The BJH average pore size initially increases with the increase in time, but decreases with prolonged activation. Prolonged activation results in an increase in burn off. The BJH average pore size of rice hulls char activated at 600°C was 25.1 A, 67.1 A, and 45.1 A with an activation time of 0.5 hr, 1.0 hr, and 2 hr, respectively. It is imperative that the larger the BET surface area and the larger the BJH cumulative pore areas, the higher the adsorption efficiencies must be. Typical BJH adsorption cumulative pore areas for different samples are shown in Fig.3. From the graph, it can be inferred that activated carbon made from rice hulls has a large BJH adsorption cumulative pore area, close to that of coconut shell , but significantly larger than two other biomass samples shown. Because of the superior surface area and average pore size measured for rice hulls activated carbon than those of wheat straw and peanut shells, this study concentrates on rice hulls carbon. Optimal pyrolysis and activation temperatures and times for carbon preparation were determined based on the amount of NOx that can be adsorbed by the activated carbon. The adsorption capacities of rice hull activated carbons generated by different pyrolysis and activation temperatures are shown in Fig.4.
A simulated flue gas containing 250 ppm NO, 5% O2, 10% CO2,maceta de plastico cuadrada with N2 as the balance was passed through the tubular reactor containing 2 g of activated carbons at a flow rate of 250 ml/min at 25°C. It is evident from the plots that the NO removal efficiency increases in the order of RH-2- 300-1-350, RH-2-400-2-450, RH-2-500-1-550, RH-2-600-1-650, RH-2-700-1-750, and RH-2-800-1-850. Activated carbon derived from the highest tested pyrolysis and activation temperatures exhibited the best adsorption efficiency. The better adsorption efficiency is attributed to higher micro-porosity obtained at higher carbonization temperatures. But samples carbonized above 700°C have a higher burn off rate than those carbonized at lower temperatures. In order to obtain a better production yield of activated carbon, 700°C and 750°C for pyrolysis and activation, respectively are chosen as optimum for the preparation of rice hull activated carbon. The NO adsorption efficiencies of samples carbonized by differing pyrolysis and activation times are shown in Figure 5. It is evident from the plots that activated carbons carbonized by prolonged pyrolysis and activation times have better adsorption efficiencies than those carbonized by shorter times due to higher pore count and BET surface area. It can be seen from these figures that the micropore count and the surface area of activated carbon increases with longer preparation time, thus explaining why the samples with the longest pyrolysis and activation times have the best adsorption efficiencies. However, prolong activation results in more burn off and the production of ash.
A balance must be reached when setting reaction parameters; one that will generate the largest surface area without a significant burn off. We found that the optimal pyrolysis and activation times are two and one hour, respectively. The removal efficiency of NO by carbon was studied at various temperatures: 10°C, 100°C, 300°C, 400°C, and 500°C. A gas mixture containing 250 ppm NO in N2 was passed through a column of carbon with a W/F of 15.4 g-min/L. Fig.6 shows that NO removal efficiency decreases with increased temperatures, when kept below 100°C. However, further increases in temperature beyond 100°C reversed the course, causing an increase in NO removal efficiency. This phenomenon is attributed to the reduction of NO by activated carbon, which results in the formation of nitrogen gas. The results from this set of experiments indicate that at the condition of W/F employed, NO can be completed reduced to N2 at temperatures equal to or above 500°C, while only some of the inlet NO is reduced to N2 at temperature below 500°C.It is expected that the high temperature flue gas will be cooled down before passing to the carbon bed to avoid the combustion of carbon due to the presence of flue gas oxygen. This cooling down results in the condensation of water vapor. Flue gas can contain up to 3.0 % v/v H2O even after cooling down to room temperature, thus it is important to study the effect of water vapor on the removal efficiency of NO by carbon. Fig. 8 shows that moisture causes a significant decrease in NO removal efficiency by carbon.
The more water vapor present in the flue gas, the lower the NO removal efficiency exhibited. This is due to the fact that water vapor competes effectively with NO for the adsorption sites on carbon particles. The detrimental effect of water vapor on NO adsorption can be overcome by the removal of water vapor either by drying agents or by cooling flue gas to low temperatures prior to adsorption. The adsorbed NO can be desorbed from activated carbon if temperature of the carbon bed is raised. Further increases of temperature results in the reduction of NO by activated carbon to produce N2. Simultaneously, the activated carbon is regenerated as a result of the reduction of NO to N2. Experiments on the reduction of the adsorbed NO by the activated carbon were performed by heating the NO saturated carbon under anaerobic conditions. In order to evaluate the behavior of the process over time, a purge gas flow of 1.0 L/min N2 was passed through the carbon bed and subsequently directed to the NOx analyzer. Desorption was conducted with a temperature ramp rate of 40°C/min from room temperature to 600o C. As the temperature of the carbon bed was increased, NO was desorbed from the surface of the activated carbon. Further increase of the temperature results in the reduction of NO by the activated carbon to N2. The fraction of the adsorbed NO that is reduced to N2 can be calculated by subtracting the NO coming out of the carbon bed from the total amount of NO adsorbed. The fraction of the adsorbed NO that is reduced depends on the temperature and the flow rate of N2 gas. Fig.10 shows the fraction of the desorbed NO integrated over the temperatures as the temperatures of the carbon bed was raised. As can be seen, the fraction of the total NO desorbed as NO reaches the maximum at 550°C and that this fraction was less than 100% of the total NO adsorbed, the difference of which being attributed to the reaction of NO with the activated carbon to form N2. The fraction of the adsorbed NO desorbed as NO is 48.2% in the case when NO adsorption was done without the presence of H2O vapor, and 64.5% in the case when NO adsorption was performed with 2% H2O. This result indicates that NO reduction by activated carbon is inhibited by the presence of water vapor. Water vapor can compete with NO for the reaction with activated carbon. From the desorption curve as a function of temperature, the NO desorption mainly took place at temperature below 300°C, while the NO reduction by carbon occurred at temperature above 300°C, the higher the temperature the more effective the reduction is. Since the ramp rate was 40°C per min., it would take 7.5 minutes to raise from 300°C to 600°C, the temperature range when most of the NO reduction takes place. During the 7.5 min. time interval, about 50% of the adsorbed NO was reduced to N2. Consequently,fabrica de macetas plasticas it can be concluded that the complete reduction of NO to N2 at 550°C can be done within 15 minutes in a closed system. Another set of experiments were performed to study the reduction of NO by activated carbon as a function of temperature and W/F, the ratio of the amount of carbon to flow rate of N2. In this study, temperatures were varied between 300 and 550°C and W/F between 10 and 40 g.min/L. Fig. 11 shows that with a feed gas containing 250 ppm NO with the balance N2, the fraction of NO reduced by activated carbon increases with the increase of temperature at a given W/F, and the fraction also increases with the increase of W/F at a given temperature.
All of NO was reduced to N2 at 550°C with a W/F above 20 g.min/L, and at 500°C with a W/F 40 g.min/L. It would require a W/F larger than 40 g.min/L to convert all of NO to N2 at temperature below 500C. The NO reduction efficiency also depends on the concentration of NO in the system. Fig 12 shows NO reduction at 500°C for two inlet NO concentrations, 250 ppm and 1000 ppm. As can be seen, higher inlet NO concentrations cause less fraction of NO to be reduced. Only 55% of inlet NO was reduced at 500°C with an inlet NO concentration of 1000ppm and a W/F of 40g.min.L-1. Experiments were conducted at room temperature using rice hull activated carbon to determine at what conditions it would prolong efficient adsorption of NO and that outlet concentrations would be less than SMAC . The principle variables manipulated were inlet oxygen concentration, ranging from 5% to 20%, and weight to flow rate ratio , ranging from 15 to 45 g.min/L . The time that the carbon bed can hold before the NOx concentration exiting the bed exceeds the SMAC, will be called SMAC time. Fig.13 shows SMAC time at different oxygen concentrations. The SMAC time increases along with increases in O2 concentrations. The SMAC time was longer than 6 hours with oxygen concentration of 10% and W/F of 45 g.min/L, while about 10 hours were obtained with an 15% oxygen and W/F of 45 g.min/L. As previously mentioned, oxygen presence enhances NO adsorption, thus allowing the SMAC time to be longer. Increasing W/F, especially above 20g.min/L, also increases the SMAC time. Experiments were conducted to determine the effects of the regeneration on activated carbon in terms of NO removal efficiency, as assessed by the carbon’s SMAC time. Fig.14 shows the SMAC time after different numbers of regeneration cycles. The results indicate that regeneration improves the removal efficiency of NO. This phenomenon is attributed to the increase of surface area and micropores of the activated carbon. However, it was observed that additional carbon burns off occurs during regeneration, which causes the overall amount of activated carbon to decrease after each regeneration cycle. The loss of mass was determined to be about 0.16% of activated carbon per cycle of regeneration. The SMAC time was 163 minutes and 372 minutes for the first and the 8thcycle run, respectively. The larger the activated carbon adsorption efficiency, the longer the SMAC time will be. The production and use of carbonaceous nanomaterials , including carbon nanotubes and graphene, have rapidly increased.Annual global production of CNTs has attained several thousand metric tons,1 and graphene production is predicted to exceed 1000 metric tons annually by 2019. Once released into the environment, CNMs may accumulate in soils.CNM-containing bio solids are applied to agricultural lands,while CNM containing products are used both to remediate soils and to fertilize and protect crops.Given the potential for terrestrial plant and herbivore exposures, there is a need to understand hazards of CNMs to crop plants.Thus far, widely varying effects of CNMs on plants have been reported,possibly owing to disparate study conditions including types and concentrations of CNMs, plant species and developmental stages, toxicity metrics, and exposure conditions. Hydroponic conditions have been used to study CNM effects at relatively high CNM concentrations .However, fewer studies simulate field conditions including using CNM concentration ranges that are predicted to occur in soils.Many complex soil components can interact with CNMs and thereby affect CNM bio-availability.Therefore, dose– response relationships for soil-grown plants cannot easily be inferred from hydroponic study results.To assess CNM hazards to agricultural plants, more soil-based research using a wider range of CNM concentrations is needed. In agricultural settings, plants not only root in the complex environment of soils, but plants also develop over time, including the maturation of root symbioses. Some CNM phytotoxicity studies have emphasized acute toxicity on seed germination and seedling growth.However, while such phytotoxicity assays offer the convenience of being standardized, they are performed over exposure periods that are too short to assess full agricultural impacts and are relatively insensitive for indicating nanotoxicity.