CFB Technologies for Combustion of Difficult Fuels Abstract In developing power markets economics are driving the utilisation of lower cost fuels that are challenging for conventional combustion systems. Whereas the application of pulverised coal technology for low grade fuels is limited to coals with higher calorific values and fusion temperatures, Circulating Fluidised Bed Technologies offers rather flexible combustion conditions and control which allows the application for a wide range of low grade fuels. The portfolio of Doosan covers the whole range of potential fuel based combustion applications. In addition, the company owns all technologies needed to comply with even strict environmental limitations as several processes to reduce sulphur dioxides, to limit the formation of NOx and the emission of particulates. Furthermore, steam turbine manufacturing is a part of the product portfolio allowing the group to provide a full power island to interested customers. A short survey on such range will be given and a more detailed presentation on the approach towards handling low grade fuels in a CFB meeting customer requirements. For the very low end of such fuels even the conventional CFB Technology requires a specific adaptation which has been developed by Doosan Lentjes on the basis of experienced boiler concepts. Often coals at this low end do not only show challenging combustion properties but as well, due to high moisture and clay content, mechanical properties challenging storage, transport and feeding of the coal. Doosan Lentjes has developed a concept tailor designed to all these properties.

Challenges of low grade coal Countries like Turkey, Indonesia and Philippines do have huge amounts of indigenous low grade coals. In well known “coal “ countries huge amounts of relatively high calorific washer residues- discard coals- arose in the past and are still arising even though at dropping calorific values. The usage of such low grade coals is very depending on applicable combustion technologies matching International standards for reliable and clean energy utilization. If this can be met, those countries can benefit manifold from the use of its domestic fuels, e.g. employing people, reducing its dependence from importing fuel, increasing energy independence and numerous others. Thus installing a environmentally compliant technology for the use of coal is essential.

Unfortunately such low grade coals usually show challenging properties to mechanical handling and combustion so that the applicability of conventional pulverized coal (PC) combustion technologies is very limited.

The following tables 1 and 2 show on examples of typical coal composition the key properties impacting the combustion and hence the applicability of technologies.

The challenging properties of those lignites can be summarised as follows: • Low calorific value -difficult combustion • High Sulphur content -high desulphurization efficiency required • High Moisture content -difficult mechanical handling- storage, feeding,

conveying, difficult combustion • High ash content: high ash/ dust flow, -ash cooling, high thermal losses • Low agglomeration/ slagging temperature -sintering, slagging, fouling • Clay content -difficult mechanical handling and fouling • Partly high elevation -low ambient pressure

Dr.Hans Piechura Doosan Lentjes GmbH Germany

Dr.George von Wedel Doosan Lentjes GmbH Germany

1

Helpful properties: • Inherent content of lime stone • high volatile matter- high reactivity Even if the high site elevation is not a coal property, it is typical for areas like South Africa/ Mpumalanga and Turkey/ Anatolia where coal sites are at elevations between 1200- 1600 m a.SL and will influence significantly the design of a combustion plant.

Table 1 Analysis of typical low grade lignites

Table 2 Thermal properties of typical low grade coals

Origin of Lignite Kangal Tufanbeyli Afsin Cankiri Riau Seival*

Ultim.Analysis a.r.

Carbon [m-%] 17 16,8 17,08 34 27,3 32,2Hydrogen [m-%] 1,2 1,6 1,33 3,26 2,1 2,3Oxygen [m-%] 10,2 7.295 8,4 14,73 9,5 6,9Nitrogen [m-%] 0,5 0.50 0,4 1,05 0,3 0,5Sulphur [m-%] 1,8 1,2 2 0,91 0,1 1,63Chlorine [m-%] 0Fluorine [m-%] 0Ash [m-%] 22,7 24.48 19,13 45,44 7,5 56,6Moisture [m-%] 46,6 48,2 53,13 50,6 53,3 20LHV [kJ/kg] 4.676 5100 4650 4800 2530 11700Volatiles [m-%] 19 17,22 21,8 20

Origin of Lignite Bosnia Kangal Tufanbeyli Afsin Cankiri Riau Seival*Ash Fus.(ox.atm.)initial deformation °C 1165-1420 1180 1400Sintering °C 805 1166-1386Softening °C 1270 1195-1490 1120-1268 1194-1511 1050-1150 1450hemispherical °c 1320 1196-1289 1270-1400 1220 1480Flow °C 1360 1207-1542 1260 1500Melting °C 1215-1500 1222-1326 1203-1315 1300-1420

Ash Fus.(red.atm.)initial deformation °C 1090 1120-1440Sintering °C 910 1090Softening °C 1200 1280-1590hemispherical °c 1280 1100 1300-1600Flow °C 1320 1100 1360-1600Melting °C

2

Application of PC combustion Even if lignite coals are highly reactive due to the high volatile matter, the high moisture content and the low combustion temperature resulting from the low calorific value, requires a high residence time for combustion in PC boilers. More over lignites with low ash fusion temperatures may cause serious slagging and fouling of boiler heat surfaces due to temperature peaks in the burner flame area and in areas of inhomogeneous flow and combustion.

The application of PC combustion technologies for low grade coals is additionally difficult in case of:

• high sulphur content - requires external and separate desulphurization • clay content - may seriously disturb pre- drying and burner operation • high ash content - results in rapid fouling of heat surfaces and the respective

need of boiler cleaning and respective design • low fusion temperature - results in rapid fouling and slagging of boiler

PC combustion technology therefore is only applicable if the calorific value and hence the combustion behavior will be improved e.g. by a form of pre- drying- what is economically difficult and for high quantities not state of the art. Additionally the applicability of the PC Technology is not recommendable for low fusion ashes- even though it has been done repeatedly- the experienced fouling and slagging in the boiler seriously impacts the availability and the resulting cleaning, repair and outage is rather expensive.

Application of CFB Combustion Technology The Doosan CFB Combustion Technology is designed to cope with the challenging properties as demonstrated above. It has been adapted for a broad range of fuels as can be seen in Table 3 and has been applied to more than 100 boiler lines in operation showing capacities up to 280 MWe. The following diagram shows the range of applied capacities and higher capacity concepts under development.

3

The different concepts with 2 to 6 cyclones are the result of the cyclone volume flows resulting in still experienced sizes of the cyclones. An additional complication results from the high elevations of local mines as mentioned above. At such levels, the increased volumetric flows are responsible for increased cyclone diameters. This implies that basically boilers for such coals are comparable large in relation to their power output.

Development of the Lurgi CFB-Process The CFB process offers, as known, the significant advantage that today’s emission limits can be met in most cases without any additional (external) equipment as

Sulphur capture is done in the system utilizing the inherent limestone in lignite and/or by externally adding additional limestone.

The formation of nitrogen oxides are inhibited due to a staged air supply in the combustion section and the moderate combustion temperature of around 850 °C.

In addition, the characteristics of the CFB technology allow for a wide fuel range of changes in the fuel composition. This is especially true for the technology offered by us due to the use of the external Fluidized Bed Heat Exchanger (FBHE) which is often characterized as a “variable heat transfer surface” and which will be explained later.

Over the years this CFB system, originally developed by Lurgi and today marketed by its successor company Doosan Lentjes, has been developed and improved with respect to fuel requirement as well as with respect to optimisation of equipment, its detailed design in specific areas and the arrangement of the equipment, features as will be highlighted as follows.

Diagram 1 Doosan Lentjes’ CFB Combustion Flow Diagram

Fluid Bed Heat Exchanger One of the most important features of the Lurgi CFB system is the Fluidised Bed Heat Exchanger (FBHE), equipment allowing heat transfer from hot ash to the water steam side. The main advantages are

constant combustion temperature at optimum level, even over a wide load range, resulting in

• suppressing thermal NOx formation

• minimising limestone consumption for desulphurization

• achieving good carbon burn-out performance over the full load range

• avoiding sintering, agglomeration or even melting of ash at low fusion temperatures

4

high fuel flexibility, i. e. use of a wide range of fuels in individual CFB boilers while showing all features mentioned above – cf. Figure 2.

high efficiency heat transfer in a none erosive atmosphere.

the option to chose the optimum position of heat transfer areas with respect to part load requirements,

economic aspects due to minimising heat transfer areas,

varying fuel qualities during boiler operation.

the possibility to place high temperature super heater surfaces into the FBHE rather than into the corrosive flue gas atmosphere in case of burning high chlorine containing fuels, thus avoiding high temperature chlorine corrosion

no additional heat transfer panels in the erosive atmosphere of the combustor

no necessity for flue gas recirculation as measure for temperature control, thus minimising the plant’s physical size

Quick and easy response to operational requirements and simple control of the combustor temperature is possible just by varying the ash flow by means of the Spiess valve- a heavy duty equipment with a history of successful application of more than 30 years, coming up in roasting and calcining plants.

Over the years the design of the FBHE has undergone significant face lifting which may be applied based on the fuel and the specific plant requirements. Were the FBHEs in the firsts units designed as metal casings with a thick multiple layer refractory lining, the casings of the next generation were already formed by membrane walls, now requiring only a thin refractory lining, far simpler to erect and to maintain. These FBHEs, however, were still separate boxes standing or hanging aside the CFB combustor, requiring numerous down comers and risers as the connection to the water / steam system. The following logical step has been executed first in the two Texas New Mexico plants (175 MWe each) in 1990: the FBHE casings were integrated into the water wall design of the combustor bottom part.

Mechanical fuel handling As mentioned above the mechanical properties of low grade coals may cause handling problems especially due to the high moisture content and in some cases clay content resulting in sticky behaviour. For such fuels conventional silo design, discharge and transport feeding is not always applicable but has to be designed according to the specific properties of the fuels i.e. Flow ability = f(σ1,σc, moisture) Time consolidation = f(σ1,moisture, t) Wall friction(φx, adhesion, moisture)

σ1= compression strength σc= pressure resistance t = storage time φx = angel of wall friction

Such properties have to be determined case by case on samples representing the required range of fuel properties to properly design the fuel storage silo and the discharge, transport and feeding systems; for high moisture contents pneumatic feeding may not be appropriate. 5

Fuel References Low quality fuels are generally the preferred feedstock for CFB units, especially fossil fuels with high ash, high sulphur, and low volatile contents, thus low reactive fuels with low heating values, but also wood, wood waste, bark, sludges and other waste materials, i.e. fuels, which cannot be handled at all in conventional units or at high additional costs only. The following table shows the variation of fuels which can be used in CFB boilers, sometimes some of these fuels have been burnt in a single unit only demonstrating the inherent fuel flexibility of this design.

Table 3 Range of fuels experienced at CFB Boiler based on Doosan Lentjes Technology

The very low end of coals As has already been mentioned above the very low end of coals need even to adapt the flexible CFB concept described above.

To burn lignite with a lower heating value of about 1000 kcal/kg only is – as proven – possible. However, most of the CFB boilers recently awarded account for a significantly higher value which requires less adoption of the CFB technology to meet owners and environmental requirements.

What is the Doosan Lentjes approach? Recognizing that the lignite combustion shall fulfil the following conditions:

safe combustion on lignite without supporting fuel during a wide load range

environmental compliance, preferably without add-on gas cleaning equipment

optimisation of carbon burnout and efficiency

a load pattern to follow grid requirements e.g. allowing for 60 % operation without supporting fuel

easy and reliable operation

Based on such conditions and knowing about the need of maintaining a temperature level in the combustion system to fulfil above requirements DL has looked to past experience and looked into where CFB came from: the combustion of much worse fuel then generally applied today. Initial concepts have

6

clearly distinguished between combustion and heat removal in the combustions systems. Thus the proposed configuration today accounts for:

an almost adiabatic combustion chamber allowing to maintain a constant optimal temperature level of 850 °C -or even lower, depending on the calorific value and the fusion behaviour of the ash- over the required load range due to the “variable heating surface” of the FBHE. Such combustion chamber allows for optimisation of sulphur capture, nitrogen oxide inhibition and carbon burn out without taking care of any needs to meet heat removal conditions as in conventional systems

A fully separate system to remove the heat, seen as a heat recovery section as a second pass including heating surfaces as all economizer and all evaporator bundles and part of super heater and reheat bundles.

Final superheating and final reheat will be accomplished in the FBHE allowing to minimize any spray as temperature control is done by variation of the ash flows across the FBHE

Basic idea behind such approach is to take care of the low adiabatic combustion temperature of such lignite in the range of 1.100 °C compared to more conventional coal at 1.600 °C. Such low adiabatic temperature requires to keep the heat in the combustion system during the combustion process and to delay the heat removal after the flue gas has left the combustion environment. By means of the FBHE, the required temperature level can be effectively controlled by the ash flow over a broad load range without any additional complication of the combustions process i.e. maintaining the target temperature of approx. 850 °C over the full load range and thus avoiding any thermal cycling of the internals.

Furthermore, such approach allows for rather high steam conditions as there is strict split between the combustion process and the heat pickup allowing for an optimization of either system, e.g. going for higher steam conditions in the heat recovery section without concerns on e.g. water circulation. This eases the whole design of the boiler.

Typical CFB boiler concept adapted to Turkish, low grade lignite Capacity range: 120– 170MWe LHV: about 4400 kJ/kg Moisture cont.: 40- 50% Elevation: up to 1600m a.S.L.

7

The boiler concept shown above takes all the issues into consideration resulting from the properties of low grade fuels:

Specifically designed storage silos with a mechanical discharge system

Final crushing downstream the daily storage

Adiabatic combustion chamber

Separate boiler pass

Combustor, boiler and cyclones designed to the specific high volumetric flow at such high elevations

Fluidised bed ash coolers allowing extracting the sensible ash heat to the steam water cycle and hence to reduce heat losses and improve the power production efficiency

Gravimetrical fuel feeding

The concept initially was realized in the first CFB plants in Germany, e.g. Lünen and Duisburg. Both of them were supplied with full adiabatic combustion chambers improving the performance for bad coals. Lünen in the meantime is shut down after more than 30 years of lifetime, while the plant in Duisburg continues to operate as a major source for providing district heating to the city and thus requiring some high availability. Duisburg is in operation since more than 20 years based on the proposed concept.

Summary CFB technology is generally well suited to burn a broad variety of different fuels. Adopting such technology to the needs of difficult to burn low grade fuels offers a beneficial chance to burn the big volumes of such fuel to the benefit of the investors. Most important is to understand the unique features of the fuel and to adopt the technology to them, not just to vary existing concepts of the CFB technology to burn the fuel. A tailor designed approach is the solution.

8