HIGH PERFORMANCE BIOMASS BOILERS FOR GREEN ENERGY PRODUCTION
Seppo Hulkkonen, Technology Director, Bioenergy Systems
Timo Kauranen, Vice President, Bioenergy Systems
Utilisation of biomass fuels for green energy production has become an increasingly important issue. Typically, wood biomass in the form of chips, bark and sawdust has been the fuel of choice. More recently, different short rotation crops such as eucalyptus, poplar, and annual grasses can be seen. The biomass fuels are very reactive and high in volatile content which make them particularly suitable for bubbling fluidised bed combustion processes.
ANDRITZ has a long history as the leading supplier of technologies and solutions for the pulp and paper industry. The company has widened its product portfolio to cover state-of-the art bubbling fluidised bed boilers for bioenergy applications. The ANDRITZ bubbling fluidised bed boiler is a modern membrane-walled natural circulation steam boiler equipped with a unique fluidising grid. The boiler incorporates several standardised features, but the boiler arrangement is designed and customised for each specific case for top performance. Boilers are supplied with up to 350 t/h capacity steam production.
BUBBLING FLUIDISED BED COMBUSTION
Bubbling fluidised bed (BFB) technology is common in many industrial processes and for combustion applications. It was originally introduced in the late 1960’s. The first applications in Finland were for peat combustion in municipal boilers and bark combustion in industrial applications. Soon BFB technology became the state-of-the-art combustion technology for biomass fuels.
The fluidised bed is a 50-100 cm high layer of sand or other inert material that is fluidised using primary air blown through the bed via nozzles in the furnace floor (fluidising grid). Primary air usually makes about one-third of the combustion air; the rest is introduced into the furnace higher up in the freeboard as secondary and tertiary air for complete combustion. This staging of combustion air makes it possible to effectively reduce NOx emissions from the boiler.
Biomass is well suited for this type of staged combustion process since the volatile content of the biomass is high and the amount of fixed carbon is low. Basically fixed carbon burns in the fluidised bed, while the gas phase volatiles are burned above the bed with secondary/tertiary air. The bed temperature is usually about 850°C, and the bed is cooled, when necessary, by diluting the primary air with recycled flue gases. This reduces the oxygen content of the fluidising gas thus lowering the bed temperature.
The heat capacity of the fluidised bed is large, which stabilises and maintains combustion and allows a very fuel flexible operation. The tolerance of this technology for moisture variations in the fuel clearly beats other combustion technologies.
A BFB boiler is simple and easy to operate. There are no moving parts and usually the high level of automation for the boilers enables even unattended operation. Availability is very high and usually no scheduled outages are required in addition to an annual service stop.
ANDRITZ BFB BOILER TECHNOLOGY
The modern ANDRITZ BFB boiler is shown in Figure 1. The boiler is a single-drum steam boiler consisting of membrane-walled furnace and second flue gas pass and a plate-walled third pass. In the fluidised bed area the walls are covered with thin refractory lining to avoid erosion.
Figure 1. Bubbling fluidised bed boiler for biomass fuels.
Fuel is fed from silos above the fluidised bed via air-cooled fuel feeding chutes. Combustion air is introduced as primary air for fluidization and into the furnace as secondary air and tertiary air. Furnace walls as well as the fluidising floor are of membrane construction and part of the evaporation circuit. The superheater in this case is divided into three stages. The primary superheater is a horizontal convective tube superheater in the second pass. The secondary and tertiary superheaters are located in the upper part of the furnace. Between the superheater stages there are two steam attemperators using feed water spraying. Economisers and air preheaters are located in the third pass. The heat transfer surfaces are equipped with effective steam sootblowing. The boiler is started up with start-up burners directed towards the fluidised bed.
An important part of the BFB boiler is the design of the furnace floor (fluidising grid) to allow effective removal of the coarse material from the bed. Large non-burning particles that are introduced into the bed with the biomass fuel, or built-up by sintering of the bed material, will disturb the fluidisation and need to be removed from the bed. This is done by discharging some bed material from the bed. In some cases this bed material is sieved and the fines returned to the boiler. ANDRITZ's fluidising grid features a unique water-cooled floor structure allowing effective coarse material removal over the whole bed surface area. This increases the availability of the plant and allows the use of low grade fuels.
Figure 2. Primary air nozzles mounted on water-cooled furnace floor.
The steam temperature in this type of BFB boiler vary typically from 480-530° C and pressure from 60-130 bar. The size scale for BFB boilers ranges up to 350 t/h of steam production.
In the case of high chlorine and high alkali fuels, fouling and corrosion of the heat transfer surfaces can become an issue. Special attention needs to be paid to flue gas and steam temperatures. The solution for these types of fuels is to lower the flue gas temperature before the superheater surfaces as shown in Figure 3.
Figure 3. BFB boiler for high alkali biomass fuels.
In this case the superheaters are located in a separate chamber behind the evaporator screen. In addition to the reduced gas temperature, this also enables removal of the possibly high alkali ash deposits from the boiler. The flue gas velocities are also lower and tube spacing wider in order to minimize fouling. The steam temperatures in this type of BFB boiler vary from 460-500° C and capacity up to 300 t/h.
WIDE FUEL APPLICABILITY
Biomass fuels cover wide range of materials with varying fuel properties. Some fuels suitable for a BFB boiler are listed in Table 1.
|- chips, saw dust, bark, forest residues||Suitable|
|- demolition wood, industrial wood waste, pallets, plywood, railroad ties||Impurities need to taken into account in boiler design and gas cleaning. Amount may be limited.|
|- prunings, olive pits, bagasse, rice husk||High alkalies affect boiler design|
|SHORT ROTATION CROPS|
|- willow, eucalyptus, switchgrass, poplar||High alkalies affect boiler design|
|- rdf/ref, waste paper, sludges, primary sludge, de-inking sludge, tyre derived fuel||Impurities need to taken into account in boiler design and gas cleaning. Amount may be limited.|
|COAL||Only co-firing up to 25%|
Table 1. Fuels for BFB boiler.
A common feature in biomass fuels is the high volatile content of the fuel, which usually ranges from 70-85%. The moisture content may vary from 10-65%, ash content between 1-10% and even up to 50% with some sludges. Fuel particles need to be smaller than 200-300 mm. Important variables in the feedstocks from the boiler point of view are also the amount of sodium, potassium and chlorine contents in the fuel. They affect the boiler performance and may cause bed sintering, fouling and corrosion of heat transfer surfaces, and therefore need to be taken into account in the boiler design. The nitrogen and sulphur contents affect the NOx and SO2 emissions. Possible heavy metals in demolition wood and wastes also may require special emission reduction technologies.
Coal has a low volatile content and therefore it is not very suitable for BFB combustion. Typically the share of coal can be up to 25% of the energy input as co-fired together with biomass fuel.
LOW EMISSIONS AND HIGH AVAILABILITY
BFB boilers can be operated with low emissions and high availability. The general performance is summarized in Table 2.
|NOx||Depends on fuel N - content. Typical value 200 mg/m3n @6%O2 with fuel N of 0.3%. SNCR (ammonia injection reduces NOx emission 40-50%. SCR can be used to further reduce NOx|
|SO2||Depends on fuel S - content and ash quality. Typical value 100 mg/m3n @6%O2 with fuel S of 0.04%|
|CO||100-200 mg/m3n @6%O2|
|Dust||After ESP 30-50 mg/m3n or bag filter 10 mg/m3n @6%O2|
|Others||Acid gases and heavy metals controlled with sorbent injection prior to bag filter|
|HIGH BOILER EFFICIENCY|
|Efficiency||Depends on fuel moisture and quality. Typical values 87-91% (LHV)|
|Unburned carbon in ash||< 5% of ash mass, < 0,5% of heat input|
|Annual operating time > 8400 h|
Table 2. Boiler performance.
ANDRITZ entered into the biomass boiler business in 2006. As background, ANDRITZ has supplied more than 70 recovery boilers for the pulp and paper industry. The power boiler business was started by establishing a separate sales and technology group, but utilizing the experienced engineering and project management personnel familiar with recovery boiler projects. A list of current biomass projects is shown in Table 3.
t/h of steam
|Ence Navia, Spain||
|123||500||Eucalyptus bark, screening fines, biomass residues||2008|
|Portucel Cacia, Portugal||
|Portucel Setubal, Portugal||
|Ence Huelva, Spain||
|100||500||Young eucalyptus, petcoke||2010|
|Fortum Pärnu, Estonia||
|117||525||Peat, wood biomass||2010|
Table 3. BFB boiler projects.
The boiler sizes range from about 50 MWth of fuel input up to about 200 MWth. The small boilers, typically less than 80 MWth, are supported from the bottom. Larger boilers are hanging (i.e. supported from the top). The project duration depends on the delivery time of critical parts as steam drum and special corrosion resistant superheater materials, and varies today from 22-26 months.
The first project, commissioned in the autumn of 2008, is located in northern Spain. The fuels for this boiler (ENCE Group, Navia) are pulp mill residues and forest residues from local sources. The boiler produces 120 t/h of 123 bar superheated steam at 500◦ C, which is being used for electricity generation/process steam. The boiler functions well and has been in continuous operation since November 2008.
The ENCE Group’s Huelva project in Spain will produce 195 t/h of steam for green electricity production. The main fuel is short rotation eucalyptus. Some petcoke may be used as secondary fuel. This boiler will be commissioned in late 2010.
Two identical boilers in Portugal will produce 58 t/h of steam using eucalyptus and pine as fuels. The boilers will be commissioned in 2009/2010. These units will be used for green electricity production.
The project in Pärnu, Estonia covers a boiler island for a combined heat and power (CHP) district heating plant with 94 t/h of steam production. The fuels in Pärnu are peat and wood biomass. The boiler will be commissioned in 2010.
Growing environmental awareness and increasing energy prices boost development of clean energy technology projects with utilisation of local fuels. The main trends in boiler applications are towards higher steam temperatures and the use of even more demanding fuels. This makes superheater corrosion prevention one of the key issues in the boiler design. Also the power plant cycle needs to be optimized for higher efficiency. One way of increasing efficiency is to use steam reheating in the boilers. The number of reheat boilers in biomass applications to date is fairly limited as the technology is applied mainly in larger boilers.
In the near future, ANDRITZ’s bioenergy product portfolio will include gasification applications. This includes gasifiers for lime kilns, for co-firing in coal boilers, and for syngas production (liquid biofuels, etc.)
BFB boilers and biomass gasifiers complement ANDRITZ’s product portfolio of recovery boilers and other equipment for the pulp and paper industry. This helps the company better serve its clients not only today, but also in the future.
Recovery Division, Bioenergy Systems
Tammasaarenkatu 1, 00180 Helsinki, Finland
tel: +358 40 8605797