With competition and fuel prices on the rise, petroleum refineries continue to look for new ways to burn petroleum coke (a byproduct of the refining process) on-site to produce steam and power. Doing so allows refineries to reduce costs, reduce stress on the local grid, and improve their environmental standing within the community by resolving a difficult waste disposal issue.

From a power generation standpoint, petroleum coke typically contains high sulphur concentrations that require more extensive emissions control equipment. But with its versatility and ability to cost-effectively control emissions, circulating fluid bed (CFB) boiler designs continue to improve the economics of burning petroleum coke.

Formosa Heavy Industries (FHI) and Alstom recently constructed two 150 MWe CFB boilers – one of the largest pet coke-fired CFB plants ever built — at the site of an FHI sister company, the Formosa Petrochemical Corporation’s Mai-Liao, Taiwan, refinery. The refinery’s by-product, petroleum coke, is to be fired as the main fuel, while a second by-product, residual oil, will be co-fired when available.

The pet-coke fired CFB designed for the Mai-Liao refinery, which is located on the southwestern coast of Taiwan, includes features which allow for the chemical and physical nature of the fuel and the combustion and operating characteristics of the CFB process, while also focusing on operational flexibility. The CFB primary loop design, furnace waterwall tube offset, and fuel-specific cyclone/vortex finder/seal pot/refractory lining system designs are incorporated to address these operational and maintenance concerns. In addition, the CFB/selective/FDA combination ensures that Mai-Liao will be one of the world’s lowest emissions solid fuel power plants. As such, the plant represents an example of a steady and growing worldwide trend towards a greater use of relatively inexpensive solid fuels for power production.

A number of innovative design measures have been implemented, including the use of a ground-breaking, state-of-the-art flue gas desulphurisation system, and new, corrosion and erosion-resistant refractory materials to protect key internal components. Upon completion in 2002, the facility will comply with some of the world’s most stringent emissions standards.

The Mai-Liao design

Crude oil contains various impurities, including sulphur and nitrogen compounds, as well as mineral contaminants like vanadium and nickel. Most are concentrated in residual wastes removed from refined products like fuels and lubricants. In recent years, the demand for lighter, low sulphur fuels has increased at a rate that exceeds the corresponding increase in demand for heavier fuel oil fractions.

The Mai-Liao CFB plant is an expansion of an existing industrial complex that includes numerous utility and cogeneration facilities, as well as the Mai-Liao oil refinery and many petrochemical factories.

Two of the project’s primary design considerations involved controlling emissions levels – which will be the lowest ever for this type of fuel – and assuring reasonable operating parameters for all of the key boiler components when burning petroleum coke.

Circulating fluid bed is an excellent technology for this project because of its inherent ability to control nitrogen oxide (NOx) and sulphur dioxide (SOx) emissions. In addition, a flash dryer absorber (FDA) system is fitted to the downstream end of the CFB to further reduce SOx emissions to 50 ppmv at 6 per cent O2. An aqueous ammonia based selective non-catalytic reduction (SNCR) system is included to further reduce NOx emissions to 50 ppmv at 6 per cent O2, with resulting ammonia slip specified to not exceed 5 ppmv at 6 per cent O2. Aqueous ammonia is injected in the cyclone gas outlet ducting, providing good mixing and dispersion of the reagent. The CFB boiler design steam conditions and the plant’s emissions levels are shown in the table.

The CFB primary loop consists of the furnace, cyclone separators, seal pots and associated connecting ducting. These components reflect the flow path of circulating solids in the CFB, forming the basis of the CFB process. The balanced draft, natural circulation waterwall furnace is designed to fire 100 per cent pet coke. But it is also flexible and can fire a 70 per cent/30 per cent mixture of pet coke and residual oil, for example.

The furnace is top-supported and features a refractory-lined lower tapered section that includes a waterwall tube offset at the refractory/waterwall interface to prevent tube erosion. The lower furnace waterwalls are covered with erosion-resistant refractory secured by studs. The furnace floor is covered by erosion-resistant castable refractory. Flue gas and solid particles exit the furnace through openings on the upper rear wall. Solids are returned to the furnace from the cyclones via the lower rear wall openings. Primary air is introduced to the furnace via the fluidisation grate comprised of a grid pattern of Alstom four-jet air fluidising nozzles to fluidise the bed.

Secondary air is introduced into the furnace through upper and lower levels of nozzles located on the front and rear furnace walls. All secondary air nozzles are furnished with automatic butterfly control dampers to maintain a steady secondary air supply duct pressure. The secondary air arrangement is staged for NOx control.

Additional evaporative heat duty is performed using in-furnace evaporator pendant panels. In-furnace superheater pendant panels are also included due to design considerations relating to the fuel fired and the steam temperature control range. The lower evaporator and superheater panels are covered with erosion resistant refractory secured by studs, similar to the furnace waterwalls.

Two refractory-lined steel plate cyclone separators — which receive the flue gas and solids from the furnace — remove almost all the entrained solids from the flue gas and return them to the furnace via the seal pots. The seal pots create a gas seal from positive pressure in the furnace to the negative pressure in the cyclone. The base of the seal pots includes a grid pattern of fluidising nozzles similar to the furnace.

The cyclones, seal pots and solids return ducts have been designed to minimise the potential for agglomerations. High cyclone collection efficiency and relatively high quantities of fluidising air and transport air have been incorporated in order to keep solids sufficiently agitated and free flowing.

Special refractories

The refractory products in use at Mai-Liao are comprised of high purity aggregates and corrosion resistant binding systems. With proper installation and curing, the selected refractory materials are expected to produce durable, high strength lining systems that are highly resistant to corrosion, erosion, and/or cracking.

For example, the cyclone design includes “enhanced target zones” where extreme duty requires extra erosion resistant materials and streamlined profiles in locations prone to erosion.

The service linings of the tangential inlet walls and target zones of the cyclone barrel are made up of an extra thick, extra-erosion resistant high alumina firebrick. The roof line just above the target zones, where solids flow is concentrated, is also enhanced with extra-erosion resistant, monolithic materials. The balance of the cyclone, hot ash ducts, and hot gas ducts are lined with high purity block insulations, insulating firebricks, insulating castables, erosion resistant high alumina bricks, and erosion resistant refractory monolithics.

SO2 removal

Another significant design feature includes the use of Alstom’s flash dryer absorber (FDA) system, which integrates several flue gas desulphurisation functions into one unit to achieve 90 per cent SO2 removal or better, irrespective of the sulphur content of the fuel.

Due to the stringent demands on the Mai-Liao plant’s sulphur dioxide emissions (200 ppm at 6 per cent O2 leaving the boiler and 50 ppm at 6 percent O2 leaving the stack) mandated at the outset, the FDA’s second-stage approach for sulphur removal was selected. First, limestone added to the CFB boiler will remove the bulk of the sulphur during combustion. Then, a secondary “polishing” step will follow – the first petroleum coke-application of the FDA system – to further reduce the plant’s SO2 emissions.

A unique feature of the FDA system is its ability to use ash (which contains surplus lime) supplied by the CFB (via the flue gas stream) for further sulphur dioxide absorption downstream.The Mai-Liao system comprises the patented FDA reactor followed by a fabric filter. The flue gas is fed into two FDA reactors (per-unit), where it is mixed with a wetted but free-flowing dust consisting of lime and recycled fly ash from the fabric filter. From the FDA reactor, the gas and dust mixture is carried to the fabric filter. Fly ash is recycled from the fabric filter to two mixers, where water and reagent are added. The water addition activates the lime content in fly ash coming from the boiler.

Separately, fresh reagent is pre-reacted with water in a dry lime hydrator, prior to being injected into the mixer. The separately added reagent is commercial quicklime, CaO. In the FDA reactor, water evaporates from the dust particles, lowering the flue gas temperature down to a level suitable for the absorption of sulphur dioxide.

The acid gas components in the flue gases react with the lime during intense contact in the reactor and in the fabric filter. The dust, with its reactive components and captured sulphur, is collected in the fabric filter. The dust collected in the fabric filter falls into two hoppers, where it is recycled to the mixers. The end product is discharged from the filter hopper and transported to a silo via a dense phase pneumatic conveying system.

The FDA system will also reduce acid halogens (HCl, HF) and sulphuric acid mist by at least 95 per cent and suppress emissions of trace elements, especially elements condensing at lower temperatures, like selenium, arsenic and cadmium. It is also expected that mercury will be reduced as it becomes absorbed on unburned ash particles at the low temperature prevailing in the FDA system.

The key parameter to be controlled in any dry FGD process is the humidity of the flue gas in the reaction zone, in this case the reactor, and the subsequent dust collector. At a relative humidity of 40 to 50 per cent, the hydrated lime is activated and readily absorbs SO2. The relative humidity of the flue gas is increased by injecting water into the flue gas.

  In a conventional dry flue gas desulphurisation process, water and lime are supplied to the flue gas as a slurry (with or without recycle) with a solids content of 35 to 50 per cent. In the FDA, the same amount of water is injected into the flue gas, but it is distributed onto the surface of dust particles with a water content of only a few percent. As a result, the amount of absorbent (which is recycled) is much greater than in a conventional dry FGD process.

This means that the surface area available for evaporation is much larger than the surface area of a conventional dry FGD. Thus, the time required to dry the dust added to the flue gas is short, which in turn makes it possible to use small reactor vessels. In fact, the volume is an order of magnitude less than the corresponding size for a conventional dry flue gas cleaning system based on spray dryer technology.

The resulting increase in relative flue gas humidity is sufficient to activate the lime for SO2 absorption at operating temperatures typical to a dry FGD: 10 to 20°C above saturation and, in practice, within the temperature range of 65 to 75°C. The activation is considered to proceed fast enough for the SO2 absorption reaction when a layer of one to two water molecules has been formed on the surface of the lime.

Water is added to the absorbent in a mixer prior to its introduction into the flue gas. A key feature of the FDA is that all recycled absorbent is subject to wetting in the mixer, which maximizes the use of the recycled absorbent. After the activation/drying step, the dried recycle dust is separated from the flue gas in a highly efficient dust collector. The separated dust is then again fed to the mixer, with make up lime added. Water is fed to the mixer in a quantity sufficient to maintain a constant outlet relative humidity.

The control system uses a feed forward signal with back trim, based on the inlet and outlet flue gas temperatures supplemented by a signal indicating the gas flow. The outlet SO2 concentration is controlled in a similar way; the inlet and outlet SO2 concentrations, plus the flue gas flow, determines the rate of lime flow into the system.

Other benefits of FDA

The FDA system was evaluated favorably when compared with both conventional dry FGD and wet FGD alternatives. Compared with conventional dry FGD systems equipped with rotary atomisers or dual fluid nozzles, the FDA process minimises the need for sophisticated and/or special equipment. There is no rotary atomiser with its high-speed machinery, nor are there any dual fluid nozzles requiring compressed air. Power requirements for the FDA recycle/reagent mixers are much lower than for the corresponding items in a conventional dry FGD system.

The FDA system was also evaluated favorably with respect to its ease of maintenance. All equipment requiring operator attention is placed near ground level, in an enclosure common with the fabric filter. And a comparison with a wet FGD system showed lower costs for the FDA since a wet system would require a more expensive wet stack and a separate dust collector (ESP or FF) for fly ash removal.

Full-scale optimisation efforts are to take place after start-up of the Mai-Liao CFB boilers in 2002. The starting point for this work is to increase the removal efficiency of the FDA beyond the base value of 75 per cent — at a lower than expected limestone flow to the CFB boiler — while still maintaining the combined system’s total required sulphur retention.

The high sulphur content of the Mai-Liao fuel makes it particularly well-suited for the CFB/FDA application. For example, pilot tests have shown limestone costs can be reduced by 10 to 30 per cent (by reducing the amount of limestone added to the CFB) while at the same time reducing the amount of disposable ash. When the first Mai-Liao CFB is placed into service, the practical limits for this technology will be investigated further.

CFB steam conditions and project emissions levels at Mai Liao