Utilization of Biomass residues for Energy production in Pulp and Paper Industry. Markus Bolhàr-Nordenkampf

Transcription

1 Utilization of Biomass residues for Energy production in Pulp and Paper Industry Markus Bolhàr-Nordenkampf Valmet GesmbH, Erdbergstraße 52-60/3/12, 1030 Vienna, Austria Keywords: Biomass, Boiler, Bubbling Fluidized Bed, Pulp&Paper Industry, Residues

2 1 ABSTRACT Energy and the utilisation of biomass based residues is gaining more and more attention in pulp and paper industry to defend it`s cost competitive level. The pressure even increases due to high primary energy costs, the EU energy savings directive as well as increasing costs for residue disposal. Valmet has built several medium and large scale combustion plants based on fluidized-bed boiler or gasification technologies to utilize residues of pulp and paper industry. These advanced technologies have supported customers to reach their targets in high efficiency energy production from biogenic residues as well as matching the recycling waste policy. In addition low emission figures as well as profitable operation could be achieved. The paper outlines different technologies and demonstrates the combustion process with different type of boiler processes meeting at the same time tightest environmental requirements. Selected experiences from high efficient combustion commercial cases will be presented. 2 INTRODUCTION The pressure to decrease green house gas emissions is increasing worldwide. Carbon dioxide (CO 2 ), which is formed when firing any fossil fuel including carbon, is typically the main focus when talking about green house gas reduction. Many countries have made their long term action plans to decrease CO 2 emission and some governments have even set mid-term targets to reduce CO 2 emissions. United Nations Framework Convention on Climate Change continues and for instance EU has set target and the governments of member countries has to find ways to meet that target. EU s target is to reduce 20% green house gas emission, decrease 20% of primary energy use and to have 20% of EU s energy consumption coming from renewable sources. Biomass firing is considered to be CO 2 neutral way to produce energy. Burning biomass naturally releases CO 2 in a same way as with fossil fuel, but it is assumed that new plants and trees continue to grow and the emitted CO 2 is absorbed by these plants and trees. When this balance remains the net CO 2 release to the atmosphere is zero. Quite often the biomass utilized in energy production has been wood based and there is a concern about the overuse of forests. Agricultural biomass residues from food production, energy crops, and recycled wood and solid recovered fuels have been seen as an option. One very efficient way to fire biomass, alone or in mixture with some other fuels, is fluidized bed technology. Fluidized bed technologies available are bubbling fluidized bed (BFB) and circulating fluidized bed (CFB). Valmet has been manufacturing fluidized bed boilers since the 1970s. Initially it built bubbling fluidized bed (BFB) boilers for bark, sludge and other biofuels. In the 1980s Valmet s products offering was added with circulating fluidized bed (CFB) boilers for fossil fuel and biomass in any combination. To date, Valmet has delivered about 250 fluidized bed boilers for different kind of fuels and fuel mixture. Valmet is a global supplier of sustainable technology and services for mining, construction, power generation, automation, recycling and the pulp and paper industries. Valmet has about 12,000 employees in more than 50 countries and 2013 net sales was approximately EUR 3.5 billion. 3 FUELS The current variety of different biomass fuels has expanded a lot since the area of fluidized bed boilers begun. The starting point was that bark and wood waste was burned in small grate fired boilers in small scale and with low efficiency. Sometimes bark and wood waste was land-filled and not used as a fuel. Pulping process creates also waste materials and most notable of these are the process sludges. Sludge combustion was not possible in the traditional grate fired boilers but in fluidized bed boilers this was not a problem. Fluidized bed technology is able to fire all side products coming from the mill and producing cheap energy to mill making it quite often energy self-

3 sufficient. After fluidized bed technology s good experience in pulp and paper industry it became very popular also in other CHP applications. New applications widened the fuel portfolio gradually towards different and often more demanding fuels. 3.1 Pulp mill fuels The superior capability of fluidized bed boilers to combust low-calorific fuels was an immediate success story. Thanks to the heat capacity of the hot bed sand fluidized bed boilers were able to combust wet bark and pulp mill sludges. Process sludges, which were earlier dumped into landfilling sites, were poor quality fuels, typically having high moisture, high ash content and low heating value. Due to higher chlorine content and simultaneous increase of steam parameters, sludges also brought challenges by increasing fouling and corrosion in the boilers. Especially eucalyptus bark is much more demanding fuel when compared to any other traditional bark. Not only having high chlorine and alkali contents but also its physical properties are challenging for fuel feeding. See Tables 1 and 2 for fuel analysis. Moisture Lower heating Ash Chlorine Sulphur Nitrogen Na+K value (as received) wt-% MJ/kg wt-% dry wt-% dry wt-% dry wt-% dry wt-% dry Bark Wood chips Pulp mill sludge Peat Table 1. Typical properties of pulp mill fuels. Bark, wood chips and sludge analysis added with additional fuel peat. 3.2 More demanding biomass qualities Paper recycling brought in a new fuel: de-inking sludge. Recycling paper plants do not have any internal bark or wood waste. Valmet has delivered three BFB boilers burning deinking sludge as a main fuel with natural gas support, one to Sachsen Papier, in Germany (32 MW th ), another to Norske Skog Golbey mill in France (90 MW th ) and the latest one to Norske Skog Parenco mill in Netherlands (37 MW th ). Recently, Valmet has delivered two identical (82 MW th ) boilers for UPM Shotton and UPM Chapelle mills burning de-inking sludge and rejects together with wood residues. Plywood and chipboard mills have a bit different side products coming from main process. Plywood residue is one good examplet. The challenge comes from glue in plywood which may cause very fast agglomeration of bed material. Valmet developed a new silica free bed material for this application it is called AggloStop and it utilizes diabase as bed material. Agglostop bed material has been used in in boilers at Ristiina and Savonlinna saw mills, both in Finland, since Forest residue chipped branches, tops and roots of trees. Depending on storage time in forest chlorine and alkali content vary a lot. A lot of fresh green parts included are an indication of higher chlorine and alkali content in the fuel, but even in brown forest residue the chlorine content is higher than in bark. Also moisture content (heating value) varies quite much in forest residue fuel. Forest residue has been used in numerous Valmet s projects. Stumps are also utilized as a fuel instead of leaving those in forest. Heating value of stumps is good and in that respect it is a good fuel. One challenge of forest residue and stumps is the amount of impurities, i.e. stones and gravel, coming with the fuel. This means that boiler bottom ash system must be efficient. Erosion has to be taken into account both

4 in fuel conveyors and boiler design since the fuel is crushed with all the stones in it. This may produce very abrasive ash. Eucalyptus bark has become a common fuel since pulp and paper industry has invested in many existing and new mills in South America and Asia Pacific. These mills are including several Valmet Hybex references, for instance Veracel in Brasil (72 MW th ), Celbi and Caima mills in Portugal (84 and 35 MW th ). Shredded eucalyptus bark has low bulk density compared to almost any other fuel and that sets first challenges in the designing of fuel feeding systems. Euca bark is also abrasive as it typically has high ash content due to sand and gravel coming with it. Localized erosion of conveyors and boiler parts may be higher than with more conventional bark fuels. Eucalyptus is fast growing (7-8 years to cutting age) and it contains high amount of chlorine and alkalis. Although ash has low melting temperature bed agglomeration can be avoided with low operational temperature. Due to high chlorine content the steam temperatures in eucalyptus fired boilers are moderate in order to avoid excessive corrosion rates in the superheaters. The most demanding biomasses are agricultural based fuels. The properties of agro fuels entail many challenges for the combustion process and thus the maximum agro biomass share must be evaluated based on chemical properties, typically being max 50%. Nitrogen, phosphorous, potassium, magnesium, and chlorine content are usually higher in comparison to woody biomass, due to the use of fertilizers. Potassium and chlorine vaporize easily and are known to have an important role in bed agglomeration, fouling, slagging, and high-temperature-corrosion-related processes. Phosphorus is involved in bed-agglomeration-, fouling-, and slagging-related phenomena. An increase in nitrogen content may elevate NO x emissions. The most recent HYBEX references with agro fuels are in Polish cities of Bialystok, Szczecin and Lodz (75, 183 and 129 MW th ). Next category of low carbon fuels is recycled wood and REF (recycled fuel) which is good quality RDF. Nowadays, these fuels are seen more often due to lower fuel cost. In practice the usage of lower quality fuels will lead to increased plant maintenance cost - thus the actual savings in plant operation cannot be directly calculated based on fuel price. In recycled fuels alkali and chlorine contents are order of magnitude higher than in traditional woody biomass. Combustion of recycled fuels may lead to increased fouling and corrosion of furnace walls, superheaters and economizers. When using high steam parameters, the share of these fuels is usually limited (to approx. 20%) in order to manage all the technical challenges. These challenges are associated mainly with alkalis, chlorine, zinc, and lead. Lead content in recycled wood typically varies between 50 to 800 mg/kg in dry solids - this is huge amount compared to virgin biomass values which are typically < 5 mg/kg. Another challenge associated with combustion of recycled wood is accumulation of impurities and formation of molten metal at the lower part of the furnace. Moisture Lower Ash Chlorine Sulphur Nitrogen Na+K heating value (as received) wt-% MJ/kg wt-% dry wt-% dry wt-% dry wt-% dry wt-% dry Forest residue Plywood residue Euca bark Straw De-inking sludge Recycled wood REF Table 2. Demanding low carbon fuels

5 4 STEAM PARAMETERS AND CORROSION During the last twenty years understanding of the superheater corrosion phenomena has increased significantly. The knowledge has increased in several ways: learning from reference plants, test measurement campaigns, laboratory tests. The calculation of combustion and ash chemistry is very complex and still today contains many assumptions or simplifications based on empirical knowhow. In the rough estimation of the corrosion risk some key parameters are often used. One of the traditional key parameters is the molar ratio between sulphur and chlorine. Typical feature for biomass is that fuel chlorine content is higher than sulphur content. Ash content is usually low, but on the other hand potassium and sodium contents are high in ash. Ash alkali metals tend to react with chlorine since the sulphur content is too low to use all alkali and to form alkali-sulphates. This may result in alkali-chloride rich deposits on superheaters. The alkali and heavy metal chlorides vaporize at fluidized bed combustion temperatures and as flue gases reach the convective section they come into contact with the heat transfer surfaces by direct condensation or sublimation. Alkali chloride plays a significant role in initiating the high-temperature (>450 C) corrosion process. The presence of lead and zinc chlorides has been shown to increase the corrosivity of the deposits even at relatively low metal temperature ( C). These chlorides lower the first melting point of the deposit and thus introduce the possibility of molten-phase corrosion. However, the presence of alkali chlorides in the deposit has caused accelerated corrosion well below the melting point of KCl. Figure 1. Simplified fuel ranking The conventional ways to control high-temperature corrosion are via boiler design, steam temperatures, superheater arrangement and through the selection of materials. The individual superheater section can be put in a less corrosive environment or the steam flow direction (counter-currently or co-currently) in individual superheater sections can be optimized in relation to flue gas flow. Valmet has developed a solution to reduce high temperature corrosion. The solution is called CorroStop and the principle is that the flue gas chemistry and environment are changed by sulphur additive feeding into the furnace. 5 HYBEX - BFB BOILER DESIGN FEATURES Valmet HYBEX boiler utilizes a state of the art bubbling fluidized bed technology. The current design is a result of thirty years of R&D development and many lessons which have been learned the hard way too. The boiler, as wells as main auxiliary equipment around it, have went through major development milestones and hundreds of small detail improvements during past years.

6 The heart of the boiler is the HYBEX grate which replaced the conventional grate type in mid 1990s. The grate construction consists of rigid water-cooled hydrobeams which are part of the natural circulation loop of the boiler. Cooling of beams is thus secured also in all situations. Each beam is furnished with wear resistant fluidization air nozzles. All nozzles are equal in height which ensures even fluidization of the whole bed area. The open area is over 30% of the total floor area this provides capability to efficiently remove stones, scrap metal and other impurities which occasionally may come in with the fuel. Large bottom ash hoppers below the hydrobeams are collecting the removed material to conveyors and to bottom ash sieving station. The eventual bottom ash amount to be taken away is rather small since only the coarse oversized material will be removed. All fine sand is sieved and recycled pneumatically back to the furnace. In the waste fired units the scrap metal is typically be separated from bottom ash and recycled. Figure 2. Left: A photo of the Hybex grate with > 30% of open bottom ash removal area. Right: Hybex bottom ash system with sieving and sand recycling station. Minimizing the fluctuations in fuel feeding and heat effect is a fundamental key to good performance of any boiler. The fuel feeding systems have been developed together with equipment manufactures in many ways since the early days. In the fuel silos there is a rotating spreader in order achieve even level of fuel. This way the silo reclaiming screw is able to produce more constant fuel flow. Some fluctuations which still may exist are smoothened out by a balancing bin just above the final feeding screws.. Air distribution is of key importance when it comes down to controlling CO and NO x emissions. Combustion air is divided into primary (fluidization) air and staging air. Staging air is split into secondary air and tertiary air. In HYBEX boilers the staging air nozzle arrangement is based on CFD modeling and long experience from operating units. The staging air nozzles are in so called semi-interlaced arrangement where a larger nozzle is opposing a smaller nozzle on the other side of the furnace. The powerful jets coming from the larger nozzles penetrate long into the furnace. The opposing smaller jets act as a counter force and prevent the larger jet from hitting the opposite wall. The result is a stabile flow pattern and effective mixing of combustion air, fuel and flue gas. Larger staging air nozzles are constant velocity nozzle type. Constant velocity nozzle was invented in the 90 s in order to get good jet penetration and mixing into the furnace also at small boiler loads. The nozzle has a throat that is

7 self-adjusting according to the air flow. As the air flow decreases the throat blades will start to gradually close thus keeping the jet velocity nearly constant throughout the whole operational range. During the past ten years Valmet has supplied more than 50 new HYBEX boilers and more than 20 boiler conversions based on HYBEX technology (see Figure 3). The current trend is that fuel spectrum has widened since many projects are located on the southern hemisphere but also due to different types of recycled fuels have become common sources of energy. That means the battle against high-temperature corrosion and emission limits continues. Those two items together with plant reliability will be the main development topics around BFB technology. Valmet is currently supplying two world s largest BFB boilers, in 100 MW e scale, for renewable energy projects in the USA. First one is Nacogdoches Power in Texas (Figure 4) which is currently under commissioning stage. Second one following is the Gainesville Renewable Energy Center in Florida (Figure 8) Net steam ouput [MW] Year of start-up Figure 3. Total number of HYBEX boiler references worldwide is 188. Two thirds are new installations and one third is boiler conversions. Figure 4. Nacogdoches Power HYBEX is the world s largest biomass fired BFB when starting-up in spring Flue gas cleaning system consists of a bag house filter with sorbent feeding. 5.1 High reliability and low maintenance The reliability of the power plant is an important matter to the plant owners. High reliability requires well proven design of all components in the power plant as well as regular preventive maintenance. Depending on fuels and many

8 other things, all boilers eventually will have wear and tear to some extent. Critical wear parts must be always available at the plant in order to keep reliability at the high level. Basis for good availability is built on the following things: well proven design, conservative dimensioning, continuous fuel quality control, sensible operation of the plant and planned annual maintenance. Sensible daily operation of the plant is crucial and monitoring of the process values should not be neglected. Controlling of fuel quality should be done on regular basis - in particular when having demanding fuel portfolio. Valmet offers services for planned outages such as comprehensive inspection of the boiler and its main auxiliaries. Proactive maintenance shall be done in each outages to keep the boiler availability at high level. When taking all these above mentioned things into consideration boiler reliability is typically maintained over 97 % of requested operation time (Figure 5). 100,00 % 99,00 % 98,00 % 97,00 % 96,00 % 95,00 % 99,4 % 99,6 % 99,1 % 98,7 % Figure 5. Typical reliability figures for a HYBEX boiler with biomass fuels. This boiler started up in SUMMARY The use of biomass and residues instead of fossil fuels can reduce the cost of land filling dramatically and also reduce the CO2 emissions. The developed bubbling fluidized bed technology of Valmet is a reliable combustion technology to combust even difficult fuels from the pulp and paper industry and turn them into valuable products like steam or electricity. The fuel portfolio that can be used in BFBs is extremely wide. Fuels have changed from virgin wood to forest residues, agricultural fuel and even recycled fuel. At the same time when fuel qualities are more demanding the flue gas emission limits are much more stringent than in early days. During the last thirty years Valmet has been a leading supplier of bubbling fluidized bed boilers. Valmet HYBEX technology utilizes a unique and patented grate construction which enables the use of most demanding fuels. The scale-up from the first units to today s largest BFB s in the world has been a success story. Simultaneously, numerous design features and details have been developed and introduced. Close to 200 delivered references, new boiler and boiler conversions, all over the world prove that Valmet HYBEX boiler has been able to meet increasing technical requirements for modern boiler design.