxygenation of lignocellulosic solid wastes (LSW) to as high as 79% compared to traditional torrefaction (AP) with the oxygen removal of 40% at the same temperature. However, the deoxygenation and chemical structure evolution mechanisms of LSW during GP torrefaction are currently unclear. In this work, the reaction process and mechanism of GP torrefaction were studied through follow-up analysis of the three-phase products. Results demonstrate gas pressure causes over 90.4% of cellulose decomposition and the conversion of volatile matter to fixed carbon through secondary polymerization reactions. Above phenomena are completely absent during AP torrefaction. A deoxygenation and structure evolution mechanism model is developed through analysis of fingerprint molecule and C structure. This model not only provides theoretical guidance for optimization of the GP torrefaction, but also contributes to the mechanism understanding of pressurized thermal conversion processes of solid fuel, such as coal and biomass.

Flue Gas-Enhanced Water Leaching: AAEM Removal from Agricultural Organic Solid Waste and Fouling and Slagging Suppression during Its Combustion.

Alkali and alkaline earth metals (AAEMs) in agricultural organic solid waste (AOSW) contribute to the fouling and slagging during its combustion. In this study, a novel flue gas-enhanced water leaching (FG-WL) method using flue gas as the heat and CO2 source was proposed for effective AAEM removal from AOSW before combustion. The removal rate of AAEMs by FG-WL was significantly superior to that by conventional water leaching (WL) under the same pretreatment conditions. Furthermore, FG-WL also obviously reduced the release of AAEMs, S, and Cl during AOSW combustion. The ash fusion temperatures of the FG-WL-treated AOSW was higher than that of WL. The fouling and slagging tendency of AOSW greatly decreased through FG-WL treatment. Thus, FG-WL is a simple and feasible method for AAEM removal from AOSW and suppressing fouling and slagging during its combustion. Besides, it also provides a new pathway for the resource utilization of power plant flue gas.

Effect of Al modification on the adsorption of As2O3 on the CaSiO3(001) surface: A DFT study.

CaSiO3 is highly resistant to sintering and can trap arsenic at high temperatures in the boiler furnace. However, the trapping capacity of CaSiO3 for arsenic does not meet the requirements of practical applications, and it is easy to react with acidic gases, which significantly affects the adsorptive property of arsenic. In this paper, the effect of Al modification on the As2O3 adsorption behaviour on the CaSiO3(001) surface was systematically investigated using a density functional theory. By comparing the magnitude of adsorption energy of different sites, the active site of As2O3 adsorbed on the surface of CaSiO3(001) was determined to be Ca, and the adsorption activity of As2O3 by the silicon oxygen chain composed of [SiO4] tetrahedron is deficient. The Si atoms in the [SiO4] tetrahedral structure are directly replaced by Al atoms, the difference in bond length and bond energy between Al-O bond and Si-O bond is used to promote the redistribution of surface charge and the increase of local structural bond angle of CaSiO3(001), leading to the exposure of new active sites (Si-top and Al-top sites) on the silicon oxygen chain. The new active site can realize the chemical adsorption of As2O3, the higher adsorption energy of the Al-top site is attributed to the stronger s-p orbital hybridization between Al and O atoms after doping, which is more conducive to the charge transfer between As2O3 and the adsorbent surface. In this work, influence of SO2 and HCl gases on the adsorption of As2O3 by modified silicon oxygen chains was also discussed. The results show that SO2 and HCl in the flue gas may occupy the Al-top site on the silicon oxygen chain through chemical adsorption, and reduce the activity of this site, thereby affecting the adsorption of As2O3. However, the exposed Si-top sites owing to Al doping show good acidic gas resistance, which in turn help the surface of Al-CaSiO3(001) can also maintain stable adsorption of As2O3 in SO2 and HCl atmosphere.

A critical review on lead migration, transformation and emission control in Chinese coal-fired power plants.

Coal is widely utilized as an important energy source, but coal-fired power plant was considered to be an important anthropogenic lead emission source. In the present study, the distribution characteristics of lead in coal and combustion by-products are reviewed. Specifically, lead is mainly transferred to ash particles and the formation and migration mechanisms of particulate lead are summarized. Also, targeted measures are proposed to control the formation of fine particulate lead as well as to increase the removal efficiency during the low-temperature flue gas clean process. In detail, interactions between gaseous lead and some coal-bearing minerals or added adsorbents could obviously suppress the formation of fine particulate lead. On the other hand, some efforts (including promoting capture of fine particles, reducing resistivity of particles and strengthening the gas-liquid contact) could be made to improve the fine particulate lead removal capacity. Notably, the formation mechanism of fine particulate lead is still unclear due to the limitations of research methods. Some differences in the removal principles of fine particles and particulate lead make the lead emission precisely control a great challenge. Finally, the environmental potential risk of lead emission from flue gas and ash residues is addressed and further discussed.

Fine particulate-bound arsenic and selenium from coal-fired power plants: Formation, removal and bioaccessibility.

The arsenic (As) and selenium (Se) in fine particulate matter (PM10) have attracted increasing attentions due to their health effects. However, the emission control of fine particulate-bound arsenic and selenium (fine particulate-bound As/Se) from coal-fired power plants still faces various challenges. Understanding the formation and characteristics of fine particulate-bound As/Se is crucial for developing specific control technologies. This study clarifies the formation mechanism, removal characteristics, and inhalation bioaccessibility of fine particulate-bound As/Se from industrial coal-fired power plants through methods including aerosol generation, As/Se speciation determination, and in vitro bioaccessibility testing. The findings demonstrated that PM1 from pulverized coal-fired (PC) boilers was enriched with As/Se in terms of concentration and mass distribution. Instead, As/Se was mainly distributed in PM2.5-10 from circulating fluidized bed (CFB) boilers. Limestone injection in CFB boilers promoted As/Se enrichment in coarse PM. Fine particulate-bound As was mainly formed by chemical adsorption of As vapors by Ca-minerals, while the formation of fine particulate-bound Se was closely related to active Ca-minerals and Fe-minerals. Furthermore, Ca-bound As was easy to remove by electrostatic precipitator (ESP) and the removal of physically adsorbed SeO2(s) was difficult, which was caused by the specific resistivity of different mineral components. Importantly, finer particulate-bound As/Se posed higher inhalation bioaccessibility, following the order of PM1 ≥ PM1-2.5 > PM2.5-10. In particular, Ca-bound Se in fine PM owned high bioaccessibility. Based on these findings, measures were proposed to suppress the formation of fine particulate-bound As/Se in the furnace and/or strengthen its removal in the post-combustion stage.

Yield prediction of "Thermal-dissolution based carbon enrichment" treatment on biomass wastes through coupled model of artificial neural network and AdaBoost.

The "Thermal-dissolution based carbon enrichment" was proven as an efficient and homogenizing treatment method in converting biomass wastes into similar high-quality carbon materials. However, their yields varied significantly with respect to the different experimental parameters employed. It is therefore imperative to establish the correlation between product yield and experimental parameters for material selection and condition optimization. In this study, Adaboost was coupled with an artificial neural network algorithm to precisely describe the abovementioned correlation. The results demonstrated the effectiveness of this model through its outstanding predicting performance for all the products, especially, the coefficient of determination in predicting the yield of Residue was as high as 0.97. Additionally, the coupling effect of temperature and time was observed. This study not only validates a close correlation between selected experimental parameters and product yields, but also provides a quick and reliable way for material selection and condition optimization.

Condensation and adsorption characteristics of gaseous selenium on coal-fired fly ash at low temperatures.

Gaseous selenium is of high saturated vapor pressure, making its retention in solid phases quite difficult during coal combustion. The selenium transformation from gaseous form into solid phases at low temperatures can be essential to control selenium emission. To understand the migration of SeO2 (g) on ash particles in the low-temperature zone, this study investigated the speciation of selenium in fly ash and simulated the physical retention of SeO2 (g) on fly ash. The results demonstrated that there was a large proportion of physically-bound Se in the fly ash of pulverized-coal-fired boiler (22.62 %-58.03%), while the content of physically-bound Se in fly ash of circulated fluidized-bed boiler was lower (∼6%). The physically-bound Se was formed through selenium condensation and physical adsorption. The decrease of temperature or the increase of cooling rate could promote the transformation of gaseous selenium to solid phase and the presence of HCl might suppress SeO2 transformation into Se in the condensation process. Meanwhile the compositions of fly ash had a great influence on the selenium adsorption process. Among typical coal-fired ash components, mullite showed the best performance in the selenium capture in the temperature range of 90-200 °C, contributing to the high content of physically-adsorbed selenium in PC fly ash. These findings provided new ideas for improving the removal rate of volatile selenium.

Kinetic Study on Continuous Sampling of Coal Char from a Micro Fluidized Bed.

We self-design a micro fluidized bed reactor (MFB) with combination of an online char particle sampling system to study the kinetics of coal char combustion and gasification. The system mainly contains two parts: a micro fluidized bed and vacuum online sampling. Vientiane coal was continuously sampled from the MFB. Both combustion and gasification reactivities of the sampled chars were tested in a thermogravimetric analyzer. Kinetic parameters of the sampled char were analyzed. Char reactivity in oxy-fuel combustion in the MFB obeys the rule of decrease-increase-decrease behavior with the sampling time. Pre-exponential factor A and activation energy E of the sampled char increase with the sampling time. The gasification reactivity of the sampled char increases with the sampling time even though there is a minor decrease in an initial gasification stage. The new designed MFB combining with the online sampling system will pave the path for the investigation of gas-solid reaction evolution in the future.

Potential hazards of novel waste-derived sorbents for efficient removal of mercury from coal combustion flue gas.

Novel waste-derived sorbents synthesized through one-step co-pyrolysis of wood and PVC (or brominated flame retarded plastic) were demonstrated as cost-effective sorbents for mercury (Hg) removal in our previous studies. To introduce magnetism and improve porosity, Fe species were further doped into such waste-derived sorbents. The ultimate fate of Hg-laden sorbents after their service is mainly disposed in landfill. Therefore, the stability of Hg/halogens on the spent sorbents is an important topic. In this work, the leachability of Hg/Cl/Br from four waste-derived sorbents was evaluated using toxicity characteristic leaching procedure (TCLP). Three traditional sorbents (Cl-impregnated activated carbon, Br-impregnated activated carbon and commercial activated carbon) were also tested for comparison. Experimental results suggested that the stability of Hg/Cl/Br on four waste-derived sorbents was far higher than that prepared by chemical impregnation. For four waste-derived sorbents, little Hg was leached out whereas certain amounts of Cl/Br escaped into the leachate. Interestingly, Fe-doping effectively improved the stability of Hg/Cl/Br on the waste-derived sorbents. Kinetic analysis revealed that diffusion process and surface chemical reaction were respectively the rate-limiting step for waste-derived sorbents before and after Fe-doping. Water-washing pretreatment could remove loosely-bonded Cl/Br from the waste-derived sorbents, while the Cl/Br essential for Hg removal was retained.

Do FeCl3 and FeCl3/CaO conditioners change pyrolysis and incineration performances, emissions, and elemental fates of textile dyeing sludge?

The pyrolysis and incineration performances of sulfur-rich textile dyeing sludge (TDSS) were determined in response to the additions of FeCl3 or FeCl3 + CaO. The emissions of eight air pollutants from the incineration and pyrolysis were systematically identified. The 3-to-8% FeCl3 additions increased the comprehensive combustibility index by 2.14 and 1.62 times, respectively, as opposed to the 5-to-10% FeCl3 + 8-to-15% CaO additions. The CaO addition inhibited the TDSS incineration, while the FeCl3 addition increased HCl emission. NOx, SO2, and H2S emissions decreased initially and increased between 600 and 950 °C. SO2 and NOx emissions rose with FeCl3 but FeCl3 + CaO. FeCl3 catalyzed NOx, while CaO retained SO2. The main pyrolysis gas/liquid products were alkane, alkenes, nitrile, heterocyclic compounds, benzene, and its derivatives. Benzene and its derivatives accounted for 55.33% of the control group and 42.25-57.23% of the treatment groups. The FeCl3 and FeCl3 + CaO additions did not significantly influence the pyrolysis products. The measured versus thermodynamically simulated SOx and HCl emissions were consistent. Neural network-based simultaneous optimizations of the non-linear dynamics of eight kinds of gases pointed to 50% and 14.4% reductions in the emissions and the pyrolytic temperature, respectively, with the 3% FeCl3, relative to the control.

Fate of chromium with the presence of HCl and steam during oxy-coal combustion: Quantum chemistry and experimental study.

Quantum chemistry combined with kinetic simulation and drop tube furnace (DTF) experiments were conducted to reveal the transformation behavior of chromium at the presence of steam and HCl under oxy-coal combustion. A completed kinetic system Cr‒O‒H‒Cl containing 107 elementary reactions was firstly proposed. The unknown microcosmic reaction paths and corresponding Arrhenius parameters were calculated via quantum chemistry. Kinetic simulations on the basis of Cr‒O‒H‒Cl system clarified that HCl promoted the oxidation of chromium to hexavalent CrO2Cl2. Coexistence of HCl and steam divided the transition of chromium into two stages. At the early stage, reaction rate between chromium and steam was faster than chromium with HCl, chromium mainly transformed to CrO(OH)2. Hereafter, HCl was dominant in the transformation of chromium, then chromium mainly presented as CrO2Cl2. Moreover, DTF experimental results indicated that introduction of HCl into combustion atmosphere induced more chromium release. Presence of steam reinforced the effect of HCl on chromium attributing to the significant transition of CrOx(OH)y to CrO2Cl2. Although CaO and Fe2O3 both exhibited good reactivity with chromium, presence of HCl largely suppressed chromium capture by CaO and Fe2O3. Moreover, the inhibition effect of HCl on Fe2O3 was stronger than CaO for Cr capture.

Gas-pressurized torrefaction of biomass wastes: Co-gasification of gas-pressurized torrefied biomass with coal.

Co-gasification of coal and biomass offers a relatively cleaner utilization way of fossil fuel. The fuel property improvement of biomass can not only improve the property of syngas but also enhance the synergistic effect during the co-gasification. In our previous work, a novel gas-pressurized (abbreviated as GP) torrefaction was proposed to effectively upgrade the biomass under mild condition. In this work, the co-gasification of GP torrefied biomass and coal were conducted to explore the synergistic effect and kinetics. Significant synergistic effect during the co-gasification was proved. The CO yield of co-gasification increased to as high as 70.70 mol/kg, resulting from the promotion of carbon in coal converting into CO by GPRS. Furthermore, the kinetic model of RPM was most fitting for the co-gasification, and the activation energy of co-gasification was reduced. Thus, the coal gasification was promoted significantly by GP torrefied biomass through obvious synergistic effect during the co-gasification.

Surface modification of fly ash by non-thermal air plasma for elemental mercury removal from coal-fired flue gas.

The fly ash from a coal-fired power plants was modified with non-thermal plasma in air to improve the elemental mercury (Hg0) removal performance. The Hg0 adsorption experiments were implemented via a bench-scale fixed-bed reactor system. Brunauer-Emmett-Teller (BET), X-ray photoelectron spectroscopy (XPS), ultimate, X-ray diffraction (XRD) and XRF analysis were employed to characterize the fly ash. The effect of non-thermal plasma voltage and time on Hg0 removal efficiency was investigated. The results showed that fly ash had better Hg0 removal performance when treatment voltage was 3.0 kV and treatment time was 7 min. Furthermore, the results showed that non-thermal plasma treatment had little influence on the specific surface area. However, non-thermal plasma treatment increased the relative content of oxygen. XPS and temperature programmed desorption results indicated that Hg0 removal process included adsorption and oxidation of Hg0. Moreover, ester and carbonyl groups played extremely vital roles in the improvement of Hg0 removal performance, and their temperature programmed desorption peak occurred at around 250°C and 320°C, respectively.

Migration and emission behavior of arsenic and selenium in a circulating fluidized bed power plant burning arsenic/selenium-enriched coal.

Arsenic (As) and selenium (Se) pollution caused by coal combustion is receiving increasing concerns. The environmental impacts of As/Se are determined not only by stack emission but also by leaching process from combustion byproducts. For a better control of As/Se emission from As/Se-enriched coal combustion, this study investigated the migration and emission behavior of As/Se in a circulating fluidized bed (CFB) power plant equipped with fabric filter (FF) and wet flue gas desulfurization (WFGD) system. The results demonstrated that arsenic was both enriched in bottom ash (41.4-47.6%) and fly ash (52.4-58.6%), while selenium was mainly captured by fly ash (73.9-83.4%). Limestone injection into furnace promoted As/Se retention in ash residues. Arsenic was mainly converted into arsenate in high-temperature regions and partly trapped in bottom ash as arsenite. In contrast, selenium capture mainly occurred in low-temperature flue gas by the formation of selenite, because of the poor thermal stability of most selenite. Triplet-tank method can totally remove arsenic in WFGD wastewater. And 18.4-58.7% of selenium was removed, resulting from the precipitation of Se4+ anions with highly soluble Se6+ anions remaining in wastewater. The concentrations of As and Se in the stack emission were 0.25-1.02 and 0.96-2.24 µg/m3, receptively. The CFB boiler equipped with FF + WFGD was shown to provide good control of the As/Se emission into the atmosphere. Leaching tests suggested that more attention should be paid to As leachability from fly ash/gypsum, and Se leachability from gypsum/sludge.

Mercury stable isotope fractionation during gaseous elemental mercury adsorption onto coal fly ash particles: Experimental and field observations.

Mercury (Hg) stable isotopes have a great potential to track coal combustion Hg emissions, but mass-dependent fractionation (MDF) during Hg adsorption onto fly ash particles could significantly alter isotope signatures of emitted Hg species. The detailed processes causing this MDF, however, are not well understood. Here, we simulated how isotopes fractionate during gaseous Hg0 adsorption onto fly ash at different times and temperatures. Kinetic MDF that preferably transfers light Hg isotopes to fly ash dominated Hg0 adsorption processes. The magnitude of MDF during Hg0 adsorption was invariable in the time-series experiment but increased significantly with increasing temperature in the temperature-series experiment. The external mass transfer and chemisorption are suggested to be the controlling processes for isotopic fractionation. Relative to diffusion-driven Hg0 adsorption, chemisorption is suggested to be a more important Hg0 adsorption step causing MDF, especially at high temperatures. The chemisorption involves Hg redox change from Hg0 to HgII and is likely enhanced with increasing temperature (50-180 °C). The proposed kinetic MDF model reveals that MDF in modern coal-fired power plants is likely driven by temperature-induced redox processes during Hg0 adsorption, and has great implications for developing MDF models in coal-fired boilers and tracing coal combustion Hg emissions.

Gas-pressurized torrefaction of biomass wastes: The optimization of pressurization condition and the pyrolysis of torrefied biomass.

A novel gas-pressurized (GP) torrefaction with high oxygen removal efficiency at mild temperature was proposed in our previous work. However, the optimal condition of the GP torrefaction and subsequent pyrolysis of the torrefied biomass were not clear. In this work, the effect of pressure on the GP torrefaction and pyrolysis product properties of the torrefied biomass were studied in detail. The results show that the pressure increasing from 1.7 MPa to 5.0 MPa just slightly contributed to further oxygen removal, and 1.7 MPa was thus selected as the optimum pressure. The GP torrefaction significantly improved the product property of biomass pyrolysis compared to the conventional torrefaction (AP torrefaction). The acids content in bio-oil was reduced from 15-20% to less than 5%, and the calorific value of biogas increased to as high as 16.57-19.31 MJ/Nm3. Furthermore, an overall conversion mechanism of combined GP torrefaction and subsequent pyrolysis of biomass was proposed.