Trials Conducted on the Bioregional Minimill Pulping and Black Liquor Recovery Technology

Trials Conducted on the BioRegional MiniMill Pulping and Black Liquor Recovery Technology 2005-2006

Final Report for DEFRA

by

BioRegional MiniMills (UK) Ltd

BioRegional MiniMills (UK) Ltd is a spin-out company of the BioRegional Development Group, a visionary environmental organisation dedicated to developing practical solutions for sustainable living.

Executive Summary

Autumn 2006

Table of Contents

1.Introduction

2.Pulping Trials

2.1.Initial trials at Dassett

2.2.Pulping Trials at Ahlstrom

2.3.Pulping Experimental Results

2.4.Bleaching Trials

2.5.Pulping Conclusions

3.Black Liquor Trials

3.1System Design

3.2Results

3.3Conclusions

4.Life Cycle Analysis

4.1Discussion of impacts, ‘hotspots’ and comparative results

4.2Recommendations

5.Market Research on the UK Paper Industry

5.1Introduction

5.2Market Research

5.3Findings

5.4Conclusion

1.Introduction

The MiniMill is a modular continuous paper pulping and effluent recovery process that is expected to be cost-competitive with traditional large pulp mills, at less than a tenth of the scale.

To achieve this aim a number of new areas of technology have been developed both in laboratories and at pilot plant scale.

This project has constructed and operated the new elements of the equipment at a development scale and conducted a work programme to optimise the process.

The ultimate goal of the project is to make possible the introduction of a technically and economically viable method of small-scale cellulose pulp production that will facilitate the uptake of alternative uses of wheat straw, hemp or flax in the market place.

The key part of the small scale pulping process that is missing from current technology is the ability to treat the black liquor effluent on a small scale.

The MiniMill process aims to treat the black liquor whilst recovering both energy and cooking chemicals.

This report comprises of the following chapters: Twin screw capacity trial work at Dassett (2); Cranfield feeding designs (3); Twin screw trials at Ahlstrom (4); Black liquor treatment (5); Laboratory work with Ahlstrom black liquor (6); Full scale MiniMill (7); Life Cycle Assessment (8); Paper industry market research (9). It includes the following appendices:

• Appendix 1Twin Screw Configurations
• Appendix 2Kappa Test Method
• Appendix 3SulphurGas Removal Technology
• Appendix 4Distribution of UK Graphics Paper Makers

The main objectives for each area of the work carried out are summarised below:

Pulping

• Optimise the twin screw extruder for continuously feeding and pulping fibres, test and measure the effect of different operating conditions in the new pulping system and determine the optimum pulping regime with particular regard to pulp quality.
• Determine a non-chlorine bleaching regime for straw pulp and produce bleached paper samples.
• Gather operating data to assist in the scale up to a commercial sized unit.

Black Liquor

(The trials conducted at Ahlstrom used a 400mm diameter pilot unit owned by Torftech. This unit has the advantage of being available for rental and being portable.)

• Investigate the potential to run the plant with a hot gas recycle.
• Examine agglomeration and sintering issues inside the reactor.
• Investigate feeding mechanisms for introducing black liquor onto the bed.
• Measure the volume and constituents of the off gas produced.
• Examine the spent bed material with regards to chemical recovery potential.
• To use the 400mm diameter pilot unit to provide information to enable a larger unit to be specified and designed.
• To gather operating data to assist in the scale up to a commercial size unit.
• To demonstrate the plant running to potential investors.

Life Cycle Analysis

• Carry out a quantitative analysis of the environmental impacts associated with processing materials in a MiniMill including direct burdens from the process and transport, indirect burdens from “background” operations (providing energy and ancillary materials), and avoided burdens displaced by recovered materials and energy.
• Identify the “hot spots” in the process giving rise to disproportionate environmental impacts, and identify strategies to reduce these impacts.
• Provide information on the scale of operation appropriate for mini-mill.

Market Research

• To maintain and build on current contact links with the industry
• To build an ongoing idea of attitudes within the industry towards straw paper
• To identify areas of concern to the industry and establish ways in which these can be addressed
• To raise awareness of the BioRegional MiniMill and build support for it

2.Pulping Trials

2.1.Initial trials at Dassett

The first tests of the 83mm diameter twin screw were conducted on site at Dassett. There were three sets of trials conducted at Dassett with the twin screw being reconfigured between each set. In total twelve sets of experimental conditions were evaluated.

The purpose of the trials was to investigate the performance of the twin screw and provide design data for the feeding mechanism and its installation at Ahlstrom.

For reasons of safety and waste disposal no cooking chemicals were added during the trials at Dassett.

It was identified in the initial stages of the project that one of the key issues was to get a system that allowed straw to be fed effectively into the twin screw.

For this reason special new flight sections were designed and manufactured to provide a large open area for feeding straw. It was also anticipated that some dilution might be required in the final stages of the twin screw and mixing elements were made to facilitate this.

As an initial configuration the twin screw was set up in its original configuration but with the modified feeding section at the start and the mixing section at the discharge end.

The main objectives were:

• To assess potential throughput of twin-screw
• To evaluate power requirements against throughput
• To establish what pressures can be achieved in the unit
• To find out how easily the straw feeds into the twin-screw
• To check the heating capacity of the heating elements
• To test effectiveness of new mixing elements

Straw samples were prepared through an off site hammer mill identical to the one purchased for the project. The hammer mill had a basket fitted with rectangular holes 140 x 90mm. This straw was used for all the trials at Dassett unless otherwise stated.

This photograph shows an early pulp sample from the initial runs. Water was being added at mains pressure and the pulp produced was too dry and had a fluffed appearance.

2.2.Pulping Trials at Ahlstrom

The pulping system installed at Ahlstrom consists of a raw material preparation section, a feeding unit, the twin screw extruder and a post cooking vessel. A diagram of the system is shown below.

Diagram of Pulping System

The straw is fed into the hammer mill unit via a feed conveyor, the speed of which is controlled from the twin screw area.

The straw is drawn into the unit by a number of rotating knives and is then forced through the screen basket by rotating hammers. The screen basket is a specially made large basket with rectangular holes 140 by 90mm.

With this large basket the mechanical work done on the fibre is fairly light and an analysis of the dust content after the hammer mill gave figures around 7%. The hammer mill uses around 30kW of power when running.

The straw that has passed through the screen is then fed into a fan that conveys the straw down a 150mm diameter duct to the separator at the twin screw.

The straw reaches the separator where the air velocity drops allowing the straw to drop out of suspension and fall into the feeding unit. The dust is extracted out of the separator top and removed in a filter before discharging to atmosphere.

The early work at Dassett had identified that feeding sufficient straw into the twin screw was an issue, and that a mechanical feeding mechanism was required that could feed significantly more straw than was possible by hand.

A feeding unit was designed and manufactured by CranfieldUniversity with the objective of providing a consistent high flow of straw into the twin screw. This feeding unit operates by forcing straw onto the flights of the twin screw via two pneumatically operated rams.

This system of feeding straw has suffered from issues of plug formation. This happens in the feeding zone on top of the twin screw flights and once a plug has formed the feed rate drops to zero.

The ram feeding will only work if the straw is chopped extremely short by putting a basket with 10mm diameter holes into the hammer mill. Under these conditions the pneumatic feeding unit will feed straw at a rate about 30% higher than can be achieved by hand. However, the small holes in the hammer mill cause extensive fibre damage and a dust analysis of the straw after this treatment showed the dust content had increased from 7% to 34%.

Consequently all the experimental trials which aimed at producing pulp suitable for paper production were undertaken utilising hand-fed straw. The straw is put through the hammer mill with the large basket fitted and then collected in sacks, where it was stored in batches prior to processing in the twin screw.

The straw is then fed down a co-rotating twin screw extruder. This is a small, compact, continuous device that could be installed as an alternative pulping technology. One of the main advantages is that the pulping is conducted with low amounts of water, making the production of black liquor at higher concentrations possible. This is beneficial in the black liquor treatment stage, as it reduces the need for evaporation.

The co-rotating nature of the twin screwis beneficial in giving the fibre less mechanical treatment and thus minimising damage to the fibres. The material in the barrel travels in a figure of eight-shaped path, and thus takes a longer route than if the screws were counter-rotating.

The final part of the pulping system is a cooking vessel. This is used to provide further cooking of the pulp samples after processing in the twin screw.

The main pulping trials were divided into five experimental programmes, the objectives of which were:

• Experiment 1 – Familiarisation with the installed twin screw system and feeding trials
• Experiment 2 – Investigation into the effect of speed and temperature
• Experiment 3 – Investigation into the effect of caustic concentration
• Experiment 4 – Investigation into the effect of changing the twin screw’s internal profile
• Experiment 5 – Investigation into the effect of post cooking after the twin screw

2.3.Pulping Experimental Results

Experiment 1

During this phase a number of different conditions and configurations of the feeding systems were investigated.

The decision was taken to conduct the main experimental programme feeding the twin screw by hand for the reasons discussed above.

Experiment 2

A full factorial experimental matrix was conducted to investigate the effect of speed and temperature on the degree of pulping taking place in the twin screw.

The speed was varied in the range 20 to 180 rpm and temperature was varied between 120 and 180 degrees Centigrade.

The pulp samples were tested for the residual lignin (a measure of the degree of chemical pulping taking place) using the Kappa number test.

At the very slow speed, 20rpm, the twin screw did not feed well and erratic results were obtained. At all other speeds and temperatures there was no statistically significant difference between the amounts of chemical pulping taking place at the different conditions.

The relationship between throughput and speed was examined and is shown in this graph.

It can be seen that throughput increases in a reasonably linear manner up to 100rpm. Above this the throughput increases less quickly with speed.

This is probably due to inefficiencies in hand feeding at the higher speeds, although it is possible that a slippage effect is coming into play at the higher speeds. This is a known problem when feeding a twin screw with plastic.

An analysis was conducted to see if there is a relationship between speed and specific power (kWHr/Tonne of straw).

The relationship between speed and absorbed power was also investigated and found to be linear as shown in this graph.

These two graphs indicate that the specific power (kWHr/Tonne of pulp) is less good at speeds above 100 rpm, because the absorbed power rises at a higher rate than the production.

To confirm the surprising lack of effect of temperature further trials were conducting increasing the twin screw temperature to 200 deg. C. This also showed no difference in exiting Kappa number.

For these reasons a standard set of conditions of 100 rpm and 140 deg. C. were adopted for future trial work.

Experiment 3

Experiment 3 consisted of the data from Experiment 2, together with some additional runs where the caustic concentration was changed.

The purpose of the experiment was to investigate if varying caustic concentration affects the Kappa number after the twin screw.

Caustic additions were limited to a number that would be economically viable in a full scale process.

The results showed that caustic concentration does have an effect, but that it is not yet possible to get to a full chemical pulp in the twin screw without using uneconomic concentrations of sodium hydroxide.

Experiment 4

After this initial set of experiments the twin screw was taken apart and reconfigured with more tri-lobal elements.

This had a dramatic effect on the quantity of reject after the twin screw, as shown in this graph.

The average reject dropped from around 50% to less than 10% as a result of the rebuild. However, the rebuild made no statistical difference to the Kappa number of the screened fibre after the twin screw.

An analysis of specific energy consumption was conducted to establish the difference between the twin screw before and after its rebuild, to increase the mechanical pulping. This showed that on average changing the twin screw configuration increased the energy consumption from 276 kWHr/Tonne of straw to 318kWHr/Tonne.

The lowest energy recorded during the trials was 170 kWHr/Tonne.

Experiment 5

A set of experiments was now conducted to investigate the effect of post cooking on the fibre produced in the twin screw.

This graph shows that post cooking reduces the Kappa number to about 30 in around 20 minutes. However, to reach Kappa numbers as low as 14 takes two hours.

A full chemical pulp would normally have a Kappa number of around 15. But work done by Trevor Dean and confirmed by the University of Washington suggests that Kappa numbers this low can not be achieved without damaging the fibre when straw is the raw material. Future work will investigate these avenues.

Some work was also undertaken to investigate whether adding caustic either in the twin screw or in the cooking vessel, or a combination, affected the resulting Kappa number. No statistical difference as to where the caustic was added was demonstrated by this work.

2.4.Bleaching Trials

The University of Washington Paper Science Centre was contracted by BioRegional MiniMills to perform a bleaching study on wheat straw pulps supplied from the BioRegional Process.

Preliminary investigations into a totally chlorine free bleaching process (TCF), and an elementally chlorine free (ECF) process were performed to establish baseline data and information.

Subsequently, a more in-depth study was completed on both processes using a variety of bleaching sequences to determine a possible optimum. An optimum was never achieved but there were good indications of the direction to continue research, or process trials.

TCF bleaching has an adverse effect on the strength properties of the bleached pulp samples. This is due to the peroxide used in TCF bleaching. This is further evidenced by a comparison of the ECF samples.

There is no statistical difference in the strength of the five stage ECF bleaching sequences. The increase in brightness from 87.7 to 89 has substantial economic consequences. It would be cheaper to manufacture a 90- brightness paper from 87.7 brightness pulp with the addition of high brightness fillers than it would be to bleach it to 90 brightness.

The results showed that it is more economic to use ECF than TCF to achieveadequate brightness, and so most of the bleaching work investigated ECF bleaching.

Using ECF bleaching, 85 brightness with good strength was achieved utilising a three-stage process. With four-stage bleaching 87 brightness was reached. The trials utilising peroxide in the final stage showed reductions in strength, with minimal or no increase in brightness.

The work showed that 90 brightness wheat straw pulp is a viable possibility using BRMM unbleached pulp. There are changes that need to be made to the pulping process.

Pre-processing of the wheat straw has been shown to have better pulp yields and strength. The

removal of the leaves and nodes gives more efficient use of the pulping chemicals for attacking the inter-nodal portion of the straw. The use of a delignifying agent along with the sodium hydroxide in the pulping has shown a more efficient use of the bleaching chemicals. The University of Washington process is an unpressurised process using pre-processed wheat straw. This technique generates stronger, brighter pulp. A version of the BRMM process along with the UoW process could possibly generate a better quality pulp than each individual process.