► Low-cost carbons with high CO2 uptake and selectivity are produced from coffee grounds. ► Chemical activation is compared to physical activation. ► KOH activation leads to carbons with higher CO2 ...uptake. ► Coffee grounds activated with CO2 present higher CO2/N2 selectivity.
In this work spent coffee grounds from single-use capsules were used as the starting material for producing low-cost activated carbons. The activation conditions were selected and optimised to produce microporous carbons with high CO2 adsorption capacity and selectivity, thus with potential to be used as adsorbents in postcombustion CO2 capture applications. Two activation methods are compared: physical activation with CO2 and chemical activation with KOH. The first method is considered less contaminant; however, leads to carbons with lower textural development and thus lower CO2 adsorption capacity than those obtained by activation with KOH. On the other hand, multicomponent adsorption cyclic experiments pointed out that the CO2/N2 selectivity of physically activated carbons is higher than that of chemically activated carbons.
Microwave-assisted dry reforming of methane Fidalgo, B.; Domínguez, A.; Pis, J.J. ...
International journal of hydrogen energy,
08/2008, Letnik:
33, Številka:
16
Journal Article
Recenzirano
The microwave-assisted dry reforming of methane over an activated carbon, which acted as catalyst and microwave receptor, was investigated. As a preliminary study, the CO
2 reforming of CH
4 was ...carried out using conventional heating and microwave heating in order to compare both heating devices. Higher conversions of CH
4 and CO
2 were achieved by microwave heating. Under microwave heating, various operating variables were studied in order to determine the best conditions for performing dry reforming with high conversions and the most suitable H
2/CO ratio. Thus, the dry reforming reaction was studied at different temperatures. An optimum range of working temperatures (between 700
°C and 800
°C) was established. In this range of temperatures, the dry reforming reaction is believed to take place as a combination of CH
4 decomposition and CO
2 gasification. Carbonaceous deposits from CH
4 decomposition are gasified by CO
2 and, as a result, active centres for the dry reforming reaction are constantly regenerated. The effect of the proportion of CO
2 fed in on the CH
4 and CO
2 conversions was also investigated. Small increases in the percentage of CO
2 fed in gave rise to large increases in both conversions, but especially in the case of CH
4. The volumetric hourly space velocity was also studied. It was found that the lower the space velocity, the higher the conversions obtained.
The use of biomass to produce energy is becoming more and more frequent as it helps to achieve a sustainable environmental scenario. However the exploitation of this fuel source does have drawbacks ...that need to be solved. In this work, the torrefaction of woody biomass (eucalyptus) was studied in order to improve its properties for pulverised systems. The process consisted in a heating treatment at moderate temperature (240, 260, 280 °C) under an inert atmosphere. The grindability of raw biomass and the treated samples was compared and an improvement in the grindability characteristics was observed after the torrefaction process. Thermogravimetric analysis of the samples was carried out in order to study their reactivity in air. The DTG curves of the torrefied biomass showed a double peak nature. The kinetic parameters were calculated for each reaction stage. The torrefaction process was found to influence the parameters of the first stage, whereas those corresponding to the second remained unaffected.
A fundamental investigation has been conducted on the combustion behavior of single particles (75–150μm) of four coals of different ranks: anthracite, semi-anthracite, medium-volatile bituminous and ...high-volatile bituminous. A laboratory-scale transparent laminar-flow drop-tube furnace, electrically-heated to 1400K, was used to burn the coals. The experiments were performed in different combustion atmospheres: air (21%O2/79%N2) and four simulated dry oxy-fuel conditions: 21%O2/79%CO2, 30%O2/70%CO2, 35%O2/65%CO2 and 50%O2/50%CO2. The ignition and combustion of single particles was observed by means of three-color pyrometry and high-speed high-resolution cinematography to obtain temperature–time histories and record combustion behaviors. On the basis of the observations made with these techniques, a comprehensive examination of the ignition and combustion behaviors of these fuels was achieved. Higher rank coals (anthracite and semi-anthracite) ignited heterogeneously on the particle surface, whereas the bituminous coal particles ignited homogeneously in the gas phase. Moreover, deduced ignition temperatures increased with increasing coal rank and decreased with increasing oxygen concentrations. Strikingly disparate combustion behaviors were observed depending on the coal rank. The combustion of bituminous coal particles took place in two phases. First, volatiles evolved, ignited and burned in luminous enveloping flames. Upon extinction of these flames, the char residues ignited and burned. In contrast, the higher rank coal particles ignited and burned heterogeneously. The replacement of the background N2 gas of air with CO2 (i.e., changing from air to an oxy-fuel atmosphere) at the same oxygen mole fraction impaired the intensity of combustion. It reduced the combustion temperatures and lengthened the burnout times of the particles. Increasing the oxygen mole fraction in CO2 to 30–35% restored the intensity of combustion to that of air for all the coals studied. Volatile flame burnout times increased linearly with the volatile matter content in the coal in both air and all oxygen mole fractions in CO2. On the other hand, char burnout times increased linearly or quadratically versus carbon content in the coal, depending on the oxygen mole fraction in the background gas.
The reduction of anthropogenic CO2 emissions to address the consequences of climate change is a matter of concern for all developed countries. In the short term, one of the most viable options for ...reducing carbon emissions is to capture and store CO2 at large stationary sources. Adsorption with solid sorbents is one of the most promising options. In this work, two series of materials were prepared from two commercial activated carbons, C and R, by heat treatment with gaseous ammonia at temperatures in the 200-800 deg C range. The aim was to improve the selectivity and capacity of the sorbents to capture CO2, by introducing basic nitrogen-functionalities into the carbons. The sorbents were characterised in terms of texture and chemical composition. Their surface chemistry was studied through temperature-programmed desorption tests and X-ray photoelectron spectroscopy. The capture performance of the carbons was evaluated by using a thermogravimetric analyser to record mass uptakes by the samples when exposed to a CO2 atmosphere.
Biofuel pellets were prepared from biomass (pine, chestnut and eucalyptus sawdust, cellulose residue, coffee husks and grape waste) and from blends of biomass with two coals (bituminous and ...semianthracite). Their mechanical properties and combustion behaviour were studied by means of an abrasion index and thermogravimetric analysis (TGA), respectively, in order to select the best raw materials available in the area of study for pellet production. Chestnut and pine sawdust pellets exhibited the highest durability, whereas grape waste and coffee husks pellets were the least durable. Blends of pine sawdust with 10–30% chestnut sawdust were the best for pellet production. Blends of cellulose residue and coals (<20%) with chestnut and pine sawdusts did not decrease pellet durability. The biomass/biomass blends presented combustion profiles similar to those of the individual raw materials. The addition of coal to the biomass in low amounts did not affect the thermal characteristics of the blends.
The thermal characteristics and kinetics of coal, biomass (pine sawdust) and their blends were evaluated under combustion conditions using a non-isothermal thermogravimetric method (TGA). Biomass was ...blended with coal in the range of 5–80
wt.% to evaluate their co-combustion behaviour. No significant interactions were detected between the coal and biomass, since no deviations from their expected behaviour were observed in these experiments. Biomass combustion takes place in two steps: between 200 and 360
°C the volatiles are released and burned, and at 360–490
°C char combustion takes place. In contrast, coal is characterized by only one combustion stage at 315–615
°C. The coal/biomass blends presented three combustion steps, corresponding to the sum of the biomass and coal individual stages. Several solid-state mechanisms were tested by the Coats–Redfern method in order to find out the mechanisms responsible for the oxidation of the samples. The kinetic parameters were determined assuming single separate reactions for each stage of thermal conversion. The combustion process of coal consists of one reaction, whereas, in the case of the biomass and coal/biomass blends, this process consists of two or three independent reactions, respectively. The results showed that the chemical first order reaction is the most effective mechanism for the first step of biomass oxidation and for coal combustion. However, diffusion mechanisms were found to be responsible for the second step of biomass combustion.
The ignition temperature, burnout and NO emissions of blends of a semi-anthracite and a high-volatile bituminous coal with 10 and 20 wt.% of olive waste were studied under oxy-fuel combustion ...conditions in an entrained flow reactor (EFR). The results obtained under several oxy-fuel atmospheres (21%O2–79%CO2, 30%O2–70%CO2 and 35%O2–65%CO2) were compared with those attained in air. The results indicated that replacing N2 by CO2 in the combustion atmosphere with 21% of O2 caused an increase in the temperature of ignition and a decrease in the burnout value. When the O2 concentration was increased to 30 and 35%, the temperature of ignition was lower and the burnout value was higher than in air conditions. A significant reduction in ignition temperature and a slight increase in the burnout value was observed after the addition of biomass, this trend becoming more noticeable as the biomass concentration was increased. The emissions of NO during oxy-fuel combustion were lower than under air-firing. However, they remained similar under all the oxy-fuel atmospheres with increasing O2 concentrations. Emissions of NO were significantly reduced by the addition of biomass to the bituminous coal, although this effect was less noticeable in the case of the semi-anthracite.
► Coal and biomass blends combustion behaviour evaluated under oxy-fuel conditions. ► Biomass addition had a greater effect on the ignition temperature than on burnout. ► Lower NO emissions by blending olive waste with a bituminous coal.
The pyrolysis of sewage sludge was investigated using microwave and electrical ovens as the sources of heat, and graphite and char as microwave absorbers. The main objective of this work was to ...maximize the gas yield and to assess its quality as a fuel and as a source of hydrogen or syngas (H
2
+
CO). Both gases were produced in a higher proportion by microwave pyrolysis than by conventional pyrolysis, with a maximum value of 38% for H
2 and 66% for H
2
+
CO. The oils obtained were also characterized using FTIR and GC–MS. The use of conventional electrical heating in the pyrolysis of sewage sludge produced an oil that could have a significant environmental and toxicological impact. Conversely, microwave pyrolysis still preserved some of the functional groups of the initial sludge such as aliphatic and oxygenated compounds, whereas no heavy PACs were detected.
•Low-cost microporous biochars are obtained by single-step activation with oxygen.•This process entails energy savings compared to conventional two-step activation.•Activation at high temperatures ...with low oxygen content favours narrow porosity.•Biochars obtained in these conditions present high CO2/N2 selectivity.•These biochars present CO2 working capacities competitive to that of zeolite 13X.
There is an urgent need to develop materials and processes that reduce the energy penalty associated to the CO2 capture step. Biochars are appealing adsorbents for post-combustion CO2 capture applications due to their low cost, stability in moisture conditions and microporous nature. Series of carbon adsorbents were prepared from almond shells and olive stones by single-step activation with air at 400–500°C, and with lower O2 concentration in the activating gas, 3–5%, at higher temperatures (500–650°C). This process entails energy savings compared to conventional activation with carbon dioxide or steam. It has been found that the pore size distribution can be tailored by adequately selecting the activating conditions. Carbons obtained under lower oxygen partial pressures and higher temperatures present narrow microporosity, which is essential for the adsorption of CO2 at low partial pressures. These appealing low-cost adsorbents have competitive CO2 working capacities and high CO2/N2 equilibrium selectivity in conditions that can be considered representative for post-combustion CO2 capture, thus showing potential for this application.
PDF
