Metal Recovery from Sewage Sludge Ash for Clc Oxygen Carriers Production

Metals like Fe, Ca, Mn present in sewage sludge ash have been deposited on high surface -alumina with high mechanical resistance used as bed materials during sewage sludge fluidized bed combustion to produce low-cost CLC oxygen carriers. The produced oxygen carriers have been characterized using ICP-MS, X-ray diffraction (XRD), Scanning Electron Microscopy (SEM) and surface area (BET) analysis. H2 Temperature Programmed Reduction (TPR) experiments have been performed to study the redox behaviour of the oxygen carrier and reduction/oxidation cycles have been carried out to assess the carrier stability. The performances of the oxygen carrier in methane combustion/air regeneration cycles in a fluidized bed reactor have been evaluated. Introduction Chemical looping combustion (CLC) is a promising strategy to produce energy from fossil fuel coupling the CO2 sequestration [1,2]. The CLC process is typically realized adopting two interconnected fluidized beds with a solid, the oxygen carrier, circulating between them. The solid, i.e. the oxygen carriers (OC), promotes fuel oxidation without direct contact with atmospheric oxygen. Typical OCs consist of an active phase, which could be a suitable metal oxide, supported on an inert material for ensuring chemical, thermal and mechanical stability [3-5]. The process involves two cyclic reactions which take place alternately in the Fuel Reactor (reaction 1) and in the Air Reactor (reaction 2): 1. CnH2m + (n+1⁄2m)MexOy = nCO2 + mH2O + (n+1⁄2m)MexOy-2 2. MexOy-2 + O2 = MexOy where CnH2m is a generic fuel, while MexOy and MexOy-2 are the metal oxide in the oxidized and reduced form, respectively. The oxygen carriers should have high reactivity with gaseous fuels and oxygen and oxygen transport capacity, low fragmentation, attrition and agglomeration tendency and high stability to a large number of redox cycles. However, the choice of the proper oxygen carrier is still open and it deserves further investigations. Usually the oxygen carrier is made of a metal oxide. Among transition metals, Fe-based oxygen carriers are considered an attractive option for the chemical looping combustion owing to their low cost, abundance and environmental compatibility XXXVIII Meeting of the Italian Section of the Combustion Institute [4,5]. The aim of this work is the production of low-cost CLC oxygen carriers by sewage sludge fluidized bed combustion. In particular, the metals (Fe, Ca, Mn) present in sewage sludge ash have been deposited during sewage sludge fluidized bed combustion on high-surface -alumina which is characterized by high mechanical resistance and it was used as bed material. The produced oxygen carrier has been characterized using ICP-MS, X-ray diffraction (XRD), Scanning Electron Microscopy (SEM-EDX) and surface area (BET) analysis. The redox behaviour of the oxygen carrier has been investigated using H2 reduction experiment. Reduction/oxidation cycles have been performed to assess the stability of the systems. The reactivity of the carriers and their oxygen transport capacity have been also evaluated under repeated cycles of methane combustion/air regeneration carried out in a fluidized bed reactor. Methods The metals (Fe, Ca, Mn) present in sewage sludge ash and identified by ICP analysis have been deposited on porous -Al2O3 particles (1mm in diameter) supplied by Sasol used as bed material during sewage sludge fluidized bed combustion. Different amount of metals have been loaded on the support changing the duration (45, 90, 220 minutes) of the combustion test. The experimental apparatus consisted of a 110mm ID stainless steel fluidized bed combustor 2.7m high. Bed temperature was measured by means of a chromelalumel thermocouple placed 3cm above the distributor. The gas distributor is of conical shape and is equipped with a port and a valve at its bottom in order to enable rapid discharge of the bottom ash and bed material. A high efficiency cyclone for the capture of fine particles generated during combustion and elutriated in the exit gasses is present. A data acquisition unit is used to process signals from pressure transducers, from the thermocouples and from the on-line gas analyzers (O2, CO, CO2, SO2 and NOx). Bed inventory was 3 kg of porous -Al2O3 particles (diameter 1mm, Umf =0.223 m/s @ 20°C) supplied by Sasol, corresponding to a static bed height of about 0.3m. The experiments were carried out at a fluidization velocity of 1m/s and at an air excess of 47%. The gas was fed to the column via high-precision digital mass flowmeters. The fuel , fed under-bed, consists of semi-dried sludge particles of about 1-3mm, whose proprieties are reported in table 1 in terms of proximate and ultimate analysis, of lower heating value and of inorganic elemental analysis. The heating value and the proximate (TGA 701 LECO) and ultimate analyses (elemental analyzer CHN 2000 LECO) were based on standard ASTM procedures. The inorganic elemental analysis of the sewage sludge and the prepared oxygen carriers was accomplished by ICP-MS using a Agilent 7500CE instrument after dissolving the fuel samples by means of microwave-assisted acid digestion according to US-EPA 3051 and 3052 methods. The experimental campaign consists of steady combustion tests with continuous feeding of the sludge particles. The gaseous and particulate emissions were monitored during both the start-up XXXVIII Meeting of the Italian Section of the Combustion Institute stage and once steady state conditions were reached. Table 1. Sewage sludge properties