Fluidised combustion

The essential features of a fluidised bed reactor is that the solids are held in suspension by the upward flow of the reacting fluid this promotes high mass and heat-transfer rates and good mixing. Heat-transfer coefficients in the order of 200 W/m2oC to jackets and internal coils are typically obtained. The solids may be a catalyst a reactant in fluidised combustion processes or an inert powder, added to promote heat transfer. 

INSTITUTE OF Fuel Symposium Series No. 1 Fluidised Combustion 1 (1975). 

Technique Multiple hearth furnace (500 °C) with sieving and classification Fluidised bed furnace (780 X ), mechanical size reduction and dedusting Fluidising combustion chamber with magnetic preseparation, simultaneous processing of sand and dust fiom mechanical furan sand regeneration... 

Thermal treatment average airflow (fluidisation + combustion) (NmVh) 700 1200 3000... 

Formation of emissions from fluidised-bed combustion is considerably different from that associated with grate-fired systems. Flyash generation is a design parameter, and typically >90% of all soHds are removed from the system as flyash. SO2 and HCl are controlled by reactions with calcium in the bed, where the lime-stone fed to the bed first calcines to CaO and CO2, and then the lime reacts with sulfur dioxide and oxygen, or with hydrogen chloride, to form calcium sulfate and calcium chloride, respectively. SO2 and HCl capture rates of 70—90% are readily achieved with fluidi2ed beds. The limestone in the bed plus the very low combustion temperatures inhibit conversion of fuel N to NO. ... 

Combustion of coal may take place in conventional fixed beds using lump coal and in which temperatures up to 1 300°C may be reached by entrained flow in which pulverised coal is injected into the combustion zone with the air, reaching temperatures up to 1 500°C or in the more recently developed systems of fluidised-bed combustion, again using pulverised coal but with... 

Fluidised-bed systems produce higher combustion intensities at lower temperatures than combustion of pulverised fuel in conventional fossil-fuel-fired boilers. The mineral matter for corrosion does not form fused salts and is not expected to release corrosive species. Fluidised bed combustors can, therefore, burn lower grade, cheaper fuel in smaller plant with better pollution control than traditional boilers... 

Minchener el al. report that the bubble phase of atmospheric fluidised bed combustion has a pOi in the range 2 x 10 to 2 x 10 Combustion in the dense phase is sub-stoichiometric, with the /Oj as low as 10 and SO2 and SO3 present in the range 500-5 000ppm. Low Cr-Mo steels show heavy scaling in these conditions, whereas 9-12% Cr steels show good resistance to sulphidation up to 650°C. Roberts et however, report that for pressurised fluidised-bed combustion, ferritic steels at or below 9% Cr show heavy general corrosion above 540-560°C. 

In Fig. 7.68 the oxidising and sulphiding potentials of four different atmospheric environments, i.e. conventional coal combustion (A), fluidised bed combustion (B), conventional coal gasification (C) and coal gasification using nuclear heat (D), are shown on the thermochemical phase stability... 

This paper introduces the Twin-Interchanging Fluidised Bed Incinerator (TIF) from EBARA Corp. of Japan, and describes a combustion test carried out by the company in collaboration with the Plastic Waste Management Institute, on waste plastic separated from municipal refuse, verifying the level of non-polluting combustion and high-efliciency energy recovery. The results of the test are presented, with considerations and conclusions. JAPAN... 

This paper demonstrates the technical feasibility of a plastics energy recovery plant using circulating fluidised bed technology from Ahlstrom of Finland. Full details are given of a two-phase test run conducted at Ahlstrom s pilot plant in Karhula, in order to obtain information on the process behaviour when combusting different types of plastics waste. Results are presented and conclusions drawn. 

The use of plastics as an energy source was demonstrated on a commercial scale at ICl Materials plastics manufacturing site in Dumfries, UK. This paper covers the preparation and use of pre- and post-consumer plastics as supplementary fuels in a circulating fluidised bed boiler specially designed for co-combustion with coal. Full emissions data on the 15% mixtures of individual plastics with coal are given, together with calculations of thermal efficiencies. Measurements by an independent body (British Coal Research Establishment) confirmed that the co-combustion of coal and plastic reduces some emissions compared with coal alone. Thermal efficiencies of around 80% were achieved and this heat was used effectively during the production of plastics. 7 refs. 

Details are given of the development of energy and material recycling processes for thermosetting polymer composites. Applications in the cement industry and in coal fired fluidised bed combustion plants are discussed. 3 refs. 

Energy recovery from packaging waste is discussed, with particular reference to the co-combustion of mixed plastics with other conventional fuels such as wood, coal and peat. Experimental work is described in which a project was established to evaluate the possibility of energy recovery from a circulating fluidised bed boiler using packaging from different sources as fuel. The role of sulphur in the formation of PCDD/F in the combustion process was also studied. 

Mixed plastics waste appears to be well suited for use in energy recovery, either as a co-eombustion fuel in a power plant designed for solid fuels, or as the sole fuel in speeially designed plants. This paper reports test results on the co-combustion of mixed household plasties with eoal. The tests were performed in a bubbling fluidised bed low-pressure steam boiler. The results show that both inorganic and organic total specific emissions were lower for mixed household plasties than for coal. Tabulated data are presented. 3 refs. 

CLC consists of two fluidised bed reactors, namely reducer and oxidiser. In the reducer reactor, fuel is fed along with the metal oxide containing oxygen, which is transferred from the metal oxide to the reactor as the combustion occurs (Figure 7). A flue gas containing over 99%v/v of C02 can be obtained by a simply condensation stage because of the fact that the exhaust gas at the reducer outlet is primarily formed by C02 and water vapour. This stream is then sent to further compression and permanent storage. 

Lind, Terttaliisa. Ash formation in circulating fluidised bed combustion of coal and solid biomass. 1999. 79 p. + app. 88 p. 

The effect of pressure on the behaviour of the bed is important because many industrial processes, including fluidised bed combustion which is discussed in Section 6.8.4., are... 

An important application of fluidisation which has attracted considerable interest in recent years is fluidised bed combustion. The combustible material is held in a fluidised bed of inert material and the air for combustion is the fluidising gas. The system has been developed for steam raising on a very large scale for electricity generation and for incineration of domestic refuse. 

Much of the basic research, development studies and design features of large-scale fluidised bed combustors is discussed in the Proceedings of the Symposium on Fluidised Bed Combustion, organised by the Institute of Fuel as long ago as 1975 134 and subsequently in the literature 135-139). Pilot scale furnaces with ratings up to 0.5 MW have been operated and large-scale furnaces have outputs of up to 30 MW. 

One of the major advantages of fluidised bed combustion of coal is that it is possible to absorb the sulphur dioxide formed. Generally limestone or dolomite is added and thus breaks up in the bed to yield calcium oxide or magnesium and calcium oxide, which then react with the sulphur dioxide as follows ... 

It appears likely that fluidised bed combustion of coal may, in the near future, be one of the most important applications of fluidised systems and it may well be that many new coal-fired generating stations will incorporate fluidised bed combustors. 

A semi-industrial pilot plant has been developed in which air-borne ultrasound has been applied to the reduction of particle emissions in coal combustion fumes [62]. The installation basically consists of an acoustic agglomeration chamber with a rectangular cross-section, driven by four high-power and highly directional acoustic transducers operating at 10 and/or 20 kHz, and an electrostatic precipitator (ESP). In the experiments, a fluidised bed coal combustor was used as fume generator with fume flow rates up to about 2000 m /h, gas temperatures of about 150 °C. and mass concentrations in the range 1-5 gm. The acoustic filter reduced fine particle emissions by about 40 %. 

As costs of precombustion hydrodesulphurisation and post combustion flue gas clean-up have escalated and as environmental regulations have further limited the sulphur dioxide emission rates, there has been a growing interest in technology designed to effect fuel desulphurisation during the combustion process. Desulphurisation during fluidised bed combustion of coal has been a leading technique in these developments. 

The principle of fluidised bed combustion with simultaneous desulphurisation is based on the thermal decomposition of limestone carbonates to yield oxides which then react with the sulphur oxide products of combustion of both inorganic and organic sulphur compounds in the hydrocarbon fuel. 

Ash related problems such as agglomeration occur, and exchange of the sand bed may be needed after some interval (Bridgwater, 2003). One way to overcome some problems with agglomeration is to use a mixture of biomass and peat in order to increase the melting point of the raw material in the furnace. It is also possible to use lime (contains CaO, Ca, etc.) instead of quartz in the fluidised bed reactor to improve the agglomeration temperature in the combustion atmosphere of some biomass (Natarajan et al., 1998). 

---