Recent biorefinery activities

Written by Roxare on October 8th, 2007

Mill implementations of biorefinery concepts are gathering momentum. It appears that the thermochemical concept (biomass solid fuel to gaseous fuel) is the preferred route at present. This is no surprise, as thermochemical gasification processes have been in use in North America for 25 years and there are at least 19 biomass gasifiers in commercial use [1]. The next step for the kraft sector will be to develop new products and markets either from the extraction and fermentation of hemicellulose, or from gasification syngas, but these involve higher risk than the thermochemical concept, where the savings from reducing energy requirement may justify the installation of the gasifier. However, it does appear that mid to long term plans for the kraft pulp industry are to find ways to benefit from the syngas route (biomass solid fuel to syngas to value-added chemical feedstocks).

Reported activities at the demonstration/mill stage include a gasification demonstration plant for the Kamloops, Domtar mill in BC. A Government of Canada grant towards the demonstration plant was a key factor. Here, the purpose is saving energy in the lime kiln. The wood-based syngas is intended to reduce natural gas consumption in the kiln and reduce CO2 emissions. The commercial-size gasifier planned for Kamloops is expected to cost around $8M and is similar to the gasifier used by Tolko, in that there are two gasification vessels [2]. The gasification process at the Tolko plywood plant in Heffley Creek, BC, started in 2006. This system recovers thermal energy from the gasification process to dry the plywood. The design for the Kamloops lime kiln requires the direct firing of the syngas in the kiln.

Other installations include a high temperature, atmospheric pressure kraft black liquor gasifier in the Weyerhaeuser mill in New Bern, NC, and a low temperature gasifier used to process carbonate spent liquor at the Norampac mill in Trenton, ON. In the case of sulphur-based black liquor, the main issue is the materials of construction. There are also three non-integrated small mills in North America that have become fossil fuel free by using biomass to create thermal energy [1]. Projects in other mills include a biomass gasification process, and ethanol production. One such example is that of Flambeau River Papers in the US, where soaring energy prices were the impetus for considering a biorefinery [3]. Here also, the involvement of the state in providing finances was a key factor. The biorefinery was considered for the mill’s steam production needs, as well as installing a state-of-the-art biorefinery pulping line. Additionally, an improved technique for extracting ethanol from sulphite pulping was planned. The recent news on Flambeau River is that they have implemented a project to recover waste energy and reduce water demand on the river. This project, which cost $190,000, will save the mill an estimated $2,100,000 in natural gas costs each year, increase production by 12 tons per day and improve operations on all its paper machines. The mill also reduced its demand on the Flambeau River by up to 1,500 gallons per minute.

1. Paper 360o, p.18, June/July 2007

2. Pulp & Paper, p.28, June 2007

3. Paper 360o, p. 18, Feb. 2007.


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Condensate Use

Written by Roxare on September 1st, 2007

The most common use of combined condensate is in brownstock washing, although some mills use condensate in bleach plants as well. The contaminated, or foul, condensate is either sewered, or steam stripped and re-used. Although heated water discharged from surface condensers can be used in other parts of the mill, a mill practicing water reduction will have excess hot water. The quality of condensate depends on the efficiency of the evaporators and the stripping systems. Mills use various methods to improve the quality of condensate, so that it can be used instead of water. For example, Irving Pulp & Paper in St. John, N.B., installed a reverse osmosis (RO) system in 1998 [1]. At that time, the mill had a pulp production of about 800 admt/d, and discharged untreated effluent. The mill has now added a moving bed bioreactor, which brings it into compliance with the regulatory limits on BOD, toxicity and suspended solids. The RO system treated about 3600 L/min of condensate from the evaporator’s fifth effect. Clean permeate (approximately 99% of the flow) was used as wash water on the second post-oxygen washer dilution conveyor. The installation of the RO system resulted in the removal of 88% BOD and 89% COD from the condensate, producing a permeate for use in post-oxygen washing with about 67 mg/L BOD and 115 mg/L COD.  

A different approach was used at Stora Enso’s Skutskar kraft mill [2]. The mill embarked on an extensive modernization plan in 1995 to increase production (from about 1230 to 1570 admt/d), and to minimize environmental impact. The changes included a new falling-film evaporator line, with a stripper column to remove COD. The new evaporator line was designed with extended condensate segregation to improve the efficiency of condensate cleaning, and reduce COD emissions. An in-line condensate stripper and a new external stripper for foul condensate were, therefore, included in the plans. The treated condensate from the external stripper also went to the in-line stripper. The accept stream from the in-line stripper had a COD concentration of 400 to 700 mg/L. As a result of these changes, a COD reduction of 20% in mill effluent was achieved.  

In closed bleach plants, the evaporator condensate, displaced by Eo filtrate, must displace fresh water to achieve water reduction. Possible uses could include a closed-water cooling system, wire cleaning showers on any caustic stage, and replacing fresh water in bleach plant showers. Södra Cell’s Värö kraft mill, with a bleaching sequence of Q(Paa)EopQ(PO), installed a system for stripping moderately contaminated condensate from the evaporation plant [3]. Together with a previously installed stripper for foul condensate, the mill had two separate stripping systems. The new stripper was integrated into a new evaporator train, and treated 200 m3/h of secondary condensate with a COD concentration of about 2000 mg/L. The system removed about 90% of COD, 95% of H2S and over 95% of DMS and DMDS, enabling the mill to use all the condensate in the bleach plant. Since the condensate does not contain any metals, the peroxide consumption decreased by about 30% and pulp strength increased considerably. 

   

1.         Dube, M., McLean, R., MacLatchy, D. and Savage, P., Reverse Osmosis Treatment, Effects on Effluent Quality, Pulp Pap. Can., 101(8), pp. 42-45, (2000). 

2.         Backman, K., Lindberg, H., Sjoberg, H., Recovery Modernization at Stora Enso’s Skutskar Kraft Mill, Tappi J., 83(7), p. 69, (2000). 

 3.         Emilsson, K., Håkansson, M. Danielsson, G., Extended Stripping and Usage of Evaporator Condensate at Värö Mill, Sweden, 1997 Minimum Effluent Mills Symposium, Proceedings, San Francisco, CA., October 23-24, TAPPI Press, pp. 191-197, (1997).


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Bleach Plant Washing

Written by Roxare on August 17th, 2007

The aim of pulp washing in the bleach plant is to remove dissolved organic and inorganic materials, that would interfere with subsequent bleaching stages and raise the consumption of bleaching chemicals. Pulp washing in the bleach plant is mostly evaluated in the same manner as brownstock washing: i.e., using soda loss or COD as an indicator.  Improving the efficiency of the bleach plant washers becomes even more critical in a closed bleach plant, where the consumption of bleaching chemicals is expected to rise. Process modeling using different indicators, obtained from laboratory tests, can be used to predict the effect of closure in the bleach plant and the recovery cycle.

In a 1995 survey of bleach plant washing practices in Canadian mills [1], the chemical demand in the following stage was the most frequently monitored variable, followed by mat consistency. The survey showed that, generally, chloride is an effective variable for monitoring the first C/D washing stage whereas, in extraction stages, sodium can be used to determine washer performance. Filtrate recycle and split showers can, however, complicate the determination of washer efficiency. Bleach plant effluent in Canadian mills surveyed ranged from 12 to 70 m3/adt, with a median of 28.4 m3/adt. Since bleaching effluents are sent to the biological treatment system, which can handle large variations in volumes and dissolved solids, bleach plant washer control has not been a major issue in open mills. As the trend towards reduction of effluents in the bleach plant continues, however, better control strategies will be required. Feedback control strategies used in brownstock washing can be used in bleach plant washers, if the required sensors exist. The carryover of a specific contaminant from the washing stage can be monitored by using the appropriate sensor, and shower flows can be manipulated to maintain pulp cleanliness [2].  

The use of a displacement press, rather than a vacuum filter washer, in the bleach plant gives the opportunity for a reduction in the consumption of bleaching chemicals, water and steam. With higher discharge consistencies, it is easier to control pH, temperature and metal ions in the process. The consumption of alkali in the extraction stage is also reduced, because the volume of residual alkali-containing dilution liquor added to a high consistency pulp (~30%) after washing is much higher than the amount added to a pulp with a consistency of ~12%. The effluent volume of an open bleach plant can be reduced by more than 50% when filters are replaced by wash presses.  

1.         Towers, M. and Turner, P.A., Survey of bleach plant washing practices in Canadian mills, Pulp Pap. Can., 99(7), pp. 44-49, (1998). 

2.         Dence, C.W. and Reeve, D.W., Eds, Pulp Bleaching, Principles and Practice, Tappi Press, Atlanta, GA, pp. 569-596 and pp. 649-673, (1996). 


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Brownstock Washing

Written by Roxare on August 5th, 2007

Brownstock washing is a key unit operation, affecting the efficiencies of both bleaching and the recovery cycle. Washing becomes even more critical with system closure, where alkaline/acid bleaching filtrates are recycled to the brown fibre line. The operation of brownstock washing has to be optimized prior to closure, to prevent excessive carryover and significant increase in the consumption of bleaching chemicals. Several approaches can be used to improve brownstock washing and reduce the carryover of black liquor to the bleach plant. These include: increased shower flow, additional shower bars, closed screen rooms and addition of defoamer and washing aid chemicals. Reconfiguring existing stages to employ filtrate segregation may also provide incremental improvement in washing efficiency. Process modeling can be used to predict the results of washing optimization and various system closure scenarios, ahead of mill trials. Process control is another cost-effective approach for improving washer efficiency. Sufficient filtrate surge tank capacity is recommended to balance accumulations in pulp storage tanks, especially during production rate changes. Sensors are now available to measure the entrained air in pulp suspensions and, therefore, provide better control in washing operations*. Many older mills are using original washers, which tend to be overloaded. However, modifications made to original washing equipment can allow mills to run at increased production levels. The new generation of washing equipment provides improved efficiency, reduced operating and capital costs and minimal environmental impact. Some of the new alternatives to the rotary vacuum drum washer include the Drum DisplacerTM, the Pressure Diffuser, the Chemi-Washer, the Compaction Baffle Filter and the Displacement Press.  
*R. Thompson and T. Mahmood, Recent developments in mill implementations of water reduction strategies, in “Water Use Reduction in the Pulp and Paper Industry”, 2nd edition, PAPRICAN, December 2001.


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Overliming

Written by Roxare on July 22nd, 2007

The critical factor in the operation of pressure filters is the quality of feed. Dregs and overliming cause blinding of the white liquor filter media, requiring more frequent acid washes. Overliming happens when the molar ratio of CaO/Na2CO3 is larger than that required to reach the equilibrium causticity, for a given ionic strength of the liquor (CaO+ Na2CO3+H2O?2NaOH+CaCO3). However, in actual mill practice, problems associated with overliming tend to occur when the molar ratio is 3-5% below the equilibrium causticity. Improved green liquor clarification and good control at the slaker are required to prevent problems in filters.  Mills may increase the use of fresh lime due to poor control, or in order to increase causticizing efficiency. To improve causticizing efficiency without adding excess lime, advanced causticizing control or partial borate autocaustcizing is recommended.   Poor settling of lime mud is attributed to a surge of fresh lime added as make-up. The fresh lime make-up should be added on a steady basis, dependent on the losses experienced by the system. The quality of the purchased lime is also important. The slaking reaction (CaO+H2OàCa(OH)2) generates a lot of heat, and proceeds quickly with a maximum temperature rise in 2-8 minutes, depending on the type of reburned lime*.  Reburned lime with a longer slaking time always has better settling and filtering properties than fresh lime. Research has shown that reburned lime is an agglomeration of small CaO crystals, with large voids between the crystals. Since slaking and causticizing reactions take place on the surface of solid particles, the reaction rates depend on the available surface and are inversely related to the degree of agglomeration due to sintering. This indicates that producing a kiln product with a high degree of sintering is desirable for optimum results in causticizing.  * C.F. Cornell, Current recausticizing practice, Part 2-Principles, 1996 Tappi Kraft recovery short course


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Borate autocausticizing

Written by Roxare on July 8th, 2007

The main reactions in causticizing are as follows:

CaO+H2O ? Ca(OH)2 + heat

Ca(OH)2 + Na2CO3 ? 2NaOH+ CaCO3

The second reaction is an equilibrium one, with a typical conversion of about 80%; this varies with TTA concentration and sulphidity of the liquor. This reaction can be driven to the right by adding more lime. However, this will create problems for the performance of the equipment (i.e., pressure filters) due to the presence of free lime. A lot of mills with overloaded kilns would like to increase their causticizing capacity and reduce the amount of purchased lime, without capital investment. The option here is to use partial borate autocaustcizing (AC). In this method, sodium tetra-borate pentahydrate (Na2B4O7.5H2O) is added to the liquor cycle. The borate AC reaction occurs in the recovery boiler, and caustic forms in the smelt dissolving tank:

Na2CO3 + NaBO2 ? Na3BO3 + CO2

Na3BO3 + H2O ? NaBO2+ 2 NaOH

Laboratory studies* have shown that sodium borates can readily react with molten Na2CO3, at any Na/B molar ratio larger than 3, to form tri-sodium borate. Tri-sodium borate (Na3BO3) hydrolyzes to form NaOH, and regenerates NaBO2. For every mole of NaOH produced, half a mole of NaBO2 is required. This means a small amount of borate can partially causticize Na2CO3 in the recovery boiler. The causticizing reaction can then be completed in the recaust plant, with a reduced amount of lime. Borates are water soluble and remain dissolved in the alkaline liquors, repeating their AC function continuously. The concentration of borate in the mill liquors is then determined by the rate of borate addition and loss (about 2.5%).

Mill trials have shown that the presence of borate in the mill liquors has little effect on the mill operation and the quality of the pulp produced. There are indications that the yield may be enhanced due to the possible retardation of cellulose peeling reactions. Partial borate AC is now used in several mills in Europe, the U.S., Brazil and Indonesia.

*Tran, H. et al, Partial autocausticizing of kraft smelt with sodium borates; Part 1: Effects on recovery boiler performance, Tappi, Vol. 1, No.1, September 2001.


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Practical implications of viscosity

Written by Roxare on June 28th, 2007

Pulping methods and conditions have a pronounced effect on the viscosity of black liquor. For example, black liquors from (PS+AQ) pulping are lower in carbohydrate content, and are expected to have lower viscosities (if the liquor REA stays relatively constant). They also have lower organic/inorganic ratios and lower heating values. If liquor firing parameters are held constant, the likely effect in the recovery boiler is the formation of smaller droplets. Smaller droplets have a higher propensity for being entrained in the flue gas, and could contribute to carryover problems and boiler plugging. Reducing the liquor firing temperature by a degree or two can increase viscosity and remedy these problems. For boilers which have problems maintaining a hot bed, this may not be an option, as decreasing the liquor firing temperature could also lower the bed temperature. The alternative is to increase the viscosity by alkali profiling, or to adjust both the liquor gun firing angles and liquor firing temperature or pressure, to compensate for the changes in droplet size distribution. If the mill has excess evaporation and steam capacity, the black liquor solids can be increased to increase viscosity. This has the added advantage of increased thermal efficiency and liquor throughput, and reduced TRS emission.  

If the REA content is not controlled during the cook, higher viscosity black liquor may result. Alkali profiling, and firing at higher temperatures, are two available options for reducing liquor viscosity and any operating problems that might be encountered. When the firing temperature is increased, there is always the possibility of liquor flashing. Flashing decreases the flow of black liquor though a nozzle at a given pressure, and decreases the median droplet size of the spray, which can increase carryover and boiler plugging problems. 

Some mills fire soap with the black liquor; this will increase the viscosity of the liquor and lower its swelling index. High viscosity liquor would generate larger droplets upon firing. These slow-burning (lower surface area) larger droplets may reach the bed before drying, and cause blackouts or smelt-water contact. Increasing the firing temperature by a few degrees (depending on the level of soap) can reduce the liquor viscosity and improve the situation. High viscosity liquor will also reduce the capacity of the evaporators. 


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The effect of soap on black liquor viscosity

Written by Roxare on June 9th, 2007

Mills that do not separate soap for tall oil production can have high soap content (around 2.5%) in their black liquor. The soap will increase the heating value of the black liquor, which may be advantageous if the boiler is not steam limited. Other beneficial effects include a reduction in chemical make-up, increased bed temperature and increased reduction efficiency. However, if soap is not removed it can adversely affect the washing and recovery operations. Burkeite (2Na2SO4. Na2CO3) and CaCO3 scaling in the evaporators is greatly accelerated in the presence of soap. In the recovery boiler, blackouts have been associated with the entry of slugs of soap containing weak black liquor. To avoid this, it is recommended that the separated soap should be first acidified to crude tall oil. However, mills that do not have an acidification plant add soap to the strong or as-fired black liquor, even though this may reduce the liquor throughput. In this case, it is important to insure a homogenised mixture and a uniform flow to prevent unstable recovery boiler operation.

The viscosity of tall oil soap varies widely, depending on the soap composition and the entrained black liquor. However, the viscosity of soap is often an order of magnitude higher than that of black liquor. Addition of soap increases the viscosity of black liquor, but increasing the firing temperature by a few degrees can reduce the viscosity to acceptable levels. Another factor involved in increasing viscosity with high levels of soap addition may be the metal content of the soap. The Ca content of the soap is generally much higher than that of the black liquor. High levels of Ca and Mg promote the flocculation of lignin, and cause an increase in liquor viscosity. The addition of soap also reduces the swelling propensity of the liquor.


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Black liquor viscosity

Written by Roxare on May 26th, 2007

The viscosity of black liquor determines its handling properties, and is a critical parameter for the control of a recovery furnace. Viscosity is also an important factor in black liquor evaporation, as it affects the heat transfer rate. An increase in viscosity can lower the evaporation capacity. Softwood black liquors (e.g., pine) generally exhibit near-Newtonian behaviour (i.e., viscosity is independent of shear rate) but some hardwood black liquors (e.g., birch) can be thixotropic (i.e., viscosity is both time and shear dependent).

The viscosity of black liquor is affected by such factors as the wood species, the solids content of the liquor, the cook’s kappa number, the residual alkali and sulphidity of the liquor, as well as the temperature. As high solids firing can improve a boiler’s thermal efficiency, increase liquor throughput and reduce environmental emissions, many mills have increased their as-fired liquor solids concentration. However, black liquor viscosity increases significantly at solids concentrations over 70%, where NOX emissions also increase. If a mill is not equipped with a pressurized liquor handling system, in which high temperatures can be used to reduce viscosity, a viscosity reduction method may have to be employed in order to facilitate high solids firing. A viscosity of 200 cp and over is considered high, since 200 cp is a common practical limit for centrifugal pumps.

The simplest and most cost effective way of keeping viscosity in an optimal range (if the solids are not too high) is by alkali profiling. All you need is a suitable benchtop viscometer for black liquor, and a reliable way of measuring residual effective alkali (REA) and black liquor solids. To control black liquor viscosity in real time, you would need an on-line viscometer (or an on-line probe which provides an indirect measure of viscosity) and an on-line REA probe in a control loop.


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Posted in Black liquor viscosity | 4 Responses »

Barium sulphate scaling in the bleach plant

Written by Roxare on May 13th, 2007

Barium sulphate is another problematic scale in the bleach plant. Barium enters the mill with wood/bark (especially hardwood) and water. Barium scaling depends on the kappa number, the pH and the concentration of Ba and SO4 in the bleaching stage. The problem of BaSO4 scaling has been observed when switching from chlorine to ECF bleaching. In the D0 stage, pH is controlled by the addition of H2SO4 or generator waste acid, thereby increasing the concentration of SO4. Scaling normally takes place in acid stage washers. Some BaSO4 scales contain radium sulphate; radium is a radioactive element which can be found in wood and ground water.

The solution to inorganic scaling is to reduce its source; i.e., have a wood supply low in metals (good debarking), control pH in the D0 stage, have good white liquor clarification, use good quality chemical make-up, use chelant or scale inhibitors and use treated water for showers. If scales have formed, the required removal measures are hydroblasting or chemical boil-outs.


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