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Ind. Eng. Chem. Res. 2009, 48, 102–109
Preliminary Assessment of a Concept of Looping Combustion of Carbon Piero Salatino* and Osvalda Senneca Dipartimento di Ingegneria Chimica, UniVersita` degli Studi di Napoli Federico II, Istituto di Ricerche sulla Combustione, Consiglio Nazionale delle Ricerche, Piazzale V. Tecchio 80, 80125 Napoli, Italy
A novel concept of looping combustion of carbon (CarboLoop) is presented. It is based on the feature of carbons to extensively uptake oxygen upon exposure to air at moderate temperatures. Surface oxides of carbon are eventually released as combustion products (CO, CO2) as the oxidized fuel is brought to moderate-tohigh temperature in an oxygen-free atmosphere. This concept is pursued to the formulation of a preliminary scheme of a looping combustor of carbons based on a dual interconnected bed reactor system. One of the reactors is air-blown and acts as the fuel oxidizer. The second reactor, operated with partial recycle of gaseous effluents (CO2 + impurities), acts as the fuel desorber. Operating conditions of the oxidation and desorption stages may be properly tuned, leading to alternative looping strategies. The present study lays down the basic mechanistic background for analyzing the process, based on a simplified semiglobal approach to combustion and oxidation of solid carbons. The soundness of the CarboLoop concept has been verified by purposely designed experiments. The alternated oxidation/desorption stages typical of a looping combustor are simulated in a thermogravimetric analyzer. Experiments were directed to monitoring the oxidation and desorption steps under simulated looping conditions. Graphitized coke has been used as a surrogate carbon fuel. The experimental results confirm the soundness and potential of the CarboLoop concept and lay the path for its further development. 1. Introduction Rising concerns about climate change have become prime drivers of scientific and technological development in the field of solid fuel combustion and gasification.1,2 Studies in this field are targeted either to the reduction of carbon dioxide emissions per unit energy generated or to the development of novel combustion concepts that make CO2 capture and sequestration inherently more economical and feasible. The first goal is mainly pursued through the improvement of combustion and gasification technologies to achieve higher overall energy conversion efficiencies.1 Combustion under supercritical and ultrasupercritical conditions and integrated gasification and combined cycles are currently the main pathways to achieve this result. The second goal is pursued by rethinking combustion technologies in such a way that carbon dioxide is highly concentrated at the exhaust, possibly free of contaminants, so that it can be more easily compressed and disposed of through the sequestration path.2 Oxyfiring and chemical looping combustion are two examples of these new-generation “capture ready” combustion technologies. The first has now reached the demonstration stage, though many open issues still remain. The second is currently being explored with some success at the laboratory scale, but application to solid fuels is still at a very embryonic stage. Chemical looping combustion (CLC) of gaseous fuels (light hydrocarbons, syngas from gasification of coal and heavy residues) has been demontrated at the laboratory scale and is fastly approaching pilot-scale demonstration. The concept underlying chemical looping combustion is fairly simple and is based on the combined operation of two interconnected reactors: a fuel reactor and an air reactor.3-6 A solid oxygen carrier (most typically a metal oxide, but other carriers have been envisaged) is transferred in the fluidized state from the fuel reactor to the air reactor and vice versa. Fuel oxidation is * To whom correspondence should be addressed. Phone: +390817682258. Fax: +390815936936. E-mail:
[email protected].
accomplished in the fuel reactor by the solid carrier, MyOx, which enters the reactor in the oxidized form and is reduced therein: CnH2m + (2n + m)MyOx f nCO2 + mH2O + (2n + m)MyOx-1 (1) The reduced form of the carrier, MyOx-1, is eventually transferred to the air reactor where it undergoes oxidation (2n + m)MyOx-1 +
(n + m2 )O f 2n + m M O 2
(
)
y
x
(2)
by contact with air. Provided that proper recirculation of the carrier can be established between the two reactors and gas leakage from each reactor to the other can be prevented,7 this operation makes it possible to meet the requirement of producing a stream of nearly pure carbon dioxide issuing from the fuel reactor, completely separated from the vitiated air stream leaving the air reactor. Alternatively, Fan and co-workers5 suggested that oxidation of the carrier MyOx-1 may be accomplished by steam according to the process (2n + m)MyOx-1 + (2n + m)H2O f (2n + m)MyOx + (2n + m)H2(3)
with net production of hydrogen. CLC based on the use of oxygen carriers is inherently suitable for gaseous fuels, as effective contact between the solid carrier and the fuel can be easily achieved in this case. Application of the same process to solid fuels is more problematic. Direct fuel oxidation by the oxygen carrier implies an inherently inefficient reaction between two solid phases: C(s) + 2MyOx(s) f CO2(g) + 2MyOx-1(s)
(4)
Recent attempts to overcome this limitation and to extend the CLC concept to capture-ready combustion of solid fuels have been based on the idea that solid fuels can be either in situ or ex situ gasified in the fuel reactor by steam8-10 or carbon dioxide,11-13 yielding a syngas that eventually acts as the
10.1021/ie800295q CCC: $40.75 2009 American Chemical Society Published on Web 10/01/2008
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reducing agent of the oxygen carrier. Accordingly, the following heterogeneous reactions take place simultaneously in the fuel reactor: carbon gasification:
{
C(s) + H2O(g) T CO(g) + H2(g) C(s) + CO2(g) T 2CO(g)
(5)
Table 1. Properties of the Graphitized Coke Proximate Analysis wt %, As Received moisture volatile matter fixed carbon ash
Ultimate Analysis wt %, As Received
reduction of oxygen carrier:
{
CO(g) + MyOx(s) f CO2(g) + MyOx-1(s) H2(g) + MyOx(s) f H2O(g) + MyOx-1(s)
0.01 1.89 89.09 9.01
(6)
Oxidation of the carrier MyOx-1 eventually takes place in the air reactor (or by contacting with steam) as a heterogeneous gas-solid reaction, much like chemical looping combustion of a gaseous fuel. In situ gasification of carbon in the fuel reactor has the advantage over external generation of syngas in a separate gasifier that both the kinetics and thermodynamics of the gasification are favored by the continuous subtraction of the gasification products as they are oxidized by the carrier. Chemical looping combustion of syngas produced by ex situ gasification of carbon, on the other hand, has the advantage that fuel ash and the oxygen carrier are unmixed all along the process, and no separation is required to extract ash from the process carrier. Both steam and dry reforming of carbons are relatively slow at temperatures of practical interest for CLC, especially when gasification is carried out ex situ; hence, heterogeneous gasification turns out to be the rate-limiting step of the whole process. In the present study, a novel and different solid fuel looping combustion concept is presented, referred to as CarboLoop. The concept is based on the well-established properties of most carbons to extensively uptake oxygen to form surface oxides. Accordingly, in the CarboLoop process, the carbon itself acts as the oxygen carrier between two reactors: an oxidizer, where it is oxidized by air at a temperature and for holding times that prevent the parallel course of carbon gasification; a desorber, where rapid desorption of oxidation products takes place in an oxygen-free atmosphere under properly optimized desorption conditions. The analysis is directed to the establishment of operating conditions of the oxidizer and of the desorber, based on a semiglobal kinetic model of carbon combustion. Moreover, the soundness of the CarboLoop concept has been checked by means of experiments in which looping combustion has been simulated by cyclic exposure of carbon samples to oxidation and desorption conditions in a thermogravimetric analyzer. 2. Experimental Section The validity of the CarboLoop concept has been checked by means of discontinuous experiments in which periodic exposure of carbon samples to oxidation and desorption conditions, similar to that experienced by fuel particles in looping combustion, has been accomplished in a thermogravimetric analyzer. A thermogravimetric analyzer Netzsch 409C coupled with a CO/CO2 IR analyzer (HB URAS 3E) has been used for this purpose. Graphitized coke with particle size below 200 µm has been used as a surrogate fuel. Its properties are reported in Table 1. Approximately 30 mg of sample have been used for each test. Tests consisted of sequential oxidation/desorption steps carried out as follows: Step 1 (sample heating up and oxidation). The sample was initially loaded into the TGA and heated in a flow of air (200
C H N ash
90.3 0.26 0.40 9.0
mL/min) up to the desired oxidation temperature TO (ranging between 300 and 500 °C in the present campaign) at the heating rate of 50 °C/min. The sample was then kept in the flow of air for a time tO. Step 2 (desorption). The gas was switched over from air to an oxygen-free stream (either pure nitrogen or pure CO2). At the same time, the temperature was stepwise raised to the desorption temperature TD (600 or 700 °C in the present campaign), at the heating rate of 50 °C/min. The sample was held at TD for a time tD before the temperature was again stepwise decreased for a new oxidation cycle. Step 3 (oxidation). Once the temperature reached the preset oxidation temperature TO, the gas feeding to the TG was again switched over from either nitrogen or carbon dioxide to air and the sample was oxidized for a time interval equal to tO. Steps 2 and 3 were then iterated several times. During the experiments, the sample weight loss as well as the CO and CO2 concentrations at the exhaust were constantly monitored. For comparison, an isothermal combustion test was carried out on the same sample in air at the combustion temperature of 600 °C. In this case, the sample was loaded in the TGA and heated up to 600 °C in a flow of nitrogen at the heating rate of 50 °C/min. Once the preset temperature was approached, the gas was switched over from nitrogen to air and the sample held at 600 °C for ∼150 min. 3. Mechanistic Basis of the CarboLoop Process The role of surface oxides as reaction intermediates in the combustion of solid carbons has long been demonstrated, though the nature of chemisorption/reaction sites on carbon is the subject of debate and considerable research effort. The formation and thermal decomposition of solid oxygenated complexes and the generation of gaseous oxidation products at very low temperatures (
