Parametric studies on the production of activated carbon and the adsorption of NO by the carbon have been conducted. The optimal conditions and effectiveness of this procedure to regulate NO emissions have been determined.The activated carbons were characterized by the measurement of their average pore size and surface area. There are three types of pores which can develop in the carbon particles: micro-pores , mesopores and macropores . BET surface area of wheat straw char carbonized at temperatures between 300°C and 700o C is shown in Fig.3. The surface area passes through a maximum of 105.2 m2 /g at a pyrolysis and activation temperature of 600o C and 650o C respectively. The BET surface area of sweet potato stalk exhibits similar behaviors as that of wheat straw, being 88.1 m2 /g, 84.7 m2 /g, and 132 m2 /g at a pyrolysis temperature of 300°C, 400°C, and 600°C, respectively. The decrease in surface area beyond 600°C is caused by sample burning off. Using temperatures much lower than 600o C would compromise the maximum amount of effective adsorption surface area attainable . It is not only important to run reactions at temperatures low enough to prevent burn off and ash formation but also high enough to generate effective surface areas, which would be at 600o C for wheat straw. Fig.4 shows the BJH adsorption cumulative pore area of wheat straw activated carbon generated at different pyrolysis and activation times. It is evident from the plot that samples derived from longer pyrolysis and carbonization times exhibited a higher micro-pore count compared to shorter times. As with temperature generation optimum, however,greenhouse pot too long pyrolysis and activation reaction times cause an adverse increase in burn off percentage. Though wheat straw carbonized with a pyrolysis time of six hours and activation time of 2 hours still demonstrated a higher cumulative pore area than the shorter times, it also produces larger amounts of burn off. Since reaction temperatures were kept below 600o C, ash formation that would have diluted effective surface area was prevented.
Even though ash does not form, pyrolysis and activation times must still be chosen to create a balance between pore formation and burn off, one that would generate a high micro-pore count but at the same time, minimize material loss. The optimal pyrolysis and activation times for wheat straw are 2 hours and 1 hour respectively.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 wheat straw activated carbons generated by different pyrolysis and activation temperatures are shown in Fig.5. A gas mixture containing 250ppm of NO, 5% O2, 10% CO2, with N2 as the balance was passed, at a flow rate of 250ml/min, through a turbular reactor containing 2g of activation carbon at 25o C. It is evident from the plots that the WS-2-600-1-650 activated carbon had the best adsorption efficiency. Samples carbonized above 600o C have higher ash concentrations than those carbonized below, while those carbonized below have lower percent micro-pore counts and surface area—both explaining why wheat straw generated at 600o C had the best adsorption efficiency. The NO adsorption efficiencies of wheat straw samples carbonized by differing pyrolysis and activation times are shown in Figure 6. 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. The micro-pore count and the surface area of activated carbon increases with an increase of the preparation time, which explains why the samples with the longest pyrolysis and activation times have the best adsorption efficiencies. However, prolong activation results in more burnoff 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 burnoff. We have found that the optimal pyrolysis and activation times for wheat straw are two and one hour, respectively. The hydroponic biomass possesses high mineral content. The activated carbon was first soaked in water to dissolve the soluble minerals and then dried to remove the moisture from the carbon particles. The adsorption experiments using the mineral-free activated carbon were performed and the results indicate that the NO adsorption efficiency was substantially improved .
A 100% NO removal efficiency was obtained for the entire 2 hr experiment, using a gas mixture containing 250 ppm NO and 10% O2 and at a W/F of 30g.min.L-1. The improved NO removal efficiency by the activated carbon with mineral removed is attributed to the increase of the carbon surface area, which was otherwise covered by the minerals. Because of this finding, the parametric study on the NO removal efficiency was performed mostly with the activated carbon having mineral content removed by water dissolution. The adsorbed NO can be desorbed from the 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.13 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 was less than 100% of the total NO adsorbed, 38.5% and 9.1% for sweet potato and wheat straw activated carbons, respectively. The difference of which is attributed to the reaction of NO with the activated carbon to form N2. From the desorption curve as a function of temperature, the NO desorption mainly took place at temperature below 450°C, while the NO reduction by carbon occurred at temperature above 450°C,30 plant pot the higher the temperature the more effective the reduction is. 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 500°C and W/F between 10 and 40 g.min/L. Fig.14 shows that, in the case of the sweet potato activated carbon, 100% of NO was reduced to N2 at 500°C with a W/F above 10 g.min./L, while at 450°C it would require a W/F above 30 g.min./L. For wheat straw activated carbon, NO can be reduced to N2 at lower temperatures and smaller W/F ratios than those of sweet potato . All inlet NO was reduced to N2 at 450°C with a W/F above 15g.min.L-1. Higher temperatures or larger W/F ratios are favorable for NO reduction. The NO reduction efficiency also depends on the concentration of NO in the system. Fig. 16 shows NO reduction by sweet potato and wheat straw activated carbons at 450C with two inlet NO concentrations, 250 ppm and 1000 ppm. The results indicate that the higher the inlet NO concentrations, the smaller the fraction of the inlet NO is reduced at a given W/F. All of inlet NO was not reduced to N2 by the sweet potato activated carbon until the W/F reached 30 g.min/L and 40 g.min/L for 250 ppm and 1000ppm of inlet NO respectively. The activated carbon made from wheat straw can reduce NO more effectively than that from sweet potato. The entire inlet NO was reduced by the wheat straw activated carbon with a W/F of only 15 g.min/L and 30 g.min/L for 250ppm and 1000ppm of NO, respectively. The space maximum allowable concentration of NO in a human occupied cabin is 4.8ppm. Experiments were conducted to determine conditions for NO adsorption on SP-2-600-1-650 at room temperature such that the NO concentration exiting the carbon bed is less than 4.8ppm. The activated carbons before and after the removal of minerals with water dissolution were both examined. The parameters studied included inlet oxygen concentration , and carbon weight to flue gas flow rate ratio . The time that the carbon bed can hold before the NOx concentration exiting the bed exceeds the SMAC will be called SMAC time. Fig.17 and 18 show that the SMAC time for both activated carbons increase with the increase of W/F and oxygen concentrations. The removal of minerals from the activated carbon increases the surface areas and results in a much longer SMAC time. The SMAC time was 22 min., 30 min., 36 min., and 62 min. with a W/F of 45 g.min/L when flue gas contains 5%, 8%, 10%, and 15% O2, respectively using the activated carbon containing mineral. However, the SMAC time was substantially increased to 145 min, 215 min, 330 min, and 500 min at a W/F of 45 g.min/L with flue gas containing 5%, 8%, 10%, and 15% O2 using the activated carbon with mineral removed. 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. The results indicate that regeneration improves the removal efficiency of NO. This phenomenon is attributed to the increase of surface area and micro-pores 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 were determined to be about 0.99% for wheat straw and 0.49% for sweet potato stalk activated carbon per cycle of regeneration at 600°C. Lastly, due to the extent of the mission, the sufficiency of the amount of inedible biomass in providing the activated carbon for the life support systems should be assessed. With a six person crew, such as the mission planned for Mars, the average food consumption is about 0.25kg/d per capita . Six people would need 1.5kg of wheat per day. Wheat straw yield depends on the specific varieties harvested and is widely affected by agronomic and climatic factors. An average ratio of 1.3 kg of straw per kg of grain is found for most common varieties . Burnoff accounts for about 66.7% at 600o C in pyrolysis and 650o C in activation, while the loss of carbon mass from soaking and drying was determined to be about 14.4%. So as a result, 203kg of wheat straw activated carbon can be produced per year. By incinerating feces, inedible biomass and trash, between 3g and 30 g of NOx can be produced per day, depending mostly on the amount of nitrogen in the inedible biomass. The range in the amount of NOx produced covers 90% pre-treatment removal of nitrogen to no pretreatment . . About 10% of nitrogen in the waste forms NOx . Regeneration of the adsorbing activated carbon shall occur once a week via the reduction reaction. The loss of carbon mass was determined to be about 0.99% at 600o C per cycle of regeneration, After one year, all the activated carbon should be incinerated and replaced. Calculations based on the above assumptions yield 210g NO emissions per week, and the adsorption capacity of NO by the activated carbon is 5.46mg/g carbon, which will need at least 38.5kg of wheat straw activated carbon all year long. These can be distributed among one adsorption tank, just with a load of 64.6kg of wheat carbon in the beginning of the year. Every week, the tank should been regenerated.