Sludge stabilization and energy recovery by hydrothermal carbonization process

Hydrothermal carbonization (HTC) is a thermal conversion process that converts high-moisture biomass into hydrochar. HTC was applied to stabilize and process sludge collected from septic tanks into hydrochar for practical energy recovery. Experiments were conducted with a 1-L high-pressure reactor operating at different temperatures and reaction times in which the sludge was mixed with catalysts and biomass at different ratios. The effects of catalysts (i.e., acetic acid, lithium chloride, borax, and zeolite) and biomass (i.e., cassava pulp, dried leaves, pig manure, and rice husks) mixing with sludge for hydrochar production were investigated. The experimental data showed acetic acid and cassava pulp to be the most effective catalyst and biomass, respectively, increasing energy contents to the maximum value of 28.5 MJ/kg. The optimum HTC conditions were as follows: sludge/acetic acid/cassava pulp mixing ratio of 1/0.4/1 (by weight), at a temperature of 220 °C, and reaction time of 0.5 h. The relatively high energy contents of the produced hydrochar suggest its applicability as a solid fuel.     

Experimental investigation of thermal insulating aerogel composites of hydrothermal reactor for biomass-to-hydrogen conversion

The paper presents aerogel SiO₂–TiO₂ composites used for thermal insulation of hydrothermal reactor for biomass-to-hydrogen conversion and the method of their production. The technology of aerogel composites production includes the following stages: ion exchange of liquid Na-glass resulting in production of silica hydrosol; hydrosol concentration; hydrogel production and its maturing; SiO₂–TiO₂ alcogel production; modification of its surface; subcritical drying of alcogel resulting in production of SiO₂–TiO₂ ambigel; its thermal treatment, granulating and classification. The influences of infrared opacifier (titanium dioxide) and thermal treatment temperature of SiO₂–TiO₂ composite on its structural and thermal characteristics have been investigated. Possibilities of hydrogen yield increase by reduction of power waste by means of vacuumized thermal insulation for reactor walls have been examined.     

Pyrolysis-catalytic steam reforming of agricultural biomass wastes and biomass components for production of hydrogen/syngas

The pyrolysis-catalytic steam reforming of six agricultural biomass waste samples as well as the three main components of biomass was investigated in a two stage fixed bed reactor. Pyrolysis of the biomass took place in the first stage followed by catalytic steam reforming of the evolved pyrolysis gases in the second stage catalytic reactor. The waste biomass samples were, rice husk, coconut shell, sugarcane bagasse, palm kernel shell, cotton stalk and wheat straw and the biomass components were, cellulose, hemicellulose (xylan) and lignin. The catalyst used for steam reforming was a 10 wt.% nickel-based alumina catalyst (NiAl₂O₃). In addition, the thermal decomposition characteristics of the biomass wastes and biomass components were also determined using thermogravimetric analysis (TGA). The TGA results showed distinct peaks for the individual biomass components, which were also evident in the biomass waste samples reflecting the existence of the main biomass components in the biomass wastes. The results for the two-stage pyrolysis-catalytic steam reforming showed that introduction of steam and catalyst into the pyrolysis-catalytic steam reforming process significantly increased gas yield and syngas production notably hydrogen. For instance, hydrogen composition increased from 6.62 to 25.35 mmol g^?1 by introducing steam and catalyst into the pyrolysis-catalytic steam reforming of palm kernel shell. Lignin produced the most hydrogen compared to cellulose and hemicellulose at 25.25 mmol g^?1. The highest residual char production was observed with lignin which produced about 45 wt.% char, more than twice that of cellulose and hemicellulose.     

Hydrothermal carbonization of microalgae II. Fatty acid,char, and algal nutrient products

A process for isolation of three products (fatty acids, chars and nutrient-rich aqueous phases) from the hydrothermal carbonization of microalgae is described. Fatty acid products derived from hydrolysis of fatty acid ester groups in the microalgae were obtained in high yield and were found to be principally adsorbed onto the char also created in the process. With the highest lipid-containing microalga investigated, 92% of the fatty acids isolated were obtained by solvent extraction of the char product, with the remaining 8% obtained by extraction of the acidified filtrate. Obtaining the fatty acids principally by a solid–liquid extraction eliminates potential emulsification and phase separation problems commonly encountered in liquid–liquid extractions. The aqueous phase was investigated as a nutrient amendment to algal growth media, and a 20-fold dilution of the concentrate supported algal growth to a level of about half that of the optimal nutrient growth medium. Uses for the extracted char other than as a solid fuel are also discussed. Results of these studies indicate that fatty acids derived from hydrothermal carbonization of microalgae hold great promise for the production of liquid biofuels.     

Alkali-promoted hydrothermal gasification of biomass food processing waste: A parametric study

A parametric study of alkali-promoted hydrogen gas production from by-products from food-based biomass, such as glucose, molasses and rice bran, under hydrothermal conditions has been carried out. Partial oxidation of the biomass samples was aided by the addition of hydrogen peroxide and experiments were carried out in the presence of sodium hydroxide. The effects of reaction temperature, reaction time and feed concentration on the conversion of glucose, molasses and rice bran to gaseous products under hydrothermal conditions were investigated. The reaction time and reaction temperature were investigated in the range of 0–120 min and 330–390 °C, respectively. The results confirmed the positive influence of NaOH in the production of hydrogen gas via the water–gas shift reaction. In the presence of the alkali, no tar/oil and char were observed. The hydrogen gas yield increased when the reaction temperature and reaction time increased. It was observed that higher reaction temperature led to an increase in the amount of methane gas produced. With increasing feed concentration, the yields of other gases such as CO, CO₂, CH₄ and C₂–C₄ increased, while hydrogen gas production decreased for all the biomass samples. The generation of gaseous products from the molasses and rice bran showed a similar trend to that of glucose, under identical test conditions.     

Experimental investigation on the kinetics of biomass combustion in vertical tube reactor

The paper presents results of experimental investigation performed in order to examine kinetics of loose biomass combustion in vertical tube reactor. The investigation conducted included continuous measurement of the fuel mass loss rate, with two biomass combustion models (piston and batch model) proposed, each relying on appropriate theoretical postulates. Results obtained indicated that piston combustion model had shown better agreement between theoretical and experimental data and was therefore used to further analyse effects of excess-air on the combustion kinetics, as well as associated effects of flue gas recirculation. Recirculation of cold flue gases is used to lower peak temperature inside the furnace, as well as to reduce a zone where ash melting problems may potentially occur. During the investigation performed, effects of flue gas recirculation on the combustion process were simulated by simultaneously injecting nitrogen and air flows into the furnace. This was deemed appropriate to simulate real-life conditions prevailing in the furnace with gas recirculation. Experiments were conducted on specially designed and constructed apparatus that enabled kinetic parameters to be determined for the combustion of different types of biomass. Results obtained have indicated that quantity of air affects kinetics of biomass combustion and that increased recirculation leads to reduced biomass reaction rate. The same conclusion was reached based on the results of experiments conducted with two different types of agro-biomass, namely wheat straw and corn stalks, which are most commonly used for energy generation. Results achieved are deemed particularly important when it comes to design of new plants that utilize cigarette type combustion system, but also for development of numerical models used to simulate combustion of biomass bales, with special emphasis placed on the impact of recirculation gases on the combustion kinetics.     

Evolution of PM2.5 from biomass high-temperature pyrolysis in an entrained flow reactor

In this study, wheat straw pyrolysis was conducted in an entrained flow reactor at 900–1300 °C, and PM_2.5 were sampled from the flue gas through a heated sampling system. Multi-phase PM_2.5 including carbonaceous matter, potassium-containing particles, and ash particles, was separated and quantified using a thermogravimetric analyzer (TGA). The micro-morphologies and chemical compositions of these three phases were characterized by scanning electron microscopy (SEM), scanning transmission electron microscope (STEM), energy dispersive X-ray spectrometry (EDS), and X-ray diffraction (XRD). Results show that PM_2.5 yields during biomass pyrolysis are in the range of 7–34 g/kg (dry-basis biomass) and increase with the increase of pyrolysis temperature. At low pyrolysis temperatures (900–1000 °C), the carbonaceous matter is dominated by char-carbon. When the pyrolysis temperature increase from 1000 °C to 1100 °C, the production of soot is greatly enhanced and soot becomes dominant in PM_2.5, and the amorphous morphologies of soot are replaced by the concentric graphitic layers. With the further increasing in pyrolysis temperature, soot particles become more spherical and onion-like. Above 1100 °C, the KCl content in PM_2.5 declines, which is because of the capture of KCl and the formation of low-melting potassium aluminosilicates in large char particles. At 1300 °C, the fragmentation of char particles is significantly strengthened, resulting in more ash in PM_2.5.     

Hydrothermal liquefaction of biomass: A review of subcritical water technologies

This article reviews the hydrothermal liquefaction of biomass with the aim of describing the current status of the technology. Hydrothermal liquefaction is a medium-temperature, high-pressure thermochemical process, which produces a liquid product, often called bio-oil or bi-crude. During the hydrothermal liquefaction process, the macromolecules of the biomass are first hydrolyzed and/or degraded into smaller molecules. Many of the produced molecules are unstable and reactive and can recombine into larger ones. During this process, a substantial part of the oxygen in the biomass is removed by dehydration or decarboxylation. The chemical properties of bio-oil are highly dependent of the biomass substrate composition. Biomass constitutes of various components such as protein; carbohydrates, lignin and fat, and each of them produce distinct spectra of compounds during hydrothermal liquefaction. In spite of the potential for hydrothermal production of renewable fuels, only a few hydrothermal technologies have so far gone beyond lab- or bench-scale.     

Petrographic and chemical reactivity assessment of Indian high ash coal with different biomass in fluidized bed co-gasification

The reactivity of coal and biomass has been evaluated by comparing the optical and chemical changes in feed material prior and after the co-gasification. The proximate, ultimate, GCV, low-pressure N₂ sorption isotherm, micropetrography, SEM and EDX spectroscopy analyses are carried out to assess the reactivity of blends of high ash Indian coal and biomass. The relative changes in parameters like surface area, pore size, and pore volume have been correlated with reacted percentage area of coal macerals and cellulose-lignin cellular structures of biomass. The Optimas image processing software is being used to mark the reacted portion of organic constituents and calculated the reactivity percentage. The bottom ash of pure coal has shown the least reacted organic matters, indicating inefficiency of high ash coal due to a large amount of inorganic and inertinite contents that is resisting the oxidation. The reactivity percentage is determined by the petrographic and SEM images, and varies from 36.34 to 99.64% and 6.61–96.22%, respectively. It is summarised that the estimation of percentage alteration of macerals and other micro-organic constituents can be used as one of the practical approaches for the assessment of the reactivity of coal and biomass. The blending ratio 6:4 of coal and press mud has shown the highest reactivity (>99.64%). The values of petrographic and SEM reactivity have shown good correlations with the carbon contents, unreacted vitrinites, mineral matters and biomass remnants. These relations have been taken into account to formulate the proposed petrographic empirically calculated reactivity (R_PEC). The focus has also been made to investigate the influence of feed composition on carbon conversion and heating value of the product gas.     

Characterization and challenge of development of producer gas fuel combustor: A review

Producer gas, which derived from a biomass gasification process, is considered as one of the alternative fuels, which is suitable for the heating process and power generation. Due to low heating density and impurities, combustion in an external combustion chamber constitutes an obvious option for the utilization of producer gas via the combustion process. This paper reviews the technical challenges and the development of the producer gas combustor. Various combustion techniques are reviewed. A stable flame combustion with low emissions (both CO and NO_x) constitutes a main requirement of the producer gas combustion. Flame stabilization techniques such as swirl-vane coupled with bluff-body, swirl flow configuration, and staging combustion were successfully employed to enhance the stability and performance of the producer gas combustion. As shown in the results of the studies, the combustion process can operate in a wide range of equivalence ratios with the exhaust gas temperature >600 °C. This temperature is sufficiently hot for the power generation and heating applications. Overall, NOx and CO emissions were below 700 ppm and 1.3%, respectively. In the flameless combustion mode, ultra-low emission for both CO and NOx were recorded. However, higher emission can be found when operated at a higher thermal load combustor. Homogeneity of the thermal field and low polluting emissions make flameless combustion a promising lean and clean combustion technology. Integration of the benefits of flameless combustion and producer gas fuel is an outstanding contribution in reducing emissions and enhancing the efficiency of the combustion systems.     

Ash deposition behaviors during combustion of raw and water washed biomass fuels

Biomass-fired boilers have the tendency to suffer from severe problems of fouling and slagging due to the high potassium content of biomass fuel. The troublesome potassium, however, can be removed efficiently by water washing pretreatment. In this study, the ash deposition behaviors during combustion of raw and water washed biomass fuels were investigated by a one-dimensional furnace and a deposition probe. Two biomass fuels (corn stalk and wheat straw) were used, and deposition mass, deposition efficiency, composition and morphology of the deposit were studied. The ash deposition while firing raw biomass exhibits a “fast?slow?fast?slow” trend with the sampling time. After water washing, the deposition mass decreases dramatically, and the deposition efficiency reduces gradually as the sampling time increases. The analyses of elemental composition, morphology and chemical composition on the deposit from raw biomass imply that the condensation/thermophoresis is quite significant in the earlier deposition stage, whereas the chemical reaction is remarkable in the later stage. After water washing, the potassium content of the deposit decreases significantly. Morphology and chemical composition analyses indicate that the deposit from water washed biomass ascribes to the physical accumulation of non-viscous fly ash particles. The deposition mass can easily approach a maximum value. The ash fusion temperatures of deposits increase remarkably after water washing. In addition, ash deposition mechanisms during biomass combustion are discussed.     

Engine combustion and emission fuelled with natural gas: A review

Natural gas (NG) is one of the most important and successful alternative fuels for vehicles. Engine combustion and emission fuelled with natural gas have been reviewed by NG/gasoline bi-fuel engine, pure NG engine, NG/diesel dual fuel engine and HCNG engine. Compared to using gasoline, bi-fuel engine using NG exhibits higher thermal efficiency; produces lower HC, CO and PM emissions and higher NOx emission. The bi-fuel mode can not fully exert the advantages of NG. Optimization of structure design for engine chamber, injection parameters including injection timing, injection pressure and multi injection, and lean burn provides a technological route to achieve high efficiency, low emissions and balance between HC and NOx. Compared to diesel, NG/diesel dual fuel engine exhibits longer ignition delay; has lower thermal efficiency at low and partial loads and higher at medium and high loads; emits higher HC and CO emissions and lower PM and NOx emissions. The addition of hydrogen can further improve the thermal efficiency and decrease the HC, CO and PM emissions of NG engine, while significantly increase the NOx emission. In each mode, methane is the major composition of THC emission and it has great warming potential. Methane emission can be decreased by hydrogen addition and after-treatment technology.     

Combined-gasification of biomass and municipal solid waste in a fluidized bed gasifier

Due to the environmental problems associated with burning of fossil fuels and population growth, more attention has been paid to develop renewable energies in recent years. Among all options for renewable energy utilization, biomass gasification is more popular because of environmental benefits and economic issues. In the present study, a series of experiments were carried out to study the influence of blending ratio, reaction temperature, equivalence ratio (ER) on co-gasification characteristics of pine sawdust (SD) and municipal solid waste (MSW). By increasing the blending ratio from 100% SD to 100% MSW, CO and CH₄ respectively increased from 16.7 to 18.8 vol% and from 4.1 to 5.1 vol%, while an opposite trend was found for H₂ and CO₂. Over the ranges of the experimental conditions used, the tar content and gas yield varied from 5.4 to 10.1 g/Nm³ and 1.34 to 1.15 Nm³/kg, respectively.     

Low-temperature SCR of NO with NH3 over biomass char supported highly dispersed MnCe mixed oxides

Series of catalysts with MnCe mixed oxides loaded onto biomass char (BC) modified by nitric acid were prepared via impregnation method. And these catalysts were used for the selective catalytic reduction (SCR) of NO with NH₃. MnCe (7:3)/BC catalysts with loading 6% (mass ratio) MnCe oxides showed the best NO conversion ratio of 99.2% at 175 °C. The changes of the microstructure, phase composition, metal valence state and functional groups were investigated through SEM, BET, XRD, XPS and FT-IR. It showed that nitric acid modification could increase surface acidic functional groups of biomass char. And surface functional groups on BC could increase NH₃ and NO adsorption capacity. In the denitration process, oxygen was transferred from CeO₂ to Mn₂O₃, which could promote the cyclic catalytic reaction rate, then significantly enhanced the NO conversion in the MnCe/BC catalysts. Based on the experimental results and theoretical analysis, the synergetic mechanism model of surface functional groups on the surface of BC, Mn and Ce on the catalysts was set up.     

Experimental study on ash fusion characteristics and slagging potential using simulated biomass ashes

Ash fusion characteristics (AFC) affect biomass slagging significantly. Due to the complexity of biomass ash composition, simulated ashes have been used for investigating AFC. Considering the practical ash components used in power plants, the mixture of SiO₂, CaO, K₂O and Al₂O₃ were used as simulated ashes. Deformation temperature (DT) changes remarkably as the ash components of biomass change, and thus is selected as an index. The results are presented by SiO₂-CaO-K₂O ternary diagrams with three different ratios of Al₂O₃. The ternary diagrams are divided as high-temperature zone (HZ, >1400 °C), medium-temperature zone (MZ, 900–1400 °C) and low-temperature zone (LZ, <900 °C). Results also show that without Al, low melting products (K₄CaSi₃O₉) of the eutectic reactions among K₂O, CaO and SiO₂ led to a DT of 1290 °C in MZ. With the addition of Al₂O₃, DT can increase from LZ to MZ at high K content condition because Si-Al-K compounds such as KAlSi₂O₆ and KAlSi₃O₈ formed. However, DT decreased and moved from HZ to MZ with the addition of Al while Ca or Si content is high because of Si-Al-Ca compounds such as Ca₂Al₂SiO₇ and CaAl₂Si₂O₈. Besides, the difference (DSD) between deformation temperature and softening temperature has been used to predict the slagging potential. Long slag tends to occur in the zone of high K content, where SiO₂ leads to a larger DSD than CaO does. Al₂O₃ can decrease DSD and form short slag, which is suitable as additive for relieving slagging.     

Comparative study of the poisoning effect of NaCl and Na2O on selective catalytic reduction of NO with NH3 over V2O5-WO3/TiO2 catalyst

V₂O₅-WO₃/TiO₂ catalyst has been widely used in industry. Alkali metals would cause the deactivation of V₂O₅-WO₃/TiO₂ catalyst. In this paper, the poisoning deactivation of NaCl and Na₂O on V₂O₅-WO₃/TiO₂ catalyst was compared. The properties of the catalysts were characterized by BET, XPS, H₂-TPR, NH₃-TPD and in situ DRIFTS. It was found the addition of NaCl, Na₂O affected the structure, redox properties and acid sites of V₂O₅-WO₃/TiO₂ catalyst. Na⁺ would react with VOH to form VONa⁺ destroying the structure of Brønsted sites and affect the adsorption of NH₃ on the Lewis acid to restrain the generation of V⁴⁺NH₂ to decrease the SCR activity, occupying the oxygen vacancy made a decline in chemisorbed oxygen. The poisoning effect of NaCl was stronger than that of Na₂O, even if the property of weak-chemisorption of NaCl is stronger and possessed more V⁵⁺ species. There is a reason that NaCl provided HCl and then reacted with VO₂ to form ClVOClOH to adsorb NH₃. However, ClVOClOH cannot make the catalysis selectively generate nitrogen and water.     

Bio-oil production from hydrothermal liquefaction of waste Cyanophyta biomass: Influence of process variables and their interactions on the product distributions

Hydrothermal liquefaction (HTL) of waste Cyanophyta biomass at different temperatures (factor A, 260–420 °C), times (factor B, 5–75 min) and algae/water (a/w) ratios (factor C, 0.02–0.3) by single reaction condition and Response Surface Method (RSM) experiments was investigated. By single reaction condition runs, maximum total bio-oil yield (29.24%) was obtained at 350 °C, 60 min and 0.25 a/w ratio. Maximum bio-oil HHV of 40.04 MJ/kg and energy recovery of 51.09% was achieved at 350 °C, 30 min, 0.1 a/w ratio and 350 °C, 60 min, 0.25 a/w ratio, respectively. RSM results indicate that effect of AB interaction was significant on light bio-oil yield. Both AC and AB had more remarkable influence than BC on heavy bio-oil yield and aqueous total organic carbon (TOC) recovery whereas BC was noticeable on ammonia nitrogen (NH₃N) recovery in aqueous products. By model-based optimization of highest bio-oil yield, the highest bio-oil yield reached 31.79%, increasing by 8.72% after RSM optimization, and light and heavy bio-oil yield was 17.44% and 14.35%, respectively. Long-chain alkanes, alkenes, ketones, fatty acids, phenols, benzenes, amides, naphthalenes were the main components in light bio-oil. Some alcohols, phenols and aromatics were primarily found in heavy bio-oil. Solid residue after HTL consisted of numerous microparticles (～5 μm) observed by Scanning Electron Microscopy (SEM). Energy Dispersive Spectrometer (EDS) analysis shows these particles primarily contained C, O, Mg, P and microelements, derived from Cyanophyta cells.     

Application potential of vegetable oils as alternative to diesel fuels in compression ignition engines: A review

Vegetable oils have been identified as the promising alternative source to replace fossil based fuel in the compression ignition (CI) engine. It is renewable and possesses characteristics that is similar to that of the diesel. Biodiesel, transesterifiedform of vegetable oil (VO), is now being commercially used in CI engines. However, biodiesel production from VO involves use of alcohols and chemicals which results the need of skilled labor and investment for its production. In view of this, many studies are also being carried out on the direct use of VO in the engine. The direct use of VO oil in engine is as good as that of the diesel. The superior quality of diesel however makes it better performance in engine as compared to the vegetable oil. Preheating and blending of VO are found to be the most common solution to overcome its inferior properties. The use of preheated and blended VO is found to improve the engine overall performance. This paper is focused exclusively on the one-to-one basis of study pertaining to the effect of neat, preheated and blended vegetable oils on diesel engine performance and emission through supplementation of illustrative figures from the various experimental studies.     

Study of the transient release of water vapor from a fuel bed of wet biomass in a reciprocating-grate furnace

The present study investigates how sudden changes in fuel moisture affected the combustion characteristics of the fuel bed in a 4-MW reciprocating-grate furnace. The moisture content of the fuel fed to the furnace was monitored online using a near-infrared spectroscopy device, and the water vapor concentration in the flue gas was measured continuously. To obtain experimental data on fuel-bed conditions, the temperature and gas composition in the bed were measured using a probe. A simplified drying model was developed using the measured gas composition values as inputs. The model was then used to estimate the drying rate and to simulate the extent of the drying zone along the grate. Measurements indicated that a change in the moisture content of the fuel fed to the furnace was detected as a change in water vapor concentration in the flue gas with a delay of about 2 h. The model predicted that a portion of wet fuel would need about 2 h to become dry, in line with the measured time delay of the water vapor concentration change in the flue gas. Overall, there was good alignment between the measured and simulated results, supporting the validity of the model and the assumed mechanisms.     

空气源热泵低温适应性研究的现状及进展

随着改革开放的发展、人们生活水平的提高，许多传统供暖地区的居民开始有夏季制冷的要求，而且这些地区传统的以燃煤为基础的供暖模式所带来的负面影响越来越不能适应社会可持续发展的要求。空气源热泵因其特殊的优点而在长江中下游以南地区备受重视，但是传统的空气源热泵要想成功应用下地北方寒冷地区还有许多问题有待进一步解决。文章分析了空气源热泵系统在寒冷地区使用时需要解决的一些问题，以及国内外为解决这些问题所做的一些工作，对于更好地促进这空气源热泵技术的发展具有积极的意义。