Energy Estimation on CRN Process of Fly Ash as a Slow-Release Nitrogen Fertilizer

Abstract

Through carbothermal reduction and nitridation (CRN) reaction, nitrogen could be fixed into fly ash up to 28 wt %. Nitrided fly ash has the potential to be a source of slow release nitrogen fertilizer. To further investigate this potential, the energy consumptions were estimated for the nitridation process of fly ash. Reactors such as parallel rotary furnaces were simulated for the heat loss of radiation and convection. Material balance was carried out to determine the dimension of the rotary furnaces. Subsequently, on the basis of the dimension of the rotary furnace, the energy balance was performed from the reaction engineering aspects of the nitridation reaction of fly ash. Ultimately, the total energy was transformed into per unit nitrogen and compared with the energy consumption of the Haber-Bosch process for ammonia synthesis. The energy balance calculation suggested that there is great potential for nitrided fly ash to be slow release nitrogen fertilizer. 1. Introduction Coal fly ash is the incomplete combustion product from a coal-fired power plant. Currently, coal is the major source of fossil energy in mainland China. The electricity generated from coal-fired power plants accounts for 80% of the power nationwide. This situation will remain unchanged over a long period of time. Consequently, the amount of coal fly ash shall continue to increase steadily, accounting for 15-40% of the mass of coal. The proper disposal or utilization of coal fly ash brings a great challenge to the environment. Scientists have long been dedicated to exploiting novel applications of fly ash, such as construction material, 1 flocculant, 2 absorbent, 3 low-value-added ceramics, 3 soil amendment, 4,5 and so on. We had previously demonstrated successful introduction of nitrogen into fly ash via CRN (carbothermal reduction and nitridation) reactions. 6 We had also elaborated the prospect of nitrided fly ash in application as a source of slow-release nitrogen fertilizer from the aspect of reaction kinetics. 7-9 Currently, the most widely used nitrogen fertilizers are mainly based on ammonia synthesized from the Haber-Bosch process. Those commercial nitrogen fertilizers have the drawback of low efficiency due to high solubility in water. In comparison, nitrogen could be incorporated into fly ash as high as 28 wt % 6 and slowly released in the form of ammonia through hydrolysis process. 10 Nevertheless, reasonable energy consumption is essential for any further potential application of fly ash. Accordingly, this study presents the energy estimation for CRN reaction of fly ash. The estimated energy was compared with the energy consumed by the Haber-Bosch process to prepare ammonia. The comparison reveals great potential in application of fly ash as a source of nitrogen fertilizer. From the aspects of reaction engineering, the energy consumption was evaluated by material and energy balance calculations from preliminary design of the rotary furnace reactors. Rotary tube reactors have the advantage that precursor powder or aggregates are continuously replaced at the furnace tube wall, thus providing superior and more uniform heattransfer characteristics relative to fixed bed reactors or moving bed reactors. 11 Residence times are shorter (typically 30 min to a few hours) than in moving beds. In this study on gas-solid reaction in rotary furnaces, a countercurrent flow was chosen. Although many furnaces experience considerable difficulty in achieving uniform product, virtually all of the rotary furnace models are based on an assumption of perfect mixing in an angular direction. The assumption depicts that, at each axial position along the furnace, the bed is well-mixed in the transverse plane; i.e., the bed material is isothermal over any transverse section of furnace. Heat transfer within the bed reactants occurs by the same mechanisms as in any packed bed, i.e., particle-to-particle conduction, radiation, and interstitial gas-to-particle convection. 12 Steady state was assumed throughout the calculation. The nitridation reaction conditions for scaling-up are shown in Table 1 from our previous publications. 6 A more quantitative understanding of the mass balance and heat-transport phenomena within the bed material are presented hereafter.