CONVERTING A KRAFT PULP MILL INTO AN INTEGRATED
FOREST PRODUCTS BIOREFINERY
Adriaan van Heiningen
Department of Chemical and Biological Engineering University of Maine, Orono, ME
There are fundamental global developments which will make energy supply one of the central problems in the coming decades. There is also growing consensus that fossil-fuel CO2 emissions will need to be
controlled. Renewable forest material is carbon neutral. A recent study by the USDA and DOE has identified that about 1.3 billion dry ton of biomass, of which 368 billion dry ton coming from forest land, would be
available in the US on an annual basis. This total amount of biomass would be enough to produce biofuels to replace more than one-third of the current US demand for transportation fuels. Therefore, managed forests
have enormous potential to reduce green-house gas emissions by conversion of the forest material into liquid fuels, electricity and other products now derived from nonrenewable carbon.
The forest products industry in N.A. is facing global competitors who use the latest and largest installed technologies, and also have wood and labor cost advantages. As a result of the increasing competition the
prices for forest products will continue to decrease. In addition, the low level of investment over the last 10 years has brought the NA Forest Products Industry to the edge of obsolescence.
In order to remain viable, the N.A. forest products industry needs to increase its revenue by producing bioenergy and new biomaterials in addition to traditional wood, pulp and paper products. This can only be done
economically if all product lines are highly integrated, are highly energy efficient, and have a minimal or no use of fossil fuel. The development of an Integrated Forest Products Biorefinery (IFBR) which
accomplishes this goal represents a great opportunity for the industry. At the same time the IFPR concept addresses the societal need to use renewable resources rather than fossil fuels to produce commodity
products, liquid fuels and electricity. The present paper discusses the opportunities and challenges of different versions of an IFBR which uses alkaline pulping as the central wood separation technology.
Global developments such as limited oil supply, increased concern about greenhouse gas emissions and decreasing competitiveness of traditional pulp and paper producers has increased the opportunities and urgency for
pulp mills in temperate climates to increase revenue. This may be done by transforming a chemical pulp mill into an Integrated Forest Biorefinery (IFBR) which produces higher value-added products such as ethanol,
polymers, carbon fibers and diesel fuel besides pulp. Considering the present economics, these new products should be derived from hemicelluloses and lignin and not from cellulose For example, the market price for
hardwood kraft pulp (~$600/MT) is 1.43 times higher on weight basis than ethanol (~$1.25/gallon), while the economics are further degraded by a ~50 % (w/w) yield for the conversion of cellulose into ethanol.
Over the last 50 years the kraft process has been established as the most versatile and economical chemical pulping process. During pulping about 20 and 30% of the wood weight in the form of respectively
hemicelluloses and lignin dissolve in the waste pulping liquor to produce nearly undegraded cellulose fibers (1). The dissolved material in the spent liquor is combusted to produce steam and electricity, and to
regenerate the pulping chemicals, NaOH and Na2S. Based on the heating values of hemicellulose (13.6 MJ/kg) and lignin (27 MJ/kg), values of $32/MT and $63/MT are calculated for respectively hemicellulose and lignin
using a price of $35/MT for biomass (20 % moisture) at 15MJ/kg. Therefore conversion of the low value hemicelluloses into new bioproducts such as polymers valued at more than $2000/MT presents a great economic
There are two major classes of hemicelluloses; glucomannans and xylans. The dominant hemicellulose in softwood is acetyl-galactoglucomannan at about 20 w/w %, and glucoronoxylan (15 – 30 w/w %) in hardwoods
(2). At alkaline conditions glucomannan is rapidly degraded by the peeling reaction, while xylan is dissolved in oligomeric form (3). Therefore hot alkaline treatment would be suitable to extract hardwoods but not
for softwoods. Extraction with pure water causes acidic conditions due to release of acetic acid from hemicellulose. The acidity may lead to degradation of cellulose by acid hydrolysis at high temperature, which is
unwanted in an IFBR that produces pulp as one of its major product. After extraction and separation of the hemicelluloses, the spent extract will be combined with the fresh and/or spent pulping liquor. Also some of
the extract will transfer to the pulping process. Therefore the extraction solution must be aqueous and only contain inorganics compatible with the spent pulping recovery process.
Modern kraft mills are net energy producers. With further energy efficiency improvements and increased use of forest waste, part of the lignin may also be used for bioproducts in an IFBR (4). Lignin may be removed
from spent pulping liquor by acid precipitation. However, the precipitated lignin must be sulfur-free for production of for example carbon fibers or polyurethanes. Another option is gasification of spent pulping
liquor to synthesis gas followed by conversion to transportation fluids in a one-pass process, with the unconverted gas used for electricity, steam generation and as lime kiln fuel. This option would also benefit
from sulfur-free pulping because the downstream syngas processes are intolerant to sulfur.
The erosion of the global competitiveness of the NA pulp and paper industry is caused by fundamental disadvantages in terms of wood and labor cost compared to new competitors, mainly in tropical countries. At the
same time these competitors have built mills employing the latest technologies at a significant larger (and economical) size than those built many decades ago in the traditional NA pulp and paper areas. This has
lead to a continuous decrease of the (inflation corrected) price of pulp and paper products (see Figure 1), and this trend will continue in the future.
The approaches used by the industry to increase competitiveness have been to cut cost, increase the value-added of the paper product, and move into niche markets. Many of these
measures have already been taken, and their return appears to be diminishing. Although increasing the size of the mills may provide part of the solution, there are considerable
barriers in terms of environmental regulations, wood supply and availability of capital. The cost cutting approach has lead to the situation whereby for several years now the capital
expenditures of NA mills are below depreciation (see Figure 2), i.e. the mills are no longer economically sustainable. To reverse this trend the forest products industry needs more
revenue from higher value-added products besides pulp and paper products. Hardwood kraft pulp mills in North America are most in need of extra revenue because of competition from
low-cost hardwood pulps with excellent paper making properties produced in tropical countries. Thus the initial envisioned IFBR is based on sulfur-free, alkaline pulping of
hardwood, with an alkaline hemicellulose extraction step prior to pulping, and spent pulping liquor gasification and lignin precipitation after pulping. Besides pulp, new products such as
ethanol, polymers and carbon fibers may be produced from hemicelluloses and lignin. Additional energy requirements of the IFBR are met by gasification/combustion of waste biomass.
MARKETS FOR NEW VALUE-ADDED PRODUCTS OF AN IFBR
Shown in Table 1 is the 2004 market size of the US pulp and paper industry. In tonnage it is only about 1/3rd as large as the annual US corn production, and is 1/4th of the annual US
gasoline consumption. About 13 % of the US corn production is used for the production of ethanol of 10 million metric ton. Since ethanol is mainly used to replace gasoline, its market
is essentially limitless. Thus even if all pulp mills use the extracted hemicelluloses for ethanol production, the market will not be affected. However, the market for unsaturated polyesters
of 0.8 million metric tons would be fully satisfied if the extracted hemicelluloses (10% based on wood) for about 10% of the pulp mills would serve as feed stock. This scenario would
have a strong effect on the price of the unsaturated polyesters. Still, unsaturated polyesters are of interest because they may be integrated into other forest products such as wood composite materials.
The US market of polyurethanes is about 3 million metric tons at a price of approximately $2000/MT. The use of lignin as an additive (5 – 10%) in polyurethane formulations has the
potential to improve the properties of the resulting materials at a lower cost, and displace the use of both toxic (aromatic isocyanates) and petroleum-derived materials. However for
commercial use the lignin must be sulfur free to avoid any release of volatile sulfur containing gases.
The total world production of carbon fibers is approximately 15 thousand metric tons, but its use is growing rapidly. The demand for this light-weight, high-strength product is expected
to grow exponentially (for example when used in energy efficient light-weight cars) if the fiber price would decrease to $7/kg, i.e. a level which is still one order of magnitude higher
than that of pulp. Recent work (6) has shown that carbon fibers can be made from hardwood kraft lignin when mixed with commercial polymers such as polyesters, polyolefins
and polyethylene oxide. A key requirement for processing the lignin is that it contains a minimum of volatile compounds, sugars and ash.
There is another incentive for the forest products industry to become involved in the development of the forest biorefinery. The US and Canada are in a unique position globally
because they have a very large and high quality forest resource. A recent study by the U.S. Departments of Energy and Agriculture (7) projects that US forest lands can produce nearly
400 million dry tons annually of woody biomass. Adding the available agricultural biomass gives a total of more than one billion dry tons annually. Based on this estimate, conversion
of biomass into ethanol could replace one-third of the 2004 U.S. gasoline consumption. Assuming a significant increase in mileage of U.S. cars by wide adoption of hybrid technology
as well as through other efficiency improvements, it was further envisioned (8) that in 2050 gasoline in the U.S. could be replaced entirely by ethanol as a transportation fluid. Future
use of lignocellulosic rather than starchy feed stocks for ethanol production is favored because the fossil fuel energy ratio of ethanol (energy of ethanol/fossil energy input)
produced from wood is about 10 compared to 1.36-1.76 from corn (9). The manufacture of transportation fuels from renewable biomass is of interest to society because it eliminates
dependence on foreign oil imports, reduces greenhouse gas emissions by up to 90% (see Figure 3, reference 9) and preserves and creates jobs. Other developments such as a 20
fold reduction in enzyme cost to less than $0.30/gallon for cellulose conversion in last 5 years by biotechnology and improved pretreatment processes, have further improved the
potential of the conversion of lignocellulosics into ethanol. Further research aims to reduce enzyme cost to $0.10/gallon ethanol. Because the forest products industry has the
infrastructure to manage and process biomass material, the skilled labor force, and the environmental permits, it is in a unique position to evolve existing pulp mills into forest
biorefineries that produce ethanol besides traditional pulp and paper products.
PRICES OF FEED STOCKS, ENERGY AND PRODUCTS
The prices of the new products are generally mostly controlled by the price of the feedstock and to a lesser extend by the price of energy. Shown in Table 2 are the approximate prices
of the feedstocks and main products of a sugar and lignocellulosic based biorefinery.
It shows that the feed stock price of the sugar and lignocellulosic based biorefinery, respectively corn kernels and pulp wood (Northeast US), are the same. However it should be
noted that the hardwood price may be much lower in Southern US and tropical countries such as Brazil. The yield of ethanol from corn is about 32 weight % (2.7 gallon/bushel; 1
bushel of corn is 25.4 kg at 15% moisture), while the hardwood kraft pulp yield from wood is about 50 weight %. If all the sugars of a hardwood based biorefinery would be converted to
ethanol then the ethanol yield based on wood would also be about 32% weight %. Therefore it can be concluded that at the present time it is more economical for a wood based
biorefinery to make pulp from wood at the highest yield possible, and use hemicelluloses which presently end up in the black liquor for production ethanol or other bioproducts. The
glucose price is added for comparison in Table 2 (10).
Comparison of Energy Values of Different Fuels in an Integrated Forest Biorefinery
Different fuels which are relevant in a Forest Products Biorefinery are shown in Table 3.
What is obvious from this table is that the use of oil in an IFBR should be minimized or avoided altogether since its energy cost is by far the highest. In contrast the energy cost of
biomass is almost one order of magnitude lower. Thus in an IFBR it makes sense to gasify biomass and immediately use the gasification gas as fuel in the lime kiln to replace oil. It can
also be seen that the energy cost of carbohydrates is twice as high as that of lignin, thus supporting the concept of preextracting hemicelluloses before pulping. Relatively speaking
the energy cost for black liquor is quite high because the cost of pulp wood as raw material (US$100/metic o.d. ton) is used for its calculation.
Integrated Forest Biorefinery Producing Alkaline Pulp, Bioproducts and Power
The block diagram in Figure 4 describes an example of an IFBR based on alkaline pulping. The new products are indicated in blue on the right hand side of the diagram. They are: electric
power, new wood composites, liquid fuel, ethanol, chemicals and polymers. The traditional pulp and paper products are indicated in black. The red boxes represent new processes and
process streams which need to be studied.
The diagram in Figure 4 shows that hemicelluloses are extracted from residual wood chips with a caustic solution at temperatures above 100 °C. This will produce a stream of
dissolved polymeric hemicelluloses with some contamination of lignin. The extracted chips will then be subjected to alkaline cooking to liberate the fibers and produce black liquor. Benefits
of the pre-extraction besides generation of a feed stream for new bioproducts are decreased alkali consumption, increased delignification rate, and reduced black liquor load. It is also
important to maintain or even increase the pulp yield. The hemicelluloses in the wood extract stream constitute the feed for conversion processes to produce ethanol, sugar based
polymers and chemicals. The sugar-based polymers will serve to replace fossil fuel based resins in wood composites for the development of renewable bio-composite materials. Black
liquor gasification and/or lignin precipitation are an integral part of the IFPR. The synthesis gas and precipitated lignin are the feed for respectively liquid fuel and carbon fibers.
DEVELOPMENT STATUS AND RESEARCH AND DEVELOPMENT NEEDS
Extraction Prior to Pulping
The glucoronic acid side group in hardwood xylan provides significant stability against peeling in alkaline conditions (3). Thus xylan may be extracted mostly undegraded as oligomers. It is
also required that the lignin content of the extracted hemicellulose is relatively low, and that the cellulose degree of polymerization and lignin reactivity in the extracted chips remains
high during subsequent pulping. Therefore new knowledge of the reactivity and stability of the chemical bonds which control selective hemicellulose extraction is needed. Also the
kinetics of oligomeric hemicellulose extraction from hardwoods and its effect on cellulose-lignin selectivity during pulping and quality and yield of the final pulp must be established.
The benefits of pre-extraction in terms of decreased EA charge during pulping, faster delignification rate, lower rejects and decreased organic load to the recovery boiler must
also be determined. Another important aspect is to keep the liquor to wood ratio for pre-extraction to a minimum to control evaporation cost. After extraction it may be
advantageous to concentrate the hot extract by multiple effect evaporation and/or vapor recompression.
After extraction the hemicelluloses must be converted to monomeric sugars using either acid or enzymatic hydrolysis. Other enzymes which attack aromatic lignin but not carbohydrates
may also be employed to cleave the lignin-hemicellulose bonds, since essentially all lignin is covalently bound to carbohydrates (11), and dissolved lignin will negatively affect the
ethanol fermentation process. The fermentation of the monosugars to ethanol will be performed with specially selected strains of micro-organisms. All the enzymes and micro
-organisms must be robust enough to perform in commercial-sized, continuous fermentation process equipment. The maximum theoretical yield for glucose and xylose fermentation
depends on whether aerobic or anaerobic conditions are employed. For anaerobic fermentation of glucose the maximum ethanol yield is 0.514g ethanol/g glucose based on the overall stoichiometry:
The overall stoichiometry for anaerobic fermentation of xylose is proposed (12) as:
In this case the theoretical yield is 0.511 g ethanol/g xylose. Based on xylan (or poly-anhydroxylose) removed from wood, the theoretical yield is 0.581g ethanol/xylan. However,
if cell growth is considered, the maximum ethanol yield becomes 0.26 g ethanol/g xylose (12). Under aerobic conditions the theoretical yield is 0.46 g ethanol/g xylose is based on the overall stoichiometry (12, 13):
Recent literature (14) suggests that it will be possible to ferment the dissolved hemicelluloses at 90% of the theoretical yield. A problem may be to accommodate the
several days residence time needed for the conversion of xylose compared to 1 to 2 days for glucose. Thus, efficient conversion of the xylose containing extract to ethanol will still
require significant R and D efforts in both strain and process development.
Chemicals, Polymers and Structural Materials Production
Similar to a petrochemical refinery, the number of chemicals, polymers and structural materials which may potentially be produced in an IFBR is very large. However, this number
may be reduced significantly when guided by a recent DOE study (15) which identifies the top 12 building blocks that may be produced from sugars. In addition, the selection may be
further refined for an IFBR when it is required that the final product be used in existing products such as wood composites and paper. For example the petrochemical based
unsaturated polyesters in wood composites and styrene-butadiene latex in paper coating may be replaced by polymers made from the extracted hemicelluloses. The advantages of
this constraint are that there is an immediate market mostly controlled by the IFPR, and the economics are improved because of process integration. Although there are many product
possibilities, only two examples will be mentioned here. An extensive R and D program is needed in all cases to establish large scale, continuous processes which produce a high
quality final product at a high yield and cost competitive with existing conventional products.
Itaconic acid is one of the twelve building block chemicals identified by DOE (see structure in middle of Figure 5). Itaconic acid can be produced by fermentation from C5 and C6
monomers. Subsequently itaconic acid can be converted into polymers through two major distinct routes as illustrated in Figure 5. The first route (left hand side of Figure 5) involves
the radical homopolymerization of itaconic acid to polyitaconic acid (16). Polyitaconic acid is a highly water soluble and highly hydroscopic material and may be used in paper coating to
allow optimal dispersion of the pigment for paper leveling.
The second route (right hand side of Figure 5) involves the formation by step polymerization of an unsaturated polyester from itaconic acid and a sugar-derived polyol such as propane
diol, butane diol or methyl butane diol (15). Such polymers are essentially hydrophobic and can react with vinylic monomers such as styrene and methylmethacrylate to produce tough
thermosets for usage in structural material such as wood composites and Sheet Molding Compounds (SMCs). With a price of hemicelluloses of about $40/MT, a mass yield of
hemicelluloses to itaconic acid of 50% and a conversion cost of $200/MT, itaconic acid could be produced at about $300/MT. Itaconic acid's market price in large volume is around $2400/
MT, inline with acrylates. Furthermore in the form of polymeric products IA can be expected to sell around $3000- 4000/MT depending on volume and formulation. Therefore, conversion
of hemicelluloses into polymers of itaconic acid with a price more than five times that of pulp and a price margin near 90% presents a great economic opportunity for an IFBR.
Another example is the production of carbon fibers using lignin precipitated from alkaline hardwood black liquor. Kadla and coworkers (6) have shown that carbon fibers can be made
from hardwood kraft lignin when mixed with commercial polymers such as polyesters, polyolefins and polyethylene oxide (PEO). A key requirement for processing the lignin is that
it contains a minimum of volatile compounds, sugars and ash. Since the actual spinning of the fibers occurs at a temperature of about 220oC, a minimal amount of gaseous
components should released at this temperature to avoid bubbles in the fibers and thus lower physical properties and avoid spinning problems. Thus filtration to remove particulates,
carbohydrate stripping, and washing of (almost) sulfur free lignin will be needed to obtain a suitable feed stock for carbon fiber production (18). It may be possible to produce the
blending polymers from hemicellulose sugars as described in the first example. The amount of blending polymer in the lignin-polymer blend is 25% or less. The overall weight yield of the
production of carbon fibers from pure hardwood kraft lignin or a blend with 5% PEO is about 45% (16). The automobile industry would consider large scale replacement of structural steel
in vehicles by composite material containing carbon fibers if the price is decreased to $7000/MT in order to take advantage of the potential two-third weight reduction. It has
been estimated that 10% of the US kraft lignin production is sufficient to produce enough carbon fiber to replace half of the steel in all US passenger vehicles (18). As part of their
IFBR efforts, a pilot trial by STFI in Sweden has produced 8 MT of a low-ash precipitated kraft lignin at an estimated cost of about $100/MT (4). Therefore, conversion of precipitated
hardwood kraft lignin to carbon fibers at a yield of 45 %, means that the lignin feedstock would represent only about 3% of the carbon fiber price. This price margin as well as the
large potential market as a structural material in automobiles makes this an attractive product for an IFBR. In addition the resin needed for imbedding the carbon fibers in the
structural panels could be produced from hemicellulose derived polymers.
Black Liquor Gasification
Black liquor gasification should be an integral part of an IFBR because its process heat may be used in the sugar conversion unit operations, and the synthesis gas may be used to
replace fossil fuels, in particular oil in the lime kiln. The gasification synthesis gas may be used as feedstock to produce transportation fluids such as Fisher-Tropsch liquid
hydrocarbons, methanol and mixtures of higher alcohols. The key requirement for implementation of black liquor gasification is to demonstrate the reliability and efficiency of
the technology at commercial scale while regenerating the pulping chemicals. Two demonstration plants using the atmospheric, low temperature (600oC), indirectly heated
steam reformer fluid bed technology of MTCI/ThermoChem are operating at Georgia-Pacific's Big Island, VA. Mill, and at Norampac's Trenton, ON mill (19). Progress has been made during
the start-up of these facilities over the last two years, but a number of technical barriers such as those resulting from tar formation still remain. The pressurized (32 bar), high temperature (1000oC), oxygen blown entrained flow technology of Chemrec is being tested
since late 2005 at a scale of 20 MT of dry black liquor solids/day in Pitea, Sweden. There are plans to scale up this technology to 300 MT of dry black liquor solids/day at Morrum,
Sweden in 2008 (20). Thus, more information about the reliability and efficiency of this enabling technology will become available in the near future. For the conditioning of the
synthesis gas and its conversion into transportation liquids the technologies already developed by the petrochemical industry may be used. A concern related to the production
of Fisher-Tropsch (FT) fluids is that the economic scale used by the petrochemical industry (10,000 - 20,000 MT/day of FT fluids) is more than one order of magnitude larger than the
415 MT/day of FT fluids produced from gasification of all black liquor of a mill with a nominal bleached pulp production of 1500 MT/day. (21). However, the large scale FT fluids
production by the petrochemical industry is caused by the large recycle of unconverted gas to obtain a high overall high conversion. In the case of the IFBR, recycle of unconverted gas
is not required because the gas may be used as fuel in other operations within the IFBR. Thus, for a 1500 MT/day pulp mill a once-through FT production from syngas overall
efficiency with use of unconverted gas as fuel and heat for steam production is 90% (22). McKeough and Saviharju (23) have recently shown that methanol and FT fluids would be
produced at the same cost because of improved integration of FT fluid production with pulp production. With 3 parallel Chemrec gasifiers of each 600 – 900 MT/day dry solids, one of
which is a spare unit, it is estimated by Chemrec that the liquid fuel production from black liquor would generate a cash flow equal to about 1/3 of that of the pulp (20). The spare unit
is included to improve process availability to match that of a recovery boiler.
Because the market for transportation fluids is an order of magnitude larger than that of pulp, and there are strong societal incentives to produce these fluids from renewable and
carbonneutral resources rather than fossil fuels, it may be considered to transform a hardwood kraft mill into a IFBR producing only ethanol and FT fluids and no pulp. However,
as can be seen from comparison of the total value of an existing hardwood kraft mill and that of a IFBR producing only transportation fluids in respectively Tables 4 and 5, the purely
transportation fluids IFBR does presently lead to a decrease in total product value. Of course this situation does not consider the net revenue which may lead to a more favorable
situation as has been illustrated by the analysis of Lynd et al. for a mature biorefinery of 5000 MT/day switch grass producing ethanol and FT fluids. Also, the price of the biomass
used in the IFBR producing transportation fluids is lower (US$35/ODMT) than that of pulpwood for the kraft mill (US$100/ODMT) partly because of lower quality feedstock requirements of the transportation fluids IFBR
When pulp as a product, ethanol produced from extracted hemicelluloses and FT fluids from black liquor, the total value increases by 41% as can be seen by comparison of Table 6 with
Table 4. However the total value increases by a factor of almost 3 if the IFBR produces structural materials such as carbon fibers and resins which may be incorporated into wood
composite materials, as well as FT fluids besides pulp. It can be seen in Table 7 that the much higher value is due mostly to the high-value added structural materials. However, even
if the value for carbon fiber and resin would be lower by a factor two, the total product value of the IFBR would still be a factor 2 higher than that of the present hardwood kraft mill in Table 4.
There presently exists a unique opportunity to convert economically marginal chemical pulp and paper mills into integrated forest biorefineries (IFBRs) that produce new biomaterials
besides traditional structural wood and paper products. This is due to the very strong alignment of the forest products industry need for added value with the societal,
governmental and global goals of rural development and jobs, energy self sufficiency, and control of green house gas emissions. Particularly attractive to improve the profitability of
the forest products industry is the IFBR which produces new structural products, diesel fuel and pulp. Other incentives for the forest products industry are that it preserves invested
capital, protects the core business, and preserves and creates jobs in rural forest-based communities. Considering the synergies when integrating traditional forest products and
renewable bioproducts and transportation fuels it is likely that IFBRs will be developed either by forest products and/or energy and (bio)chemicals companies.
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