WO2012136251A1

WO2012136251A1 - Method of controlling pulp production and a control system therefor

Info

Publication number: WO2012136251A1
Application number: PCT/EP2011/055277
Authority: WO
Inventors: Alf Isaksson; Lars Ledung; Simo Saynevirta
Original assignee: ABB Research Ltd Switzerland
Current assignee: ABB Research Ltd Switzerland
Priority date: 2011-04-05
Filing date: 2011-04-05
Publication date: 2012-10-11
Anticipated expiration: 2013-10-05

Abstract

The present invention relates to a method of controlling the production of pulp in a pulp mill for supplying pulp and steam to a paper mill. The method comprises the steps of receiving (S1) energy price data pertaining to an expected variation in energy price over a time period; determining (S2) a pulp production plan over the time period based on the energy price data and an amount of paper to be produced by the paper mill during the time period; and controlling (S3) the production of pulp in the pulp mill by means of the pulp production plan. A computer program product and a control system are also presented herein.

Description

METHOD OF CONTROLLING PULP PRODUCTION AND A CONTROL SYSTEM

THEREFOR

TECHNICAL FIELD

The present invention generally relates to production

optimization in the process industry, and in particular to production optimization in the paper industry.

BACKGROUND

In pulp and paper production large amounts of electrical energy is consumed by the pulp and paper mill. Hence, in such mills it may be desirable to utilize energy as efficiently as possible .

The article "Optimal energy management in pulp and paper mills" by H.K. Sarimveis et al . , published in Energy

Conversion and Management 44 (2003) 1707-1718, discloses the utilization of mathematical programming for optimum energy management of a power plant in pulp and paper mills. The article aims at fulfilling the total plant requirements in energy and steam with the minimum possible cost. In particular a proposal is disclosed of how to achieve optimal operation of the powerhouse satisfying the mill demands with the minimum possible cost, given the requirements of the mill for steam at different pressure levels and electricity and the prices of fuel and purchased electricity. SUMMARY

The above-mentioned article concerns energy optimization in pulping processes utilizing power boilers and recovery

boilers, e.g. chemical pulping. In particular, the

optimization of the operation of the powerhouse, including power and recovery boilers for steam delivery to the mill, is disclosed .

The present invention has been made by the inventors'

realization that energy optimization with regards to costs may be applied to a mechanical pulp mill, e.g. thermomechanical pulp mills, in combination with the associated paper mill.

A thermomechanical pulp mill and a paper mill together form an energy-integrated industrial process, because steam generated in the refiner lines of the pulp mill is provided to the paper mill for drying the paper in the paper mill. Hence, an

adjustment to the production rate of pulp in the refiner lines in the pulp mill may directly influence the production of paper in the paper mill, even if there is excess pulp

available for paper production in pulp storage towers.

Pulp and paper mills are sometimes able to negotiate a fixed electricity price. However, volatile energy prices, e.g.

market electricity prices which may vary during the day as a function of the energy demand, may become a reality in the near future. It is in fact already utilized in some

geographical areas. Therefore, energy costs have become an important issue for large scale energy consumers, such as pulp and paper mills.

In view of the above, a general object of the present

disclosure is to provide a control method for reducing the energy costs for a pulp and paper mill.

Hence, in a first aspect of the present disclosure there is provided a method of controlling the production of pulp in a pulp mill for supplying pulp and steam to a paper mill, the method comprising: receiving energy price data pertaining to an expected variation in energy price over a time period, determining a pulp production plan over the time period based on the energy price data and an amount of paper to be produced by the paper mill during the time period, and controlling the production of pulp in the pulp mill by means of the pulp production plan.

By determining a pulp production plan, taking the energy price data and the amount of paper to be produced in the paper mill into account, it may be possible to schedule the pulp

production according to the variations in the electricity price. As a result, the energy costs for the pulp and paper mill may be reduced.

The determining the pulp production plan may comprise

minimizing an overall energy cost for the pulp mill and paper mill while maintaining the production of the amount of paper to be produced.

The determining of the pulp production plan may further be based on constraints pertaining to a total steam demand in the paper mill.

By providing constraints to the total steam demand in the paper mill when determining the pulp production plan, it may be possible to evaluate whether excess steam is generated by the pulp mill. With excess steam is herein meant more steam than is needed for both pulp production and paper production at any point of time within the said time period. If the pulp mill produces excess steam at a specific time instance, it may be possible to reduce the pulp production at such a time instance if the electricity price is predicted to reach a peak level at such a time, provided that there is sufficient amounts of pulp in the pulp storage towers for supplying the paper mill such that the predetermined paper production amount may be maintained. Thereby, the pulp production may be

reduced, resulting in lower energy consumption while the paper production may be maintained at a level which allows for producing the amount determined by a paper production plan.

The determining of the pulp production plan may further be based on constraints pertaining to a capacity of a steam source other than the mechanical pulp mill. Hence an

assessment may be made whether it is possible to provide enough steam from an alternative steam source to the paper mill if the pulp mill were to produce less steam as a result of reducing the pulp production in the pulp mill e.g. for the purpose of reducing energy costs.

The determining of the pulp production plan may further be based on costs of utilizing a steam source other than the pulp mill. Hence, it may be possible to assess whether it cost-wise would be worth utilizing the alternative steam source while reducing the pulp production, and hence the steam production in the pulp mill, provided that the predetermined amount of paper to be produced may be maintained.

The determining of the pulp production plan may further be based on constraints pertaining to a maximum production of each refiner line of the mechanical pulp mill.

The determining of the pulp production plan may further be based on constraints pertaining to a minimum production of each refiner line of the mechanical pulp mill. Thereby it may be possible, for the purposes of determining the pulp

production plan, to determine whether it may be cost-efficient to completely turn off a refiner line for cost optimization. The determining of the pulp production plan may comprise adjusting a paper production plan of the paper mill by re^¬ scheduling production of different paper qualities according to energy consumption for producing the different paper qualities. The production of different paper qualities may generally consume different amounts of energy when producing the pulp for the different paper qualities. Hence, by re^¬ scheduling the production of the different paper qualities, it may be possible to avoid producing the most energy-consuming pulp and paper qualities at peak energy price levels.

One embodiment may comprise updating the pulp production plan when receiving updated energy price data.

One embodiment may comprise waiting at least one hour to perform the step of updating between receiving consecutive updated energy price data.

The pulp production plan may be updated regularly, e.g. once every hour, in order to always have an up-to-date pulp

production plan.

The determining of the pulp production plan may further be based on income of utilizing a steam source other than the pulp mill for steam generation to generate electricity to be provided to a power grid.

Thereby it may be evaluated whether it is profitable to operate the pulp mill at full capacity while also operating the energy plant of the pulp and paper mill e.g. at full capacity even though the steam from the energy plant is not needed in the paper mill. The steam generated by the energy plant may instead be used for producing electricity via one or more turbines, whereby the electricity may be sold e.g. to the power utility. Hence, if the cost for generating the electricity is lower than the price for which the electricity is sold, it may be profitable for the pulp and paper mill to sell electricity generated by the energy plant of the pulp and paper mill.

The pulp mill may be a thermomechanical pulp mill.

In a second aspect of the present disclosure there is provided a computer program product comprising a computer readable medium storing program code which when executed performs the method according to the first aspect of the present invention.

In a third aspect of the present disclosure there is provided a control system for a pulp mill, which control system

comprises : a processing unit arranged to receive energy price data pertaining to an expected variation in energy price over a time period; to determine a pulp production plan over the time period based on the energy price data and an amount of paper to be produced by the paper mill during the time period, wherein the control system is arranged to control the production of pulp in the pulp mill by means of the pulp production plan.

In one embodiment, the control system may be arranged to determine the pulp production plan by minimizing overall an electricity cost f r the pulp mill and paper mill while maintaining the pr duction of the amount of paper to be produced .

The pulp mill may be a thermomechanical pulp mill. Generally, all aspects of the present invention provide the same or similar advantages.

Additional features and advantages will be disclosed in the following . BRIEF DESCRIPTION OF THE DRAWINGS

The invention and the advantages thereof will now be described by way of non-limiting examples, with reference to the

accompanying drawings of which:

Fig. 1 is a schematic block diagram of a pulp and paper mill. Fig. 2 is a control system for controlling pulp production in the pulp mill of Fig. 1.

Fig. 3 is a flowchart describing a method of controlling the production of pulp in the pulp mill of Fig. 1.

DETAILED DESCRIPTION

Fig. 1 shows a schematic block diagram of a pulp and paper mill 1. The pulp and paper mill 1 comprises a pulp mill 3, a pulp storage container 9, such as a pulp storage tower, a paper mill 5, and an energy plant 7 comprising a boiler 7-1.

The pulp mill 3 may for instance be a mechanical pulp mill, a thermomechanical pulp mill (TMP) , or a chemical pulp mill. The pulp mill 3 will in the following be exemplified by a

thermomechanical pulp mill.

The pulp mill 3 comprises a plurality of refiner lines 3-1 and 3-2 comprising refiners. The refiner lines 3-1 and 3-2 are fed with chips C. The chips C are pre-processed in pre-steamers prior to being provided to the refiners, e.g. by means of feeding screws or similar devices. The pre-steamed chips C are processed by mechanical means, such as rotating discs in the pressurized refiners for

producing pulp P .

The refiner lines 3-1 and 3-2 consume large amounts of

electrical power in order to operate the mechanical means e.g. by large motors. As an example, a refiner may typically consume energy in the range of 10-30 MW.

When processing the chips C in the pressurized refiners, steam is produced. It is generally desired to utilize the steam generated in the refiners. To this end a portion of the steam may be provided to the refiner lines, e.g. to the pre- steamers . Another portion of the steam produced by the

refiners may be provided as steam S-2 to the paper mill 5 for paper processing in the paper mill 5. Such processing may for instance include drying of the paper in the paper mill 5.

The steam S-2 may be fed to the paper mill 5 via a steam transportation system. The pulp mill 3 and the paper mill 5 hence form an energy-integrated system.

The pulp P produced in the pulp mill 3 may be stored in the pulp storage container 9, for later use in the paper mill 5. It is to be noted that generally a pulp and paper mill

comprises a plurality of pulp storage containers. The

exemplified pulp and paper mill 1 has one pulp storage

container in order to provide a simplified view of the mill. The production flow direction of the pulp P from the pulp mill 3 to the paper mill 5 is shown by arrows Al and A2.

The paper mill 5 is arranged to receive the pulp P for

producing paper. The process of producing paper in a paper mill 5 will not be explained herein, as it is well-known to the skilled person in the art. It is however to be noted that the pulp goes through a plurality of de-watering processes involving the removal of water from the paper in the paper mill 5. One stage of the de-watering process involves drying the paper utilizing the steam S-2 from the pulp mill 3.

Moreover, the boiler 7-1 may be utilized for providing steam S-l to the paper mill 5 in addition to the steam S-2 from the refiners, if necessary.

Fig. 2 shows a control system 11. The control system 11 comprises a processing unit 13 for processing energy price data 15 received by the control system 11 in a step SI. The energy price data 15 comprises information regarding an expected variation in energy price over a time period. Such a time period may be a predetermined time period of for instance 24 hours.

The energy price data 15 utilized in step SI may be published by the power utility. Alternatively, the energy price data 15 utilized in step SI may be determined by means of e.g. daily price negotiations between the power utility and the pulp and paper company. Alternatively, or additionally, the energy price data 15 may be based on statistical data pertaining to historic variations in the electricity prices.

The amount of paper to be produced by the paper mill 5 during the time period may also be provided as an input to the processing unit 13. In a step S2 a pulp production plan is determined with respect to the energy price data 15 and the amount of paper to be produced by the paper mill 5 during the time period.

In one embodiment, the step S2 of determining the pulp

production plan may further be based on a possible income from utilizing the energy plant (7) for generating steam provided to one or more turbines for generating electricity. The generated electricity may to be provided to a power grid.

It is also envisaged that the pulp and paper mill 1 may reduce or completely stop the paper production. Thus, the amount of paper to be produced is altered, if it is determined that it would be more profitable to sell electricity, previously purchased from the power utility, on the spot market, than to produce and sell paper.

In a preferred embodiment the pulp production plan is

determined by means of minimizing an objective function. The objective function may in general be based on energy costs for the pulp and paper mill 1 when pulp supply is equal to or greater than the demand for pulp provided by a paper

production plan, and the steam supply is equal to or greater than the demand for steam in the paper mill 5.

In some embodiment the objective function may be based e.g. on the following terms: costs of purchased electricity, the cost of internal steam and electricity generated at the mill, and the income of providing electricity back to the grid. In general, any combination of the above terms may be possible.

In one embodiment costs for stoppage and for starting the energy plant 7 may also be taken into account in the objective function, in combination with any of the above terms.

In one embodiment, the future horizon used in minimizing the objective function may be 24 hours with a resolution of one hour. In another embodiment, a future horizon with the number of minutes in a day with a resolution of one minute is envisaged . In one embodiment, the pulp production plan may be updated when receiving updated energy price data. In particular, the pulp production plan may be updated according to the

resolution of the future horizon, e.g. once every hour. The horizon then moves one step forward according to the

resolution of the future horizon. The updating of the pulp production plan comprises executing step S2, i.e. determining the pulp production plan according to the updated energy price data taking the amount of paper to be produced by the paper mill 5 into account during the new time period.

In one embodiment the step S2 of determining may further be based on costs of utilizing a steam source other than the pulp mill. Thereby it can be determined whether it is more cost efficient to use excess steam capacity of the pulp and paper mill 1 in the form of steam S-l from the boiler 7-1, than to operate the refiner lines 3-1 and 3-2 in the pulp mill 3 for production of steam S-2 in case there is sufficient pulp in the pulp storage container (s) 9 for maintaining the desired paper production volume in the paper mill 5. The costs for producing steam S-2 in the boiler 7-1 may for instance depend on the fuel prices for heating the boiler 7-1. These costs are compared to the electricity price for operating the motors of the refiners. The most cost-efficient mode of operation of the pulp and paper mill 1 with respect to excess steam production may thereby be determined.

The processing unit 13 may further be provided with

information regarding constraints to one or more parameters pertaining to the paper manufacturing process in the pulp and paper mill 1. Such constraints may for instance be any of the following: Constraints pertaining to a total steam demand in the paper mill.

Constraints pertaining to a capacity of a steam source other than the pulp mill.

Constraints pertaining to a maximum production of each refiner line of the pulp mill.

Constraints pertaining to a minimum production of each refiner line of the pulp mill.

The constraints may in any combination be utilized when minimizing the objective function and determining the pulp production plan.

In one embodiment the paper production plan of the paper mill may be adjusted by re-scheduling production of different paper qualities according to energy consumption for producing the different paper qualities. The re-scheduled paper production plan may then be utilized when determining the pulp production plan. The above constraints may also be used in such an embodiment .

The pulp production plan is provided as setpoints r to a control unit 17 which may comprise controllers such as PI- controllers. The control system 11 may in one embodiment be implemented in software as an integrated unit. Alternatively, the processing unit 13 and the control unit 17 may be

implemented separately. Measurements Y from the pulp mill 1 are compared to the setpoints r in order to determine the control error. The control unit 17 can hence in a step S3 provide control outputs u, based on the control error, to the pulp mill 3 for controlling the pulp and steam production in the pulp and paper mill 1. Alternatively, control outputs may also be provided to the energy plant 7 for controlling the steam production of steam S-l, if necessary. By means of the present disclosure, it may be possible to reduce the energy costs in a pulp and paper mill while

maintaining the production of a predetermined amount of paper. In particular, the present disclosure may provide optimal energy utilization in a pulp and paper mill with regards to price peaks provided by volatile energy prices.

The skilled person in the art realizes that the present invention by no means is limited to the examples described hereabove. On the contrary, many modifications and variations are possible within the scope of the appended claims.

Claims

1. A method of controlling the production of pulp (P) in a pulp mill (3) for supplying pulp (P) and steam (S-2) to a paper mill (5), the method comprising: a) receiving (SI) energy price data (15) pertaining to an expected variation in energy price over a time period, b) determining (S2) a pulp production plan over the time period based on the energy price data (15) and an amount of paper to be produced by the paper mill (5) during the time period, and c) controlling (S3) the production of pulp (P) in the pulp mill (3) by means of the pulp production plan.

2. The method as claimed in claim 1, wherein the

determining (S2) the pulp production plan comprises minimizing an overall energy cost for the pulp mill (3) and the paper mill (5) while maintaining the production of the amount of paper to be produced.

3. The method as claimed in claim 1 or 2, wherein the determining (S2) the pulp production plan is further based on constraints pertaining to a total steam demand in the paper mill (5) .

4. The method as claimed in any of the preceding claims, wherein the determining (S2) the pulp production plan is further based on constraints pertaining to a capacity of a steam source (7) other than the pulp mill (3) .

5. The method as claimed in any of the preceding claims, wherein the determining (S2) the pulp production plan is further based on costs of utilizing a steam source (7) other than the pulp mill (3) .

6. The method as claimed in any of the preceding claims, wherein the determining (S2) the pulp production plan is further based on constraints pertaining to a maximum

production of each refiner line (3-1, 3-2) of the pulp mill (3) .

7. The method as claimed in any of the preceding claims, wherein the determining (S2) the pulp production plan is further based on constraints pertaining to a minimum

production of each refiner line (3-1, 3-2) of the pulp mill (3) .

8. The method as claimed in any of the preceding claims, wherein the determining (S2) the pulp production plan

comprises adjusting a paper production plan of the paper mill (5) by re-scheduling production of different paper qualities according to energy consumption for producing the different paper qualities.

9. The method as claimed in any of the preceding claims, comprising updating the pulp production plan when receiving updated energy price data.

10. The method as claimed in claim 9, waiting at least one hour to perform the step of updating between receiving consecutive updated energy price data.

11. The method as claimed in any of the preceding claims, wherein the determining (S2) the pulp production plan is further based on income of utilizing a steam source other than the pulp mill (3) for steam generation to generate electricity to be provided to a power grid.

12. The method as claimed in any of claims 1-11, wherein the pulp mill is a thermomechanical pulp mill.

13. A computer program product comprising a computer readable medium storing program code which when executed performs the method according to any of claims 1-12.

14. A control system (11) for a pulp mill (3), which control system (11) comprises: a processing unit (13) arranged to receive energy price data (15) pertaining to an expected variation in energy price over a time period; to determine a pulp production plan over the time period based on the energy price data (15) and an amount of paper to be produced by the paper mill during the time period, wherein the control system (11) is arranged to control the production of pulp in the pulp mill (3) by means of the pulp production plan.

15. The control system (11) as claimed in claim 13, arranged to determine the pulp production plan by minimizing an overall energy cost for the pulp mill (3) and the paper mill (5) while maintaining the production of the amount of paper to be produced.