R&D for Gasifier Optimization/Plant Supporting Systems | Coal Gasification Encyclopedia

Gasifiers operate under demanding conditions, presenting several challenges especially in regard to materials, where not only is the gasifier itself—more specifically the refractories—under severe physical and chemical stress, but so are any devices inserted to monitor and control the gasification process.

Availability is very important to the economics of a gasification plant. A shut-down gasifier halts synthesis gas (syngas) production and, therefore, final product output (electricity, liquid fuels, etc.). With current, state-of-the-art technology, many integrated gasification combined cycle (IGCC) designs incorporate a spare gasifier in order to achieve acceptable overall plant availability, even though this entails a higher capital cost. With continued operating experience and research, it is believed that an online availability of 85–95 percent in utility applications—and 95 percent for chemical production and other applications, can be achieved by a gasification plant, but currently, most plants cannot achieve that without redundancy or fuel backup:

Research and development is being conducted to increase the availability of the gasifier and decrease the cost of operation and maintenance, thereby substantially optimizing gasifier operation. Examples include advanced materials development for refractory and the development of a reliable, practical and cost-effective means of monitoring real-time temperature in the gasifier through advances in sensors and instrumentation.

In addition to development of technologies such as advanced refractories and sensors, current research efforts also include development of gasifiers for low-rank coal, creating models to better understand the kinetics and particulate behavior of fuel inside a gasifier, and developing practical solutions to mitigate the plugging and fouling of syngas coolers.

Refractory material lines a gasifier and gives it ability to withstand extremely high operating temperatures. In the most severe slagging gasifiers, refractories can require replacement every three months, in which the gasifier system needs to be shut down for one to two weeks. These shut-downs can cost a plant millions of dollars. Research and development to improve the refractory materials will lead directly to increased gasifier availability.

Some typical causes of refractory failure include:

Conventional (left) and phosphate-modified (right) chrome oxide refractory materials after rotary slag testing. New materials research and development, like this improved refractory material aims to improve gasifier availability. (Source)

Slag is fluidized ash and occurs in gasifiers where the operating temperature is above the ash fusion point. Slagging gasifiers are associated with very extreme conditions; refractory materials must withstand:

The dissolution of the refractory material begins another process of refractory failure: spalling. Spalling is the flaking away of the refractory material. The slag penetrates the refractory material, weakening it and causing significant material loss. Spalling shortens refractory life even more than chemical corrosion.

Chromium oxide (Cr2O3) is a commonly used refractory material because of its ability to withstand extremely high temperatures and relatively slow rate of inevitable dissolution (chemical corrosion) from the molten slag. However, improvement is still important towards gasifier optimization.

Spalling in Cr2O3 refractories is a major issue. Fuel flexibility can sometimes necessitate a different refractory material because Cr2O3 may not be suitable with ash and slag that is high in alkalis and alkaline earths. Other industries have methods for repairing refractories to extend life and increase availability that do not work with Cr2O3 refractories. In addition, suppliers of high Cr2O3 refractories are dwindling, making an already expensive and difficult to produce product even more expensive. Therefore, research on novel materials for use in refractories is being done to improve plant availability.

Some of these novel materials are described below. More information can be found in the NETL presentation, Refractory Materials for Slagging Gasifiers [PDF].

Phosphate Modified Cr2O3

Aurex 95P (NETL-patented refractory material, Press Release)

Failure of non-Cr2O3 refractories is expected to be similar to Cr2O3 refractories: dissolution and reaction with slag. Thermodynamics indicates that few materials will be as chemically stable as Cr2O3, but depending on ash chemistries, refractories of ZrO2 or Al2O3/MgO have potential. These are currently undergoing laboratory testing and scale-up for further testing.

Latest NETL-sponsored research aimed at refractory improvement is focused on refractory development as discussed above, and the impact of additives on carbon feedstock ash behavior and refractory wear. Refractory service life improvement through material development or modeling and control of slag chemistry will be evaluated via laboratory testing at NETL and through the cooperation of industry gasifier operators. The goal is to develop a slag model that allows operators to control slag viscosity, maximize refractory service life, and minimize downstream material issues like syngas fouling. It is expected this information will become part of an NETL developed database, allowing predictive modeling of coal and/or petcoke slags in gasifiers.

To maintain control of a gasifier under changing conditions—which allows for optimized performance—knowledge of key system parameters, pressure or temperature for example, must be available. Thermocouples are one of the most common methods of providing near real-time temperature data, but they have limited durability and can fail early in start-up. Replacement requires the gasifier to be shut down. A functioning thermocouple allows for greater control of the gasifier, which increases reliability and performance; a failed thermocouple decreases availability. These factors prompt research and development into extending the life of gasifier thermocouples.

Causes of thermocouple failure include:

Thermocouple protection system for gasifiers. (Source)

Several of these problems (e.g. slag) are possibly related to refractory issues.

NETL research has resulted in recommendation of a thermocouple fabrication procedure to reduce fabrication defects and has developed a filler material to reduce slag penetration. Improvements in well blocks in the refractory material could also better protect the thermocouples. In addition, thermocouples will be tracked in regard to cause and frequency of failure for better understanding. See the NETL presentation, Refractory Materials for Slagging Gasifiers for more information.

Accurate physical models are currently unavailable for predicting rates of slag buildup in a gasifier and ash deposition in a syngas convective cooler. Demonstration plants have experienced problems such as lower carbon conversion and plugging of syngas coolers with fly ash. These issues reduce plant heat rate, place additional strain on solids handling and grey water circuits, and reduce the overall reliability of the gasifier. Research in this area will focus on establishing how iron and vanadium oxide content, as well as oxidation state, impact slag viscosity and is aimed at negating plugging and fouling throughout the syngas cooling system. It will lead to increased capabilities for predicting gasifier performance and further determine the impacts and limits of using various coal grades and petcoke in entrained gasifiers. In addition, development of computational simulation tools will reduce uncertainty associated with the use of low-rank coal and mixed feeds. Results of this work will aid in gasifier design and performance relative to the use of these fuels, allowing for fuel flexibility in current and future gasification facilities.

Transport Reactor Integrated Gasification (TRIG™), originally developed by Kellogg, Brown, and Root (KBR) based on the company's fluidized catalytic cracking technology, has been enhanced through extensive testing by Southern Company at the Power Systems Development Facility in cooperation with NETL. Testing corroborated that the gasifier effectively handles low-rank coals (e.g., Powder River Basin lignite), which account for half of the worldwide coal reserves but are often considered uneconomic as energy sources due to high moisture and ash contents.

Research in this area has focused on the development of gasification performance prediction models to reduce uncertainties associated with the use of low-rank coals and co-feeds, including biomass. The development of a hierarchy of co-feed TRIG models with uncertainty quantification provides a practical framework for quantifying various types of uncertainties and assessing the impact of their propagation through computer models of the physical system. Reducing the uncertainty of using these feeds enables gasification designers and operators, leading to greater use of these resources.

Dry coal feeding, which would allow efficient use of low-rank high moisture coals like lignite in high pressure gasification, is among the most important recent examples of NETL-supported R&D in the area of low-rank coal optimization.

Dynamic simulation at NETL's AVESTAR Center is one of the highly specialized and increasingly sophisticated computational tools that will be used by the Gasification Team to help achieve operation and control objectives for gasification and carbon dioxide (CO2) capture technologies. Key technical challenges and gaps for developing plant-wide, pressure-driven dynamic models and corresponding control strategies for integrated gasification combined cycle (IGCC) systems with CO2 capture will be addressed. Research efforts will focus on developing a comprehensive portfolio of dynamic simulation research, development, and training activities, including optimizing the operation and control of advanced gasification-based energy plants with carbon capture. AVESTAR will be instrumental in preparing an industry workforce trained to safely and effectively operate, control, and manage commercial-scale IGCC systems with CO2 capture.

DOE/NETL co-funded large-scale demonstrations of clean coal technologies have played an important role in bringing gasification into the commercial marketplace. Tampa Electric's IGCC plant and Wabash River are notable examples of successes of the supported demonstrations R&D efforts. These continue in current projects, which are described at the DOE Coal & Power Systems Major Demonstrations homepage.

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