Technical Background Document

Hemicellulose Extraction Subtask

Pretreatment Core R&D Interim Stage Gate Review Meeting

June 9-10, 2005

Session 1

## 

## Introduction

In 1994, the American Forest & Paper Association (AF&PA) signed a
compact with the Department of Energy titled "Agenda 2020". The goal of
the compact was to develop and employ technology improving the pulp and
paper industry's economics (1,2). The panel identified six technology
platforms leading to the compact's goal of improving the economic health
of the pulp and paper industry. One of the most potentially promising is
the forest biorefinery platform. At the most basic level, the forest
biorefinery seeks to gain value from the historically discarded portion
of the tree. One of the processes considered involves extracting value
from wood chips prior to pulping, otherwise known as "Value Prior to
Pulping." In this scenario, a portion of the hemicellulose is extracted
under very mild cooking conditions to recover that fraction in the form
of soluble oligomers and monomers in a relatively clean, dilute liquor.
Ideally, the fraction of hemicellulose that is removed during this
pre-extraction corresponds to the same proportion that is typically lost
to the black liquor during typical kraft pulping . The resulting
pre-extracted wood chips may be easier to pulp, requiring lower pulping
residence times and lower pulping chemicals usage. The key to the
process is to preserve the same pulp quality and pulp yields of the
extracted chips compared to non-extracted chips or, if there is a loss
in pulp yield, a revenue source from co-product sales (or cost savings
from a less expensive pulping process) greater than the corresponding
revenue loss from lower pulp yields would result. The new revenue source
would be derived from sales of fuels and chemicals generated from the
pre-extraction liquor. The ultimate intent is to evolve current pulp
mills into forest biorefineries, thereby preserving existing
infrastructure, jobs, and supply chains (2).

The results from initial techno-economic modeling conducted in FY04 at
NREL suggested that extracting hemicellulose sugars from wood chips,
followed by fermentation to ethanol would results in revenues to create
a payback period in less than three years if the following could be
achieved:

-   Convert on the order of 50% of the hemicellulose of both hardwoods
    and softwoods to fermentable sugars without negatively affecting the
    properties of the resulting pulp.

-   Organisms are available or are developed which function well in the
    pre-extraction liquor without the need for significant conditioning
    of the liquor prior to fermentation.

-   Organisms are available or are developed which give high conversions
    of resulting hexose and pentose sugars to ethanol.

-   Utility costs (steam) are low, presumably from access to relatively
    inexpensive excess steam available from an existing pulp mill.

These findings were in line with the AF&PA vision of the "Value Prior to
Pulping" concept (1, 2), although many of the AF&PA analyses also assume
additional revenue from acetic acid that is recovered from the
pre-extraction liquor.

In August, 2004, a working group involving several pulp and paper
company representatives, university researchers, and USDA Forest
Products Laboratory (FPL) researchers met at NREL to discuss options for
doing further research and development work on the Value Prior to
Pulping concept. While some preliminary experimental work on the concept
has been performed at both the University of Maine and the State
University of New York College of Environmental Science and Forestry
(SUNY ESF), the pulp and paper companies expressed the desire for NREL
to generate further pre-extraction data on up to three wood species
(sugar maple, spruce, and loblolly pine), along with some process
economic modeling to project ethanol production costs from available
sugars in the pre-extraction liquor. In addition, tentative plans were
made for the USDA FPL to conduct appropriate pulping trials on the
pre-extracted chips generated at NREL. Finally, plans were discussed for
the development of a proposal to the DOE Office of the Biomass Program
(OBP) involving several pulp and paper companies, SUNY ESF, and NREL,
with significant cost share coming from the pump and paper company
participants. Subsequently, the DOE Office of the Biomass Program (OBP)
committed funds from the Pretreatment and Enzymatic Task FY05 Annual
Operating Plan (AOP) to conduct the initial pre-extraction runs and
associated chemical analysis at NREL, along with funds for the process
modeling work. Initial planning and preparatory work was begun in
November, 2004, with the initial experimental work for the
pre-extraction of maple chips beginning in December, 2004. Experimental
work for the pre-extraction of spruce chips beginning in February, 2005.

## Methods and Materials

### Wood Chips

In December 2004, the State University of New York College of
Environmental Science and Forestry (SUNY ESF) arranged for harvesting of
several sugar maple trees from a local tree farm in Lafayette, NY. SUNY
ESF debarked and chipped these trees in relatively small scale research
equipment and shipped the resulting chips to NREL. Upon arrival at NREL,
the maple chips were mixed via a cone and quarter method to provide
uniformity across the large chip quantity. Four random samples were
taken and subsequently analyzed for chemical composition using NREL
Standard Laboratory Analytical Procedures. The chips were stored
outdoors (frozen) in drums with plastic liners and were thawed prior to
experimental use.

Spruce chips were provided by Stora Enso North America from a commercial
pulp mill in Wisconsin Rapids, WI. Upon arrival at NREL, the spruce
chips were mixed via a cone and quarter method to provide uniformity
across the large chip quantity. As opposed to the maple chips, which
were quite uniform and contained very few fines, the spruce chips came
from a commercial debarking and chipping operation and contained
significant amounts of bark, large oversized chips, knots, and fines.
After coning and quartering, the majority of the oversized chips, bark,
and knots chips were removed via hand picking and the majority of the
fines were removed using a vibrating --1/4-inch sieve. Despite this,
there was particle size variability in the spruce chaps as compared to
the maple chips. Five random samples were taken and subsequently
analyzed for chemical composition using NREL Standard Laboratory
Analytical Procedures. The chips were stored outdoors (frozen) in drums
and were thawed prior to experimental use.

[Recirculating Chip Extractor]{.underline}

A Recirculating Chip Extractor (RCE) was fabricated at NREL specifically
for use in the pre-extraction experiments. This was designed as a larger
scale version of a M/K recirculating laboratory digester (M/K Systems,
Danvers, MA) that is often used in bench-scale pulping trials. One of
the existing Pneumapress filter filtrate receivers (originally used as a
flash tank for the countercurrent pretreatment reactor system) in the
Process Development Unit (PDU) at NREL was used as the shell of the RCE.
This vessel has an ASME Maximum Allowable Working Pressure Rating of 100
psig, so a pressure relief valve set to relieve at just under 100 psig
was installed, with venting through the PDU south exterior wall. The
NREL machine shop fabricated two removable 316 stainless steel chip
baskets (approximately 10 inch diameter by 50 inch height) with a 1/16
inch screen plate on the bottom of the basket to allow for recirculation
of liquid. A ¼ inch thick liquid distributor plate with several ¼ inch
holes was also fabricated, which rests on top of the chip bed and keeps
the chip bed slightly compressed as the chips are solubilized and the
bed height shrinks during the pre-extraction run. A chip basket lifting
device was also fabricated, which allows for connection to a hoist to
insert and remove the full chip basket into and from the RCE vessel.

A heat exchanger (borrowed from the PDU horizontal pretreatment reactor
system) was plumbed into the recirculation loop to allow for indirect
steam heating of the recirculating liquor. Temperature control was
provided through the PDU Data Acquisition and Control System (DACS)
system, via an automatic control valve for steam addition to the heat
exchanger. Two RTD temperature probes were installed (one in the
recirculation loop at the bottom of the RCE and one in the recirculation
loop at the top of the RCE). DACS programming was conducted to provide a
graphical representation of the operation of the system and to
automatically control temperature at a selected setpoint, with trend
charts for temperature readings and control valve position, along with
adjustable PID control parameters. Automatic data logging of temperature
and recirculation flow rate was also implemented. Liquor recirculation
was provided via a 0.5 HP centrifugal pump (Emerson (Model BU60A), St
Louis, MO) with a maximum flow rate of about 15 L/min. A photograph of
the RCE system is presented in Figure 1.

![](media/image1.wmf){width="3.08740157480315in"
height="3.8751968503937007in"}

Figure 1. The Recirculating Chip Extractor (RCE) system installed at the
NREL PDU.

[Pre-Extraction Experiments]{.underline}

A range of pre-extraction conditions for both maple and spruce chips
were provided to NREL by SUNY ESF under the terms of a Non-Disclosure
Agreement with NREL that was signed in December, 2004. For this reason,
specifics on the actual pre-extraction reaction conditions used in for
the maple and spruce chip experiments will not be disclosed in this
report. Published literature that discusses conceptually similar
pre-extraction reaction conditions for various hardwood and softwood
chips are listed as references (3-5).

Generally, about 15 kg (wet basis) of wood chips were loaded into the
chip basket and inserted into the RCE vessel. A prescribed quantity of
reaction liquid was added to the RCE and the distributor plate was
placed on top of the chip bed. The top flange was bolted on the RCE
vessel and the thermocouples were inserted into the recirculation loop.
The pump was started and the recirculating liquid was heated via the
indirect steam heat exchanger. The recirculating liquid flowed downward
through the chip bed and upward through the heat exchanger and back to
the top of the RCE. The temperature set point was slowly increased
manually during the heat-up time and the pump rate was set at a flow
rate to prevent pump cavitation and uneven heating. About 30-60 minutes
of heat-up time was generally required to achieve the target
temperature.

The start of the prescribed hold time began when the target temperature
was reached. At the end of the hold time, the pump and heat exchanger
was shut off and the liquor transferred to a flash tank (Jaygo reactor)
using the residual pressure on the RCE. The flange on the RCE was the
removed and the basket was lifted from the RCE vessel and weighed. The
chips were poured into a large tray and were gently mixed before
sampling. The collected liquor was analyzed for sugar (monomer and
oligomer), acetic acid, furfural, and HMF concentration using standard
NREL Laboratory Analytical Procedures. Dry weight of the extracted chips
and liquor were measured in order to calculate mass loss of the chips
resulting from the pre-extraction. The chips were refrigerated prior to
shipping to the USDA FPL for pulping tests.

**Results and Discussion**

[Pre-Extraction of Maple Chips]{.underline}

The feedstock compositional analysis for the maple chips is presented in
Table 1. This composition represents an average of the 4 random
feedstock samples taken, which were found to have excellent
reproducibility.

> Table 1. Maple feedstock composition.

  ------------------------------------- ---------------------------------
  **Component**                         **% (wt/wt)**

  Glucan                                43.4

  Xylan                                 16.5

  Galactan                              1.1

  Arabinan                              0.6

  Mannan                                2.9

  Acetate                               4.1

  Lignin                                27.0

  Extractives                           3.3

  Ash                                   0.2

  Total                                 99.1
  ------------------------------------- ---------------------------------

### 

Four different pre-extraction conditions were conducted on the maple
chips. As described above, guidance for selection the specific
pre-extraction conditions was provided under the terms of a
Non-Disclosure Agreement between NREL and SUNY ESF and are not presented
in this report for this reason. In order of increasing severity, these
runs are labeled as Run 4, Run 2, Run 1, and Run 3, respectively. The
conversion of xylan and glucan to sugar and degradation products in
shown in Figures 2 and 3. Yields are calculated based upon the measured
original xylan and cellulose content of the maple chips and is reported
on a monomer sugar, oligomer sugar and total (monomer + oligomer) sugar
basis. Photographs of the pre-extracted chips are shown in Figure 4 and
indicate some color difference in the extracted chips for the different
pre-extraction conditions as a function of severity. The overall
gravimetric mass loss resulting them each pre-extraction condition is
shown in Table 3. Mass loss generally increases as the pre-extraction
severity increases. The mass loss is used to determine the chip yield
resulting from pre-extraction. In order to maintain high pulp yields, it
is desirable to maintain high pre-extraction chip yields.

![](media/image2.wmf){width="5.995138888888889in"
height="3.477777777777778in"}Figure 2. Conversion of xylan to total
(oligomer + monomer) xylose, oligomeric xylose, monomeric xylose, and
furfural from pre-extraction of sugar maple chips.

![](media/image3.wmf){width="5.991666666666666in" height="3.48125in"}

Figure 3. Conversion of glucan to total (oligomer + monomer) glucose,
oligomeric glucose, monomeric glucose, and HMF from pre-extraction of
sugar maple chips.

![](media/image4.png){width="5.5in" height="1.75in"}

Figure 4. Photograph of the extracted chips from the 4 different
pre-extraction conditions on sugar maple chips (increasing
pre-extraction severity from left to right).

Table 2. Overall mass loss resulting from pre-extraction of maple chips.

  --------------------------------- -------------------------------------
  **Run**                           **Mass Loss (% of original chip mass
                                    removed)**

  Run 4                             6.3

  Run 2                             11.6

  Run 1                             18.9

  Run 3                             21.4
  --------------------------------- -------------------------------------

The vast majority of released xylose and glucose are in the oligomeric
form and would require further thermal, chemical, or enzymatic
conversion to produce monomeric sugars before fermentation. In Run 1,
the highest yield of recovered xylose was obtained. When accounting for
both the collected liquor and the remaining liquor entrained in the
extracted chips, about 53.4% of the xylan content was recovered as total
(monomeric + oligomeric) xylose. Approximately 40% of the total
extracted xylan was in the collected liquor fraction and 13% in the
entrained liquor. Furfural yield from xylan was 8.5% for a total removal
of 61.9% of the total xylan available in the maple chips. In Run 3,
total extracted hemicellulosic sugars dropped to 47.2% and the xylan to
furfural conversion increased to 29.5% (although monomeric xylose yields
were about twice as high in Run 3 as compared to Run 1). The higher
losses of xylan to furfural suggest that the Run 3 extraction conditions
were too severe. While the yields of glucose (on the basis of original
glucan content in the feedstock) are much lower than the respective
xylan yields, the quantity of recovered monomeric and oligomeric glucose
represents about 20% of the recovered monomeric and oligomeric xylose,
as the glucan content in the maple chips is nearly 3 times the xylan
content. The low conversion of glucan is desired, as the glucan content
of the wood chips has higher value for pulp production than for sugar
production.

### Pre-Extraction of Spruce Chips

The feedstock compositional analysis for the spruce chips is presented
in Table 3. This composition represents an average of the 5 random
feedstock samples taken, which were found to have excellent
reproducibility.

> Table 3. Spruce feedstock composition.

  ------------------------------------- ---------------------------------
  **Component**                         **% (wt/wt)**

  Glucan                                42.6

  Xylan                                 7.4

  Galactan                              2.8

  Arabinan                              1.7

  Mannan                                11.2

  Acetate                               1.5

  Lignin                                28.4

  Extractives                           3.2

  Ash                                   0.5

  Total                                 99.3
  ------------------------------------- ---------------------------------

######## 

Five different extraction conditions have been completed using the
spruce chips provided by Stora Enso, following the removal of oversized
chips and fines, as described in the Materials and Methods section.
Approximately 20 kg (dry basis) of spruce chips for each of the five
conditions (2 runs for each condition) was generated. Extracted spruce
chips, along with unextracted control chips, were provided to the USDA
Forest Products Laboratory in Madison, WI for pulp yield and pulp
quality tests, which are currently being conducted.

As described above, guidance for selection the specific pre-extraction
conditions was provided under the terms of a Non-Disclosure Agreement
between NREL and SUNY ESF and are not presented in this report for this
reason. Due to the low acetate content in the spruce feedstock, three of
the five runs included the addition of relatively low concentrations of
added acetic acid to result is a slightly lower pH than occurs in a
water-only pre-extraction. In order of increasing severity, the spruce
extraction runs are labeled as Run 5, Run 1, Run 3, Run 2 and Run 4,
respectively. In addition to xylan and glucan yields, mannan yields were
also determined due to the high mannan content in the spruce chips. The
conversion of xylan, mannan, and glucan to sugars and degradation
products is shown in Figures 5, 6, and 7, respectively. Yields are
calculated based upon the measured original xylan, mannan, and glucan
content of the spruce chips and is reported on a monomer sugar, oligomer
sugar and total (monomer + oligomer) sugar basis. The overall
gravimetric mass loss resulting them each pre-extraction condition is
shown in Table 4. Mass loss generally increases as the pre-extraction
severity increases. The mass loss is used to determine the chip yield
resulting from pre-extraction. In order to maintain high pulp yields, it
is desirable to maintain high pre-extraction chip yields.

![](media/image5.wmf){width="5.995138888888889in"
height="3.5215277777777776in"}Figure 5. Conversion of xylan to total
(oligomer + monomer) xylose, oligomeric xylose, monomeric xylose, and
furfural from pre-extraction of spruce chips.

![](media/image6.wmf){width="5.992361111111111in"
height="3.3965277777777776in"}

Figure 6. Conversion of mannan to total (oligomer + monomer) mannose,
oligomeric mannose, monomeric mannose, and HMF from pre-extraction of
spruce chips.

![](media/image7.wmf){width="5.99375in"
height="3.520138888888889in"}Figure 7. Conversion of glucan to total
(oligomer + monomer) glucose, oligomeric glucose, monomeric glucose, and
HMF from pre-extraction of spruce chips.

Table 4. Overall mass loss resulting from pre-extraction of spruce
chips.

  --------------------------------- -------------------------------------
  **Run**                           **Mass Loss (% of original chip mass
                                    removed)**

  Run 5                             10.3

  Run 1                             13.0

  Run 3                             11.0

  Run 2                             5.2

  Run 4                             16.8
  --------------------------------- -------------------------------------

The majority of released sugar from the various spruce extraction
conditions is in oligomeric form and would require further thermal,
chemical, or enzymatic conversion to produce monomeric sugars before
fermentation. In Run 2, the highest yield of recovered xylose was
obtained. When accounting for both the collected liquor and the
remaining liquor entrained in the extracted chips, about 37% of the
xylan content was recovered as total (monomeric + oligomeric) xylose.
However, furfural yields from xylan were high in all spruce
pre-extraction conditions, ranging from about 24% of the original xylan
content in the least-severe Run 5 condition to about 55% of the original
xylan content in the most severe Run 3 condition. These results suggest
that even the least severe condition tested did not achieve good
recovery of solubilized xylan without high sugar degradation losses.
Even though there is a higher proportion of oligomers to monomers in the
least severe Run 5 condition, there is a much higher proportion of
xylose monomers when compared to most of the maple pre-extraction runs,
where lower losses to furfural were generally observed. The results also
suggest that it may not be possible to achieve high xylose sugar yields
(above 30% of the original xylan content) as well as low losses of
xylose to furfural for the spruce chips under the range of general
pre-extraction conditions tested.

Mannose yields were significantly higher than xylose yields for the
spruce chip extraction conditions tested. The mannose yields were
relatively insensitive to extraction condition severity, with recovered
total xylose yields ranging from about 49% to about 58% of the original
xylan content. Again, the vast majority of recovered xylose was
generally in oligomeric form, with the exception of the most severe Run
3 condition, where a relatively equal split between oligomeric and
monomeric mannose was obtained. Importantly, mannose sugar degradation
losses to HMF were very low throughout the range on spruce
pre-extraction conditions tested.

As was observed with the maple chips, conversion of glucan in the spruce
chips was very low, with very low losses of recovered glucose sugars to
HMF. The low conversion of glucan is desired, as the glucan content of
the wood chips has higher value for pulp production than for sugar
production.

[Process Economic Modeling]{.underline}

## 

In preparation to analyze the pre-extraction data, a process economic
model was previously developed using Aspen Plus^®^ for the material and
energy balances. A simplified, existing process design for the corn
stover to ethanol process was used to model the hemicellulose extraction
and resulting conversion to ethanol. One key feature of the process was
that only ethanol was assumed to be a product (that is, no extra value
from a co-product, such as acetic acid, was examined). This would show
whether the extraction process could be economically justified with
revenues generated only from ethanol. Also, the model only included the
extraction and conversion process, assuming utilities would be supplied
from the existing pulp mill. The "cost" (or internal transfer values)
for these utilities would be site and plant specific.

A range of expected utility and enzyme costs were assumed to determine
their effect on the process economics. Table 5 shows the range of
utility costs examined. The "Very Optimistic" values assume excess steam
available at essentially no cost and cooling water available from an
adjacent river. The "Less Optimistic" values are more indicative of
costs from a new natural gas boiler and cooling tower.

Table 5. Range of utility and enzyme cost assumptions used in
hemicellulose pre-extraction process economic modeling.

![](media/image8.wmf)

Results from the process economic analysis using sugar yields obtained
from the maple pre-extraction runs are summarized in Table 6. The
economic viability of the hemicellulose pre-extraction process is
strongly dependent on the utility costs. Changing these costs over a
relatively small range can drive the process from being economic (with a
fairly short payback period) to being uneconomic. Low hemicellulose
conversion (as represented by Run 4) results in a small net annual
revenue and a long capital payback period, even under the Very
Optimistic scenario. Higher sugar yields (and assumed resulting ethanol
yields) in the other runs result in higher revenues from ethanol and
shorter capital payback times, but even in those cases, the less
optimistic economic assumptions do not result in a positive net annual
revenue. Additionally, this analysis does not account for any potential
effects on pulp yield and pulp quality. Presumably, the higher severity
conditions where high losses of xylan to furfural occurs (i.e. Run 3)
may result in lower pulp yields, so the less severe runs where
reasonably high sugar (and resulting ethanol) yields are coupled with
relatively low overall mass losses (Table 2), such as in Runs 2 or 1,
may provide the best overall economic opportunity.

Table 6. Economic analysis results using xylose and glucose yields from
four maple pre-extraction runs.

Results from the process economic analysis using sugar yields obtained
from the spruce pre-extraction runs are summarized in Table 7. As for
the maple runs, the economic viability of the spruce hemicellulose
pre-extraction process is strongly dependent on the utility costs.
Changing these costs over a relatively small range can drive the process
from being economic (with a fairly short payback period) to being
uneconomic. Over the range of process conditions tested, the projected
ethanol yields and resulting economics are relatively insensitive due to
the fact that sugar yields (especially mannose) were relatively
constant. As with the maple pre-extraction, this analysis does not
account for any potential effects on pulp yield and pulp quality.

![](media/image9.wmf){width="8.125in"
height="1.7270833333333333in"}Table 7. Economic analysis results using
xylose and glucose yields from four maple pre-extraction runs.

## 

**Conclusion**

Approximately 20 kg (dry basis) of maple chips were extracted for each
of the four conditions (2 runs for each condition). The extractions were
performed in a recirculating chip extractor with a basket strainer,
recirculation pump, and indirect steam heat exchanger that was assembled
using existing equipment in the PDU at NREL. The experimental chip
extraction conditions produced a wide range of hemicellulosic sugar
removal and sugar degradation losses, depending upon reaction
conditions. Generally recovered xylose sugars ranged from 20-40% of the
feedstock xylan content, although over 50% recovery if possible if the
liquid entrained in the chips is accounted for. Most of the xylose was
recovered in oligomeric form. Small amounts of glucose (primarily in
oligomer form) was also released, but only representing about 3% of
feedstock glucan content. Extracted maple chips, along with unextracted
control chips, were provided to the USDA Forest Products Laboratory in
Madison, WI for pulp yield and pulp quality determination.

A second hemicellulose pre-extraction campaign using softwood (spruce)
chips has also been conducted. Five different extraction conditions have
been completed, generating approximately 20 kg (dry basis) of spruce
chips for each of the five conditions (2 runs for each condition).
Generally recovered xylose sugars ranged from 30-40% of the feedstock
xylan content, although high losses of xylan to furfural were observed,
even in the least severe condition tested (over 20% conversion of xylan
to furfural). Most of the xylose was recovered in oligomeric form,
although greater proportions on monomeric xylose were seen in the spruce
pre-extraction liquors as compared to the maple pre-extraction liquors.
Relatively high yields of mannose (generally about 50-60% of the
original mannan content) were obtained, with almost all of the released
mannose recovered in oligomeric form, except at the most severe
conditions. Only small losses of mannose to HMF were observed. Small
amounts of glucose (primarily in oligomer form) was also released, but
only representing about 3% of feedstock glucan content. Extracted maple
chips, along with unextracted control chips, were provided to the USDA
Forest Products Laboratory in Madison, WI for pulp yield and pulp
quality determination.

Initial process economic analysis have been conducted using the maple
and spruce pre-extraction data and indicate that assumptions involving
utilities costs for the chip extraction process have a significant
impact on the projected Minimum Ethanol Selling Price (MESP), Capital
Expense (CAPEX), and capital payback period. Using the amore optimistic
assumption set does result in potentially attractive ethanol production
economics and short capital payback times. In many respects, these
assumptions, along with any impacts that the pre-extraction operation
may have on pulping economics and yields, will be of greater importance
than ethanol yields and revenues resulting from a pre-extraction of wood
chips

**References**

1\. Thorp, B., Raymond, D. Agenda 2020 Reachable Goals Can Double P&P
Industry's Cash Flow. Paper Age, Sept. 2004.

2\. Thorp, B., Raymond*,* D*.* Forest Biorefinery Could Open Door To
Bright Future for P&P Industry. Paper Age, Oct. 2004.

3.  Elmore, C. Method of Producing Kraft Pulp Using and Acid
    Prehydrolysis and Pre-extraction, U.S. Patent 4,436,586, March 12,
    1984.

4.  Simmonds, F.A., Kingsbury, R.M., Martin, J.S. Purified Hardwood
    Pulps for Chemical Conversion, II. Sweetgum Prehydrolysis-Sulfate
    Pulps. TAPPI, 38:3, 178-186, March, 1955.

5.  Simmonds, F.A., Kingsbury, R.M., Martin, J.S., Mitchell. Loblolly
    Pine High Alpha Prehydrolysis-Sulfate Pulps. TAPPI, 39:9, 641-647,
    September, 1956.