Article pubs.acs.org/EF
Investigating the Use of CaO/CuO Sorbents for in Situ CO2 Capture in a Biomass Gasifier Ryad Abdul Rahman,† Poupak Mehrani,† Dennis Y. Lu,‡ Edward J. Anthony,‡,§ and Arturo Macchi*,† †
Department of Chemical and Biological Engineering, University of Ottawa, 161 Louis Pasteur Street, Ottawa, Canada K1N6N5 CanmetENERGY, Natural Resources Canada, 1 Haanel Drive, Ottawa, Canada K1A 1M1 § School of Applied Science, Cranfield University, Cranfield, Bedfordshire MK43 0AL, United Kingdom ‡
ABSTRACT: The primary objective of this study was to investigate the integration of combined calcium looping (CaL) and chemical looping combustion (CLC) with steam gasification of biomass through the utilization of composite pellets consisting of limestone, CuO, and a calcium aluminate cement binder. In this process, the heat released from the exothermic reduction of CuO is used to calcine CaCO3. The technologies can be integrated by combining an oxygen carrier such as CuO with limestone within a composite pellet, or by cycling CuO and limestone within distinct particles. Using a thermogravimetric analyzer, it was demonstrated that the use of composite CaO/CuO/calcium-aluminate-cement pellets for gasification purposes required oxidation of Cu to be preceded by carbonation as opposed to the postcombustion case in which the pellets are oxidized prior to carbonation. Composite pellets were thus tested under this CO2 capture sequence using varying carbonation conditions over multiple cycles. While the pellets exhibited relatively high carbonation conversion, the oxidation conversion declined for all tested conditions likely because of the CaCO3 product impeding passage of O2 molecules to the more remote Cu sites. The reduction in oxygen uptake was particularly important when the pellets were precarbonated in the presence of steam. Limestone-based pellets and Cu-based pellets were subsequently tested in separate CaL and CLC loops, respectively, to assess their performance in a dual-loop process. A maximum Cu content of 50% could be accommodated in a pellet with calcium aluminate cement as support with no loss in oxidation conversion and no observable agglomeration.
1. INTRODUCTION More than 80% of the world’s energy is still supplied by fossil fuels.1 Reducing the resulting emissions of greenhouse gases (GHG) can be accomplished by decarbonizing the energy sector, which is achievable through the implementation of a “hydrogen energy economy”. Such a system involves the production of hydrogen through renewable sources and its use to cater to the energy demand, particularly in the domain of transportation. Paradoxically, while hydrogen itself is marketed as a clean fuel, more than 95% of hydrogen production is carried out through the use of fossil fuels, primarily through steam methane reforming.2 A clean, carbon-neutral alternative to hydrogen production from fossil fuels comes in the form of biomass gasification. To increase the yield of H2 in the gasification produced gas (H2, CO, CO2, H2O, CH4, etc.), a CO2 sorbent such as CaO can be employed in situ to capture CO2 during gasification, resulting in the thermodynamic equilibrium of the water−gas shift reaction, shown in eq 1, to move toward the products side, enhancing H2 production.
In a calcium looping process, the CaO-based sorbent circulates between two vessels: the carbonator, where CaO reacts with CO2 to form CaCO3, and the calciner, where the reverse reaction takes place to regenerate CaO.4 Using oxyfuel combustion in the calciner, regeneration of the sorbent is carried out at high temperatures (>900 °C) to generate concentrated CO2 which can subsequently be compressed and stored. The integration of calcium looping with steam biomass gasification holds numerous incentives in addition to enhancement of H2 content in the produced gas. Because the carbonation of CaO in the gasifier is exothermic, the heat released can be used to partially drive the endothermic gasification process. Part of the required heat can also be supplied by the sensible heat of the solids exiting the calciner, which is operated at high temperature (>900 °C). The water− gas shift reaction is mildly exothermic, and its enhancement will also contribute some of the heat required by the endothermic gasification process. Moreover, CaO has been shown to display catalytic behavior in the decomposition of tar at high temperatures.3 Unfortunately, calcium looping faces a few challenges. The use of oxyfuel combustion in the calciner relies on a costly and energy-intensive air separation unit which imposes a significant energy penalty on the process.5 Moreover, any unreacted oxygen from oxyfuel combustion in the resulting concentrated
CO(g) + H 2O(g) ↔ CO2 (g) + H 2(g) r ΔH650 ° C = − 35.6 kJ/mol
(1)
A leading potential technique for in situ CO2 capture in a biomass gasifier is calcium looping using CaO-based sorbents.3 The CaL cycle is based on the following reversible reaction: CaO(s) + CO2 (g) ↔ CaCO3(s) r ΔH650 °C
= −171 kJ/mol © 2015 American Chemical Society
Received: February 2, 2015 Revised: April 28, 2015 Published: April 28, 2015
(2) 3808
DOI: 10.1021/acs.energyfuels.5b00256 Energy Fuels 2015, 29, 3808−3819
Article
Energy & Fuels
tation technique16 and a sol−gel technique.17 As in other studies, the authors highlighted the high sustained oxidation conversion (>98% over all cycles tested) for all tested cases although a gradual decay in carbonation performance was still observed under mild experimental conditions. While the combination of CaL and CLC for postcombustion applications has been shown to be promising, what has not been attempted so far is integrating the technology with precombustion technologies such as biomass steam gasification. Given the inherent benefits of in situ carbon capture for gasification, the carbonator would act as the gasifier in such a process. Part of the produced gas from the gasifier could then be fed as fuel to the calciner to reduce CuO and provide the required heat to drive the calcination of CaCO3. Although just as in the postcombustion scenario, the present system would utilize three distinct reactors (carbonator, air reactor, and calciner), questions remain with regard to the best way of circulating solids in such a system. Ideally, the solids would circulate across the different reactors in a similar sequence as in the postcombustion case: carbonator (gasifier)−calciner (fuel reactor)−air reactor, and finally return to carbonator (gasifier). This sequence, which for the sake of simplicity will be labeled as sequence 1, is illustrated in Figure 1.
CO2 stream makes the material corrosive upon compression and liquefaction, making its transport by pipeline problematic.6 More critically, exposing the CaO-based sorbent to the high temperatures required in the calciner makes it liable to sintering, a phenomenon through which smaller pores fuse together to form larger pores, leading to a loss in reactivity of the sorbent.4 A plethora of methods have been attempted to counter the loss in CaO reactivity throughout the years, with the pelletization of limestone using calcium aluminate cement as a binder being particularly effective. Manovic and Anthony7 demonstrated that the formation of mayenite (Ca12Al14O33) dispersed across the pellet particles contributed in forming a framework of nanopores which delayed the sintering of CaO sites. However, although the decline in CaO reactivity can be slowed, the decline in CO2 uptake has proved to be inexorable, irrespective of any applied method.4 Because of the issues surrounding regeneration of the CaO-based sorbent, regeneration by means of CLC was proposed by Manovic and Anthony.8 In such a process, the oxygen is supplied to the fuel (either natural gas or syngas) by means of an oxygen carrier, eliminating the need for a pure oxygen stream. One of the most promising oxygen carriers is CuO, which is reduced exothermically by CH4, H2, and CO as shown by eqs 3, 4, and 5, respectively. 4CuO(s) + CH4(g) ↔ 4Cu(s) + CO2 (g) + 2H 2O(g) r ΔH900 ° C = − 208.8 kJ/mol
(3)
CuO(s) + H 2(g) ↔ Cu(s) + H 2O(g) r ΔH900 ° C = − 100.3 kJ/mol
(4)
CuO(s) + CO(g) ↔ Cu(s) + CO2 (g) r ΔH900 ° C = − 133.5 kJ/mol
(5) Figure 1. Integrated process of CaL−CLC with steam biomass gasification. Sequence 1: carbonator (gasifier)−calciner−air reactor− carbonator (gasifier) sequence.
Besides the exothermic nature of its reduction, CuO offers numerous advantages; compared to other CLC materials, it exhibits high oxygen-carrying capacity (0.20 g O2/g CuO) and high reactivity.8 In addition, while it is more expensive than the more abundant but less reactive oxygen carriers such as Fe2O3, it is still cheaper than many other oxygen carriers such as NiO and CoO. de Diego et al.9 underlined the affordability of CuO by identifying it as one of the cheapest materials available for CLC. The main downside, however, is that CuO has a relatively low melting point of 1085 °C, which makes it vulnerable to agglomeration at high temperatures and requires it to be combined with a support.10 From 2011 onward, a number of attempts at combining limestone and copper oxide in an integrated CaL−CLC process have been reported. Manovic et al.8,11,12 used a wet mechanical mixing method to produce several batches of CaO/CuO/ calcium aluminate cement pellets which exhibited excellent oxidation conversions and high CaO reactivity over the maximum of three tested cycles. Using a wet mixing method, Qin et al.13 first produced CaO/CuO sorbents supported with MgO and in more recent studies14,15 investigated the performance of the sorbents using various precursors. Although all sorbents again exhibited excellent oxidation conversions, the authors observed in their first work13 on the topic that the presence of CuO within the sorbent led to a more drastic decay in carbonation performance, which they attributed to the “wrapping” of Cu/CuO around CaO. Kierzkowska and Müller synthesized CaO−CuO composite sorbents using a coprecipi-
In contrast to flue gas from fossil fuel combustion which consists primarily of N2 and CO2, the producer gas from the steam gasification of biomass is composed, for the most part, of H2, CO, CO2, CH4, and H2O, with their relative proportions dependent on the gasifier conditions. Thus, in addition to the carbonation of CaO to CaCO3, significant reduction of CuO to Cu could occur inside the gasifier (carbonator). This depends on the relative kinetics of CaO carbonation and CuO reduction and the residence time of the pellets within the reactor. Manovic and Anthony8 examined this scenario and underlined the fast kinetics of both the reduction of CuO and the carbonation of CaO (specifically the reaction kinetic-controlled regime). However, in their study those reactions were carried out separately (i.e., carbonation in solely CO2, followed by parallel reduction−calcination in CO and H2). Thus, in their case, which of the two reactions is faster remains uncertain. While the reduction of CuO could be significant in this particular gaseous environment, there is also the distinct possibility that the fast regime carbonation would result in the formation of a CaCO3 product layer, impeding the transport of the reducing gases to the CuO core, especially if CuO core-inCaO shell composite pellets (similar to those employed by Manovic et al.11) were to be used. If the kinetics of CuO reduction are so fast that it experiences a significant conversion 3809
DOI: 10.1021/acs.energyfuels.5b00256 Energy Fuels 2015, 29, 3808−3819
Article
Energy & Fuels
which the integration of CaL and CLC with biomass gasifier can be incorporated, the aim of this study was to investigate both the feasibility of each configuration and the performance of the relevant pellets using the different sequences.
even at limited exposure times to the gasifying mixture, using the sequence illustrated in Figure 1 will be challenging. There will subsequently be insufficient CuO to be reduced and generate the required heat to drive the calcination of CaCO3. A different sequence for solids circulation would then be required: carbonator (gasifier)−air reactor−calciner (fuel reactor)− carbonator (gasifier). This sequence is identified as sequence 2 and illustrated in Figure 2.
2. EXPERIMENTAL SECTION Composite pellets consisting of CaO, CuO, and a binder were prepared in-house. Powdered (
