Dennis Y. Lu

Sintering and Reactivity of CaCO 3 -Based Sorbents for In Situ CO 2 Capture in Fluidized Beds under Realistic Calcination Conditions

Sintering during calcination/carbonation may introduce substantial economic penalties for a CO2 looping cycle using limestone/dolomite-derived sorbents. Here, cyclic carbonation and calcination reactions were investigated for CO2 capture under fluidized bed combustion FBC conditions. The cyclic carbonation characteristics of CaCO3-derived sorbents were compared at various calcination temperatures 700-925°C and different gas stream compositions: pure N2 and a realistic calciner environment where high concentrations of CO280-90% and the presence of SO2 are expected. The conditions during carbonation employed here were 700°C and 15% CO2 in N2 and 0.18% or 0.50% SO2 in selected tests, i.e., typically expected for a carbonator. Up to 20 calcination/carbonation cycles were conducted using a thermogravimetric analyzer TGA apparatus. Three Canadian limestones were tested: Kelly Rock, Havelock, and Cadomin, using a prescreened particle size range of 400-650 m. In addition, calcined Kelly Rock and Cadomin samples were hydrated by steam and examined. Sorbent reactivity was reduced whenever SO2 was introduced to either the calcining or carbonation streams. The multicyclic capture capacity of CaO for CO2 was substantially reduced at high concentrations of CO2 during the sorbent regeneration process and carbonation conversion of the Kelly Rock sample obtained after 20 cycles was only 10.5%. Hydrated sorbents performed better for CO2 capture, but also showed significant deterioration following calcination in high CO2 gas streams. This indicates that high CO2 and SO2 levels in the gas stream lead to lower CaO conversion because of enhanced sintering and irreversible formation of CaSO4. Such effects can be reduced by separating sulfation and carbonation and by introducing steam to avoid extremely high CO2 atmospheres, albeit at a higher cost and/or increased engineering complexity.

In-Situ Capture of CO2 in a Fluidized Bed Combustor

Increasing atmospheric concentration of CO2 and concern over its effect on climate is a powerful driving force for the development of new advanced energy cycles incorporating CO2 capture. This paper investigates the feasibility of CO2 capture using the carbonation reaction of CaO “in situ” in a fluidised bed combustor, where natural gas or petroleum coke (or any other fuel with low ash content) is being burned. The sorbent can be partially regenerated for CO2 capture by combustion of part of the fuel with O2 /CO2 in a separate FBC. The thermodynamic limits in the proposed cycles, in terms of CO2 capture efficiencies, are examined along with the limits imposed by the rapid decay in the sorbent activity during repeated carbonation/calcination cycles, which will be exacerbated by the presence of S. Despite these limitations, it is shown that operating windows exist where it is possible to integrate fuel combustion, CO2 and SO2 capture in a single dual reactor facility. The decay in activity in the sorbent appears to be the major practical limitation to this concept, but this can be compensated for by using a relatively large supply of fresh sorbent, which appears to be acceptable considering the low cost of limestone. Also, a novel concept to reactivate the spent sorbent using sonic energy is outlined here as an alternative to reduce the use of fresh limestone.Copyright

Waste Classification of Slag Generated in a Pilot-Scale Entrained-Flow Gasifier

Series of gasification tests have been completed in the pilot-scale entrained-flow slagging gasifier at CanmetENERGY using Canadian coals, oil-sand coke, and blends of these fuels to determine if the produced slags are nonhazardous in nature. Solid wastes generated during these tests were analyzed for their trace metals, crystallinity, and toxic constituent leaching tendency in an attempt to provide more insight into the possibility of disposal or by-product use of gasifier-produced solid waste. The gasification tests were performed at conditions representative of commercial gasifiers using a dry-fuel-feed configuration. The lower-volatility elements were found to partition between the slag and process-water solids (PWS) collected after gasification of the oil-sand coke. The less volatile group 1 elements tended to be enriched in both solid streams, whereas the slightly more volatile group 2 elements tended to exhibit higher enrichment in the PWS. Slag samples were found to be inert with regard to their leaching potential, and so these materials can be considered nonhazardous.

Hydration and Pelletization of CaCO 3 -Derived Sorbents for In-Situ CO 2 Capture

Steamhydrationandpelletizationoflimestone wereinvestigatedusinga thermogravimetric analyzer (TGA) toimprove the sorbent utilization for in-situ CO2 capture under typicalfluidized bed combustion (FBC)operating conditions. Steamhydration of CaO improves carbonation capacity but the hydratedsorbent is very fragile, which will be a problem for FBC applications. Similar sorbent improvements in terms of maintaining/enhancing reactivity were observed by sorbent fine grinding and pelletization, whichappears to be a method of using hydrated sorbent in fluidized bed applications.

Capture of CO2 with CaO in a pilot fluidized bed carbonator experimental results and reactor model

Publisher Summary Experiments in a small pilot fluidized bed reactor have demonstrated that CO 2 capture from combustion flue gases can be effective at temperatures around 650°C as long as a sufficient fraction of CaO is present in the bed. The experimental CO 2 concentration profiles, measured in the interior and at the exit of the bed during the fast carbonation period, show that the fluidized bed is an effective CO 2 absorber even after several cycles. The axial CO 2 concentration profiles during the carbonation part of the cycle have been interpreted with the KL model, adopting reactivity data from previous work and sorbent deactivation data from laboratory tests. Once the model has been validated with experimental data, it allows an extrapolation to conditions beyond those tested during the experiments, in particular, conditions resembling the simultaneous generation (combustion) and capture of CO 2 (carbonation of CaO) which show that a compact CO 2 capture process can be designed with specific benefits for highly reactive fuel, like biomass.

Combustion Characteristics of Heavy Liquid Fuels in a Bubbling Fluidized Bed

Liquid fuels such as bitumen, tars, and pitches are byproducts of heavy oil upgrading processes, and are usually contaminated with high sulphur and sometimes heavy metals contents as well. Fluidized bed combustion (FBC) appears to be a promising technology for the combustion of such fuels due to its inherent fuel flexibility and low emissions characteristics. The combustion of three liquid fuels, i.e., no. 6 oil, bitumen and pitch was investigated in a pilot-scale bubbling FBC unit. An efficient liquid fuel feeding system was developed and a bubbling FBC was successfully used to combust all three liquid fuels. The proportion of fuel escaping in the form of unburnt hydrocarbons in the flue gas was less than 0.4 percent and combustion efficiencies higher than 98.5 percent were achieved. However, combustion of liquid fuels tended to occur in the freeboard and, therefore, good mixing of the fuels in the bed was critical in achieving satisfactory combustion performance.

Role of the Water-Gas Shift Reaction in CO2 Capture from Gasification Syngas Using Limestones

The work in this paper aims at determining the effect of gasification syngas on the carbonation reaction and conversion for several naturally occurring calcium-based sorbents. Experiments were performed via the use of a thermogravimetric analyzer (TGA) and it was observed that the presence of CO and H2 caused an increase in initial rate of approximately 70.6%. The increase in reaction rate was attributed to the CaO surface sites catalyzing the water-gas shift reaction; as well, the shift reaction was assumed to be responsible for the increase in activation energy for limestone based on the formation of intermediate complexes.

Combustion Characteristics of Natural Gas in a Circulating Fluidized Bed

Recently there has been interest in extending the application of fluidized bed combustors (FBCs) to fuels with difficult handling properties or ones that are associated with non-conventional air pollutant problems. These fuels, such as biomass, plastic wastes, black liquors and heavy liquid fuels, have very high volatiles contents and, because they are often treated as easily-burned materials, they have received much less attention than has been given say to the combustion processes for char in FBCs. Understanding their gas-phase chemistry is helpful in optimizing their combustion. This paper describes the study of natural gas combustion in a fluidized bed as a simple model for studying gas-phase reactions involving C/H/N/O chemistry in the absence of char. The experimental work was conducted using a pilot-scale CFBC unit. Combustion characteristics and emissions were investigated by varying the operating conditions and in particular the combustion temperature, fluidizing velocity and bed material. The results indicated that fluidized bed combustion chemistry is associated with superequilibrium free radical processes, similar to high-temperature flame systems. In this system, prompt-NO mechanisms are the only routes for NO formation and this work shows that they can lead to significant NOx production.Copyright