Gasification of coal, petroleum residuals and biomass in integrated gasification combined cycle (IGCC) power systems provides an opportunity to produce energy more efficiently and with significantly less environmental impact than more conventional combustion-based processes. In addition, the synthesis gas byproduct of the gasification process offers the gasifier operator the option of "polygeneration;" that is, the production of alternative products instead of power if economically favorable.
However, widespread adoption of coal gasification by the power-generation industry depends on whether reliability and gasifier operation-economics issues can be resolved. Central to both increased reliability and economics is the development of materials having longer service lives in gasifier systems, which will provide extended periods of continuous gasifier operation.
The Advanced Refractories for Gasification Project at Albany Research Center (ARC) focuses on the development of improved materials capable of withstanding the harsh, high-temperature environment created by the gasification reaction. This includes materials for the refractory lining that insulates the slagging gasifier and thermocouple assemblies used to monitor gasifier operating temperatures.
Current generation refractory liners in slagging gasifiers typically are replaced every 10 to 18 months, at costs ranging up to $2,000,000. High materials and installation costs are compounded by the lost-opportunity costs during the three to four weeks that the gasifier is off-line for the refractory exchange. In addition, current generation thermocouple devices rarely survive the gasifier start-up process, leaving the operator with no real means of temperature measurement during gasifier operation. The goals of the project include the development of a refractory liner having a service life at least double that of current generation refractory materials and the design of a thermocouple protection system that allows accurate temperature monitoring for extended periods of time.
Postmortem analysis of refractory brick removed from commercial gasifiers combined with laboratory studies of refractory behavior in simulated gasifier environments indicate that slag penetration and attack of the refractory is the primary cause for the rapid degradation of the refractory lining in slagging gasifiers[1-6]. The mechanisms leading to material loss are illustrated schematically in Figure 1. Stresses generated as the brick expands during heat-up of the gasifier may lead to some initial cracking and subsequent pinch spalling at the brick corners. As the feedstock is introduced into the gasifier and slag begins to flow down the refractory wall, the slag immediately penetrates the refractory and corrosion of the material begins. Because the thermal gradient within the hot-face brick is relatively flat (~ 4C/cm, or 18F/in.) and the viscosity of the slag is typically low at the gasifier operating temperature, the slag moves deep into the refractory (> 4 cm, or 1.5 in.) via the interconnected porosity and along matrix grain boundaries. The presence of cracks vertical to the hot face also facilitates slag penetration. The resulting changes in mineralogy and/or physical characteristics of the slag-penetrated region result in the formation of cracks parallel to the hot face near the slag-penetrated/virgin refractory interface. Link-up of this crack system, which is accelerated by sudden or large changes in gasifier operating temperature, ultimately leads to large-scale material removal (Fig 2). The cycle then begins again with renewed slag penetration and attack of the fresh refractory.
When structural spalling is the principal failure mechanism for a refractory, as is the case in slagging gasifiers, there is the potential for large amounts of material to be removed as the result of single fracture events. The actual volume of material removed is defined by the depth of slag penetration, because changes induced in the material as a result of interaction with the slag lead to crack initiation behind the hot face. Therefore, the key to improving the performance of refractories in slagging gasifier environments where structural spalling is a problem is to reduce the ease of slag penetration into the material. Ways to achieve this include:
- Changing the wetting characteristics of the slag by altering the slag chemistry
- Reducing the wettability of the refractory
- Reducing the level of interconnected porosity in the refractory
- Changing the pore size distribution within the refractory
- Inducing an in-situ change in the refractory microstructure that effectively seals the refractory surface
Any changes made to the refractory must be effective in reducing slag penetration while retaining the other beneficial properties of the material.
Material development approach
The approach selected to reduce slag penetration and attack is to induce in-situ change in the refractory microstructure to seal the refractory surface. Slag penetration is reduced to less than one-fifth that observed in an unmodified high-chrome (at least 80 wt% Cr2O3) refractory under identical conditions in laboratory exposure tests by adding a small amount (< 5 wt%) of a phosphate-based material (AlPO4, CrPO4, etc.) to the refractory matrix. The amount of refractory damage and the volume of refractory material loss caused by slag penetration are significantly diminished.
Results of a laboratory cup exposure test comparing the performance of a commercial high-chrome refractory with the ARC-modified material is illustrated in Fig. 3. In the test, a cup is drilled into each refractory brick and filled with a coal ash slag having the chemical composition (in wt%) 51% SiO2, 21% Al2O3, 20% Fe2O3, 6% CaO, and 2% MgO. After exposing the slag-filled cups in a furnace at a temperature of 1600C (2910F) for 24 hours in an argon environment, they are sectioned and examined for evidence of slag penetration and attack.
The level of slag penetration in the phosphate-modified refractory is limited to within 1 mm (0.04 in.) of the refractory-slag interface indicated by the dotted lines in Fig. 3. By comparison, the unmodified refractory control samples undergoes nearly complete penetration by the slag to the bottom of the cup (> 20 mm, or 0.8 in.).
Microstructural and microchemical analyses using scanning electron microscopy shows that slag penetration occurs in the modified material along matrix grain boundaries and via interconnected porosity, but there is very little slag interaction with the fine-grain matrix and aggregate phases. In contrast, there is only limited slag penetration and attack of the modified refractory as illustrated in a chemical map of the near-surface regions, which shows the presence of silica, a major component of the slag (Fig. 4).
The mechanisms for improved refractory performance in the phosphate-modified materials have not been fully confirmed. However, it is believed that the reaction between constituents in the slag, phosphate and refractory causes the slag to freeze more quickly within the refractory. This limits penetration to a narrow surface region and, therefore, reduces the potential for the loss of large volumes of material due to structural spalling.
An additional benefit to the phosphate-modified material is that the relatively small change in chemical composition required by the process likely will not result in a large scale change in refractory mechanical and thermal performance.
Based on the laboratory test results, the modified refractory material will at least double the service life of currently available high-chrome refractories, which translates into a potential savings of millions of dollars in annual gasifier operating costs, as well a significant increase in gasifier on-line availability. Ongoing research is aimed at further optimizing the refractory and preparing materials in collaboration with a commercial refractory manufacturer for expanded pilot-scale and field tests.
Postmortem analyses of spent thermocouples removed from commercial gasifiers indicate that, as with the refractories, slag penetration and attack are the principal mechanisms of rapid thermocouple failure in the gasifier environment. In this case, elements in the slag chemical composition quickly penetrate into the thermocouple protection assembly, react with the thermocouple wire and result in rapid failure of the device. To reduce thermocouple susceptibility to slag attack, research involves designing and testing thermocouple assemblies that incorporate one or more of the following protection strategies: a slag-resistant coating of the thermocouple sheath, a more slag-resistant filler material and a slag-resistant assembly end cap.
The purpose of most of the thermocouple assembly is to provide protection to the thermocouple wire in the service environment. The assembly usually consists of an outer sheath or sheaths (frequently Al2O3 or SiC), a filler material (usually Al2O3), and the individual thermocouple wires encased in a final Al2O3 protection tube.
However, because most molten gasifier slags are undersaturated with respect to Al2O3 at the operating temperature, the high-Al2O3 protection tubes and filler materials are particularly susceptible to attack by the slag. Similarly, the reducing operating conditions combined with the presence of iron in the molten slag result in rapid degradation of SiC components. As a result, direct contact of the thermocouple device with the molten slag results in a rapid breach of the protection system.
In collaborative work with Ames Laboratory and Oak Ridge National Laboratory, ARC is examining the feasibility of using several coating techniques on the outer protection sheath to extend thermocouple life by slowing the rate of slag attack. Several coating compositions, including Cr2O3 and tungsten, will be tested to determine their relative resistance to the molten slag under gasifier operating conditions.
Also, ARC is developing an economical method to manufacture dense thermocouple filler materials having a composition similar to that of the improved refractory material previously described. Such a filler material is expected to have both increased physical (due to its higher density) and chemical resistance to attack by most gasifier slags, and slag-exposure testing will conducted to confirm its relative resistance to slag penetration and attack. Following the tests, ARC will design an optimum thermocouple protection assembly and will produce prototypes in collaboration with a thermocouple manufacturer for testing in commercial gasifiers.