REDUCTION OF ENERGY DEMAND IN PAPER MAKING USING ONLINE OPTIMISATION AND CONTROL

造纸业是一个能源密集型行业：即使是一个小规模的造纸厂每年也可以使用超过150GWh的能源，相当于一个中等规模的城镇。英国有50多家造纸厂，年产量超过500万吨纸和纸板。造纸厂生产的产品范围从精细纸张到瓦楞纸板，涉及多种具有不同特征的过程的融合，其中突出的是非最小阶段、间歇、多变量、时滞和非线性系统。该过程由按顺序排列的多个部分组成。整个生产线的控制一般由集散控制系统(DCS)来实现。因此，该工厂具有复杂的多层控制系统，每一层都有先进的、经典的和手动回路的混合。再者，造纸机是一个湿端和干端相结合的混合系统，代表了间歇过程和连续过程的混合。在本项目中，除了分段过程建模外，还将利用热力学原理(能量平衡和熵流等)和造纸生产线控制系统提供的过程数据来建立能量流模型。预计能量流模型将揭示整个过程的能量分布，分段能耗将与控制变量(即控制回路设定值)和其他分段运行变量动态关联。一旦工厂的过程和能量流动模型可用，它们就可以用于热能管理，通过模型预测控制(MPC)技术进行在线动态优化和控制。在这种背景下，预测控制可以用来实现产品质量、生产和能源效率的多个优化目标。目前，MPC已被用于实时优化(RTO)，以便在过程级为控制回路生成适当的设定值，并将单一性能函数与过程约束结合使用。目前，通过在执行优化之前指定多个目标的相对偏好，可以将多个目标合并到MPC算法中。然而，当成本/能源消耗随时间变化时，这种偏好信息将动态变化，并且应该明确地向过程监督者提供一系列可行的质量/效率权衡。基于本研究的结果，将开发简单易用的建模和信息呈现系统。该系统将与参与该项目的工业伙伴合作开发，并将用于对示范过程进行综合过程监测、控制和优化，以展示所开发方法的有效性。在两台造纸机上还将实施在线闭环多变量节能MPC控制系统：1.SK SSK纸板机是这类机器的典型：纸张排水量发生自然的无规则变化，并导致进入烘干机的纸张固体含量发生变化。进入烘干机的板材的固体含量越高，需要更多的蒸汽来烘干板材。2.另一方面，在艾尔斯福德，与能源供应商签订的一份合同约束工厂每天必须获得固定数量的蒸汽。热效率的提高将导致蒸汽不得不被倾倒，除非增加产量以使用备用蒸汽。

Paper making is an energy-intensive industry: even a small scale paper mill can use in excess of 150GWh of energy per year, as much as a medium-sized town. There are more than 50 paper mills in the UK with annual production of more then 5 million tonnes of paper and board. Paper mills produce products which range from fine papers to corrugated cardboard and involves fusion of a variety of processes with different characteristics, prominent amongst which are, non-minimum phase, batch, multivariable, time-delay and nonlinear system. The process consists of a number of sections arranged in a sequential way. The control of the whole production line is generally realized by distributed control systems (DCS). As such the plant has complex multilayered control systems with mixture of advanced, classical and manual loops in each layer. Moreover, paper-making machine is a hybrid system, combining wet end and dry end, which represents a mixture of batch and continuous processes. In this project, in parallel to the sectional process modelling, energy flow models will be established using principles in thermal dynamics (energy balance and entropy flow etc) and the process data available from the control systems along the paper making production line. It is expected that the energy flow models will reveal the energy distribution in the entire process and the sectional energy consumptions will be dynamically linked with control variables (i.e., control loop set points) and other sectional operational variables. As soon as the process and energy flow models of the plant are available, they can be used for the thermal energy management via online dynamic optimisation and control via model predictive control (MPC) techniques. In this context, MPC can be used to realize multi-optimization objectives in terms product quality, production and energy efficiency. At present, MPC has been used for real-time optimization (RTO) so as to generate appropriate set points to the control loops at the process level, and single performance function has been used together with process constraints. At present, multiple objectives can be incorporated into MPC algorithms via specifying their relative preferences prior to the optimization being performed. However, as the cost/energy consumption changes with time, this preference information will change dynamically and a range of feasible quality/efficiently trade-offs should be explicitly made available to the process supervisors.Based on the results of this research simple-to-use modelling and information presentation systems will be developed. The system will be developed in collaboration with the industrial partners involved in the project and will be applied to perform integrated process monitoring, control and optimisation of the exemplar processes to demonstrate the effectiveness of the developed methodologies. An online closed loop real-time multivariable energy-reducing MPC control system will also be implemented on two of the paper machines:1. The SK SSK board machine is typical of its kind: natural unregulated variations in sheet drainage occur and cause variations in the solids content of the sheet entering the dryer. The higher the solids content of the sheet entering the dryer, the more steam is required to dry the sheet. 2. On the other hand, at Aylesford, a contract with the energy supplier binds the mill to take a fixed quantity of steam each day. Improved thermal efficiency would result in steam having to be dumped unless production was increased to use the spare steam.