Is there a moving bed system in your future?; Alstom is developing a circulating moving bed combustion system that could make boilers of the future--even those firing low-grade solid fuels--less expensive, cleaner, and more efficient. Although a utility-scale version of the system is still as much as a decade away, the results of first tests of the concept are tantalizing.

Power, Nov/Dec 2003 by Marion, John L, Jukkola, Glen D

In the U.S., coal-fired power plants more than 20 years old account for 286 GW of generating capacity, or about 85% of the nation's total installed coal-fired capacity of 337 GW. The need to replace and repower all this aging capacity represents a tremendous opportunity for advanced combustion technologies capable of meeting the power industry's fast-changing financial and environmental targets.

Paris-based Alstom is developing a novel circulating moving bed (CMB) combustion system whose early specs promise significant capital, operating, and environmental cost savings to generators (Table 1). The CMB project--partially funded by the U.S. Department of Energy (DOE)--borrows heavily from traditional circulating fluidized bed (CFB) technology but uses an innovative method of solid fuel combustion and heat transfer. Alstom recently completed Phase I of the CMB project--proving the concept--at its multi-use combustion test facility in Windsor, Conn. The goal of this phase of the project was to identify the technical, design, and performance challenges that must be met before moving on to the next phases--scaling up the design, operating a pilot plant, and demonstrating the technology at full scale.

How it works

The key innovation of the CMB system is its use of a heat exchanger to heat the energy cycle's working fluid (steam or compressed air) to much higher temperatures than those of today's combustion systems--temperatures that will become commonplace in the next generation of advanced power generation systems. Alstom believes this will produce a step change in both boiler performance and capital costs (Table 2) relative to today's pulverized coal (PC) and CFB boiler designs.

In the CMB combustion system, two chambers separate the combustion process from heat transfer surfaces (Figure 1). The upper chamber, or combustor, consists of two zones. Coal or another solid fuel--pet coke, anthracite, sludge, wood waste, or biomass--is fed into a high-velocity bubbling bed in the lower zone, where combustion temperatures may approach 2,000F--higher than the 1,550F to 1,650F generated in traditional CFB designs.

Above the bubbling bed, in the upper zone, a reactor with a relatively long residence time transfers the heat of combustion--now in the gas flowing up from the bubbling bed--to high-density solid particles flowing downward. These particles are made of bauxite, because it is dense, chemically inert, and readily available.

The hotter the better

Once the solids have recovered the heat of combustion, they collect at the bottom of the combustor, where they are fluidized and transferred to the lower chamber through standpipes (Figure 2). Inside the lower chamber is another Alstom innovation--a counterflow, direct contact, "moving bed" heat exchanger (MBHE) (Figure 3). The MBHE transfers the heat of the low-velocity bauxite particles to the tubes carrying the working fluid through a series of finned tubes (Figure 4). In the CMB system, the nearly 2,000F temperature of combustion makes it possible to raise the temperature of the working fluid to as high as 1,750F.

The better system efficiency that these higher temperatures make possible is hardly the only advantage of the CMB design. Two others are compact size and low cost. In the MBHE, the bauxite particles encounter an environment largely free of corrosion, erosion, and plugging, and this allows the use of finned heat transfer tubes. Because finned tubes can transfer five times as much energy per linear foot as conventional boiler tubes, they can be small, reducing the footprint, weight, and capital cost of the overall system.

Yet another benefit of the CMB system's high combustion temperature and efficiency is the virtual elimination of N2O emissions. As in a conventional CFB design, the lower combustor is staged for NOX control, while flyash entrained in the flue gas is captured in a low-temperature cyclone and recycled back to the high-temperature lower combustor to reduce carbon loss. SO2 emissions are expected to be controlled primarily by a back-end cleanup system such as Alstom's Flash Dryer Absorber (FDA), which uses limestone calcined in the combustor for increased sulfur removal.

Finally, the CMB system features lower pressure drops on the working fluid side of the cycle. In addition to improving system efficiency, these lower drops could, for example, make the path length of the feedwater/steam-finned circuit in the heat exchangers 20% shorter than that of a typical PC or CFB design.

Solid performer

After surrendering their heat in the MBHE, the cooled bauxite particles are transported back to the top of the combustor to start the cycle again. This solids recirculation (or thermal looping) process within the CMB holds promise as a building block for advanced power generation processes, including chemical looping combustion and gasification. Chemical looping is an emerging concept that may, in time, develop into the foundation of (1) coal-fired power plants with near-zero CO2 emissions and system efficiencies greater than 60% and (2) hydrogen production plants. The technology is based on the oxidation, reduction, carbonation, and calcination of calcium-based compounds to chemically react with coal in two chemical loops and one thermal loop.