Production and Stability Assessment of Oxygen Carrier Produced By Sewage Sludge Fluidized Bed Combustion

CO₂ concentration has increased significantly after the industrial revolution owing to the anthropic emissions caused by the use of fossil fuels. This has been determined by an increase in the greenhouse effect and, consequently, the rising of the temperature of the earth’s surface which is known as “global warming”. In this scenario, the development of renewable energies, the energy consumption reducing, and the energy efficiency optimization, represents a valid and definitive middle-long term solution to preserve our planet.

Despite many renewable technologies reached a good level of maturity from both technical and economical point of view, these suffer of some limitations which still make our society fossil fuel-dependent. These limitations are represented by the intrinsic intermittent nature of the renewable systems and by the inability to fulfil the total energy demand; this entails that the fossil sources will be predominant for many decades. The Carbon Capture and Storage (CCS) or Utilization (CCU) technologies represent a collection of several processes for an eco-friendly employment of the fossil fuels.

Among the CO₂ capture techniques, the Chemical Looping Combustion (CLC) is one of the most promising technology, which allows to burn fuels without direct contact between fuel and oxygen in the air.

The key aspect for this process is the utilization of oxygen carriers (OC) (typically metal oxides) that undergoes two different process: the first is the fuel oxidation; the second one is the re-oxidation of the OC. These processes can be carried out by using two reactors, typically two interconnected fluidized beds, named respectively Air Reactor (AR) e Fuel Reactor (FR). In the AR the reduced OC, coming from the FR, is regenerated (re-oxidized) by the air. In the FR the fuel is burned with the oxygen supplied by the OC producing a flue gas mainly composed by CO₂ and H₂O.

However, the choice of OC is a complex aspect, because its characteristics must meet well defined physical-chemical properties such as high reactivity, high mechanical strength, and remarkable oxygen transport capacity (1,2). The cost of an OC is a very crucial feature, in particular for the synthetic materials, being the sum of several factors such as the cost of the metal oxide and of the inert support, and the cost of the manufacturing process. This latter is rather low when industrial methods are used and the final cost depends mainly on the price of the raw material.

For this reason, most research is focused on the production of low-cost OC starting from natural ores or reusing waste materials from other activities (3).

As matter of fact, the goal of this work is to develop low-cost OC by combustion of sewage sludges. These sludges, deriving from purification treatment of civil wastewater, contain a large amount of metals, in particular iron, calcium and manganese. The OCs have been prepared by depositing the metals contained in the sludge ash on a support made of high area γ-Alumina (Al₂O₃), used as bed material during the sewage sludge fluidized bed combustion. Three different combustion tests have been carried out in a 41 mm ID reactor. For each test, different operative conditions were tested, in particular: the fluidization velocity and the particle size, in order to find optimal conditions for the preparation of the OCs. Each test lasted 3 hours, and at predetermined times a small amount of bed material has been sampled, to assess the role played by the duration of the combustion process on the amount of ashes deposited on the alumina particles. These samples have been characterized by TPR (temperature programmed reduction) analysis, carrying out reduction tests with a mixture of 2% H₂/Ar, and a temperature ramp of 10°C/min up to 850°C, to study the reactivity of the carriers and to evaluate the amount of reducible material deposited on the support. The TPR analysis showed that hydrogen consumption increases, increasing the residence time of the samples into the combustor, confirming the progressive accumulation of reducible material (mainly iron oxides) on the support. Moreover, some reduction/oxidation cycles have been performed to evaluate the stability of the OC. In general, material with a higher size (1000 µm) showed better stability during the cycles, on the contrary, smaller particles (400-600µm) exhibited a decay of their reductive activity of about 70% after only 4 cycles.

Moreover, further and deeper studies on the performance of the prepared OCs have been performed using a lab-scale fluidized bed apparatus properly designed for the study of chemical looping processes as close as possible to reality, in terms of cycling of temperatures and of reaction atmospheres. This apparatus, named Twin Beds (TB) (4), consists of two identical bubbling fluidized beds that can be operated separately in batch mode. However, the two reactors are connected each other by a duct, which permits the transfer of the OC from on reactor to the other and vice-versa. The transportation is carried out pneumatically, by the utilization of a suitable valve system placed along the duct and at the outlet of both reactors.

The two reactors were employed as FR and AR respectively, and the OCs have been tested for 10 cycles of oxidation/reduction. The initial amount of OC for the looping tests was ca. 10g. The Oxidation step has been carried out at 850°C at a fixed fluidization velocity of 0.5m/s for 10 min, using methane at 2% in volume as fuel in a stream of nitrogen. The Reduction step has been carried out at the same temperature, fluidization velocity and time of the Oxidation step, but the fluidizing gas was air, in order to regenerate the reduced OC. The progress of reactions was followed by monitoring the CH₄, CO and CO₂ gas profiles at the outlet of the reactors by gas analyzers.

The activity of the OC with methane is, as expected, lower respect to hydrogen. Additionally, these tests confirm that the oxidation capacity decays with the number of the cycles and reaches an asymptotic value after the 6^th cycle.

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