However, only photosynthesis rates during leaf senescence of plant pre-cultured at low N supply reflected leaf senescence score during reproductive growth and N efficiency in the field experiments. Therefore, cultivar differences in leaf senescence during reproductive growth can only partly be reproduced in a short-term nutrient-solution experiment. Several differences between vegetative and reproductive growth might influence the induction and development of leaf senescence: first, although leaf senescence might be induced by N shortage both in hydroponics and under field conditions, the timing of N shortage is dependent upon different factors. In the field, the exploration of N sources in deeper soil layers might play the most important role for N uptake during reproductive growth . Thus in the field, root growth and morphology are the most important plant traits, which play only a minor role for N uptake in hydroponics. Secondly, source-sink relationships differ distinctly between vegetative and reproductive growth, both for carbohydrates and as a consequence also for N. The changes in assimilate flows might influence the development of leaf senescence, or at least the parameters used to characterize leaf senescence. However,potted blueberries the fact that photosynthesis rate during late stages of leaf senescence was significantly correlated to leaf senescence in the field experiments and to grain yield at limiting N supply suggests that cultivar differences in specific steps of leaf senescence related to the breakdown of the photosynthetic apparatus contribute to N efficiency in the field.
Life support systems are what make human travel a possibility. In long range space travels, such as the travel to Mars, life support cannot depend upon storage alone, it requires a fully regenerative system as well, i.e. waste must be reclaimed for reuse. Steam reformation, supercritical water oxidation, electrochemical oxidation, and incineration are a few of the solid waste reclamation technologies that are being developed and tested for use in space travel. Currently though, it seems that incineration might be the best choice among the previously mentioned, in providing a fully regenerative system. Through rapid conversion, incineration of the inedible parts of wastes and crops produces carbon dioxide, water, and minerals. Incineration is already the most thoroughly developed technology for use in a terrestrial environment. However, with the use of incineration in a closed environment, there is the eventual buildup of pollutants that are emitted in the process. The resulting NOx and SO2 pollutants need to be removed and recovered for reuse by a flue gas cleanup system. Important things to consider when developing a flue gas clean up technology for use in long range space travels are safety, energy requirements, sustainability, and doable under a micro-gravity condition. Due to the sensitivity and restrictions of space missions, a flue gas clean up system lacking in any of these considerations can be hazardous and could potentially compromise the missions. Technologies requiring things such as expendables or the use of catalysts are unsuitable for space missions due to the loss of valuable resources and the possibility of catalyst poisoning thus limiting the life-span of a catalyst. Also, due to the micro-gravity, it is difficult to use wet processes that handle liquids, such as spray absorbers.
Consequently, even though there are numerous flue gas clean up technologies developed , taking into consideration the limitation each provides, the number of reliable and applicable systems seem to be dwindling. Commercial activated carbon, made mostly from materials such as coconut shells and coal, has been studied for the adsorption and/or reduction of NOx and SO2 . In this paper, we study the use of the activated carbon prepared from hydroponic grown wheat straw and sweet potato stem for the control of air pollutants that are a result of incineration during space travel. Both wheat straw and sweet potato stalk are inedible biomass that can be continuously produced in the space vehicle. It was found that there is actually a minuscule amount of SO2 in the flue gas from the incineration of hydroponic biomass, and that most of the sulfur from the biomass ends up as sulfate in flyash. Since SO2 should have already reacted with the alkali metal, the technique entails the carbonization of the wheat straw and sweet potato stalk, resulting in an activated carbon for the adsorption of NOx and then a reduction of the adsorbed NOx by carbon to form N2. Since most NOx in flue gas from combustion is in the form of NO, and NO2 is readily adsorbed on the activated carbon, this paper concentrates on the removal of NO. 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.After the biomass was shredded, it was packed tightly into a stainless cylinder for pyrolysis and activation. Nitrogen and Carbon dioxide were used as the pyrolysis and activation gases, respectively. The gas flow rate for pyrolysis was 0.5-1.0L nitrogen per minute and activation was 0.25L carbon dioxide per minute.
The amount of biomass used was approximately 50.0-60.0g for each batch. Pyrolysis and activation temperature and times were varied during carbonization in order to determine optimal activation conditions. The activation temperature was 50°C higher than the pyrolysis temperature. The notation of the activated carbon “WS-2-600-1-650” and “SP-2-600-1-650” indicate that the activated carbon was prepared from wheat straw and sweet potato stalk, respectively with a 2 hrs pyrolysis time at 600°C followed by 1 hr of activation at 650°C. Once activation was complete, CO2 was supplied to the sample until it could be sealed in a container. Biomass on a space mission would likely come from a hydroponic system. The total mineral content of hydroponic biomass can be much higher than field-grown biomass if nutrients are supplied luxuriously to the hydroponic solution. A typical hydroponic plant solution has a nutrient ratio of N:K:Ca:P:S:Mg =16:6:4:2:1:1 and contains micro-nutrients of B, Mn, Zn, Cu, Co, Mo, and Fe. The concentration of potassium in the solution is about 6 mM. Thus, hydroponic activated carbon adsorption efficiencies may differ significantly from field-grown biomass. Field grown biomass generally has less than 10% mineral content, while hydroponic biomass can have as much as 30% mineral content. Because of this, many of the experiments were performed using char, which had been soaked in deionized water to remove the soluble minerals. After soaking, the char was drained and dried at 200o C for one hour before use. Specific surface areas of samples were determined by gas adsorption. An automated adsorption apparatus was employed for these measurements. Nitrogen adsorption/desorption was measured isothermally at -196o C. Before any such analysis, the sample was degassed at 250o C in a vacuum at about 10-3 torr. The nitrogen isotherms were analyzed by the BET equation, to determine the surface areas of the chars. BJH adsorption cumulative surface areas of pores of the samples also were determined.Most of NOx in flue gas from combustion is in the form of NO. Also, NO2 is readily adsorbed on the activated carbon. Consequently, efforts were directed to determine conditions for maximal removal efficiency of NO. Adsorption efficiency of NO on the activated carbon was studied to evaluate the effects of temperature, oxygen composition,square plastic pot moisture and flow rate on the production of effective activated carbon. The adsorption experiments were performed by using a simulate flue gas with variable concentrations of N2, carbon dioxide, oxygen, NO and H2O. NO and NO2 concentrations were analyzed by a chemilum inescent NOx analyzer . The amount of NOx absorbed by the activated carbon was determined from the difference in NOx concentration of the inlet and outlet gases. It was assumed that missing NOx was adsorbed by the activated carbon.Preparation of activated carbon was conducted by heating hydroponic grown wheat straw and sweet potato stalk under anaerobic conditions. Nitrogen and carbon dioxide were used as the pyrolysis and activating gases, respectively. Time and temperature were varied during pyrolysis and activation to determine optimal carbonization conditions. In order to determine optimal carbonization temperatures, samples of wheat straw and sweet potato stalk were heated at 100o C intervals between 300 to 800o C for two hours during pyrolysis and between 350°C to 850o C for one hour during activation. Afterwards, percent burn off was measured. Higher carbonization temperatures caused larger portions of the samples to burn off, as shown in Fig.2.
The burn off was 68% at 600°C and 86% at 800°C for wheat straw, while 66% at 600°C and 92%at 800°C for sweet potato stalk. Both samples appear to follow the same burn off trend, the percent burn off increases only slightly with the increase of temperature between 400°C to 600o C. However, pyrolysis and activation temperatures above 700o C were observed to cause significant amounts of wheat straw and sweet potato stalk to burn to ash. It is thus not recommended that reactions be run at temperatures exceeding 600o C. 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 micropore count compared to shorter times. As with temperature generation optimum, however, 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 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 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 effect of the minerals on the activated carbon on NO adsorption efficiency was studied. 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 .