Balancing steam supply with demand saves energy
Keeping a balance between steam supply and demand is a challenging task in a multiline pulp mill complex, since there are always many sources of supply and multiple users of steam energy. To make matters worse, pulp mill processes are dynamic and often in transition. So if this delicate equilibrium is disturbed on the supply side or the demand side, it is quite common for the steam network to get out of balance, which can result in steam supply variations. Some key processes may lack steam or steam may be vented, both of which waste energy and money. Typically, when process steam demand is not met by primary sources, auxiliary boilers have to make up the difference, often using more expensive non-biofuels like oil or gas. If demand drops, excess steam has to be vented, wasting valuable fuel and money while adding to the carbon footprint of the mill.
To keep that ideal balance point and with an eye on saving boiler fuel and reducing carbon emissions, Celulose Nipo-Brasileira S.A. (Cenibra) commissioned a Metso Steam Manager optimization control for the steam supply and distribution system of its ECF bleached eucalyptus fiber pulp mill in Minas Gerais, Belo Oriente, Brazil. The mill is owned by Japan Brazil Paper and Pulp Resources Development Co., Ltd. Total production from two fiber lines is 1.16 million tpy, 92% of which is exported. Cenibra was established in 1977 and has grown to be one of the largest pulp producers in Brazil.
Since commissioning the Steam Manager APC (advanced process control) system in May 2009 the company has reduced vented steam by 90% and the steam supply has stabilized. “Steam consumption is lower and Steam Manager meets our expectations, reducing costs which are reflected in oil consumption,” reports Róbinson Félix, mill manager.
Improving stability, lowering costs
The decision to deploy the control platform was part of a larger, more comprehensive plan to optimize many of the mill’s processes to improve process stability, produce more uniform quality, and lower costs. Altogether, five Metso APC systems have been commissioned in the mill.
The Steam Manager project was a collaborative effort between Metso and Cenibra after working together for many years. When asked why they chose Metso, Ronaldo Ribeiro, Cenibra automation specialist, says, “This was a long-term process. Metso was selected because we have worked seriously during those years, established a partnership, prepared a consistent contract, and most of our machinery and all control valves were from Metso. Metso knows our processes and this would help the advanced control implementation. For steam management the target was to reduce costs. The possibility to reduce steam quantity released into the air was identified. Based on this opportunity Metso prepared some calculations which helped Cenibra to get a reduction in vented steam.”
The first mill audit regarding possible APC solutions was carried out in 2006. After that, the projected results and returns were calculated. In July 2008, the mill decided to order five APC controls, one of which was Steam Manager. Before the system was installed, Metso gathered data and studied process information concerning the mill’s steam network. The operation model of the mill and its controls were studied to create a full-scale picture of the plant’s performance level in terms of energy efficiency.
Dynamic process, modeled control
This legwork was well justified by the complexity and dynamics of the network. The advance work helped to configure the structure of the multivariable predictive (MPC) models that are a cornerstone of Steam Manager. The fundamental objective of the control strategy is to guarantee both steam quality and availability—not too little for the pulp mill processes and not so much that it has to be vented. Steam quality is ensured by regulating the pressures of high-, medium-, and low-pressure headers at their target values of 65, 13, and 4 kgf/cm2 (925, 185, and 57 psi). By stabilizing and regulating the pressures, high-quality process steam is assured. Steam availability is maintained by regulating balance of the steam network by ensuring that boiler production matches consumption from the turbo-generators and mill processes demanding process steam at any instant.
Most steam at the mill is produced by three recovery boilers, with the balance from two power boilers burning biofuel. Oil burners for these power boilers are used during start-ups and at high load situations. In addition, there is an oil boiler available for steam outages. Two turbine-generators operate from the high-pressure header supplied by all five boilers. The biggest process steam consumers are two evaporators and two continuous digesters that use medium- and low-pressure steam. Also, the recovery boilers require some process steam for their operation.
The APC system controls many different unit operations and valves in order to balance supply and demand. Models take into account the interactions between several interrelated variables: manipulated variables (MVs) are managed to achieve controlled variable (CV) targets in the presence of disturbance variables (DVs), which come from a variety of sources on the supply and demand side. Figure 1 shows a matrix that represents the MPC control structure at Cenibra.
The Steam Manager controls are configured in a MetsoDNA system linked via OPC to the mill’s existing DCS, which then carries out the loop control commands.
Results from day one
Immediately after the start-up in May 2009, the Steam Manager results looked very promising and were well documented over the following months. The new system improved the network steam and energy balance management during normal production periods and, most importantly, during disturbance periods. Vented steam to the atmosphere has been reduced significantly.
Júlio Ribeiro, Cenibra recovery line and utilities coordinator, sums up the results: “Steam Manager uses algorithms including multivariable control, as it coordinates multiple inputs, measurements, and disturbances. The commissioning phase and software configuration involved a multidisciplinary team. The operational start-up was smooth. Results could be seen from the first day, and the target of 90% reduction of blow-out steam was reached in three months.” This means each month 10,000 tons of steam are not being discharged to the atmosphere, which is the energy equivalent of 715 tons of oil per month and 2,200 fewer tons of CO2.
In addition, the high-pressure steam pressure is more consistent. The stabilization of all steam pressures in the network—high, medium, and low—is the backbone of the Steam Manager concept. The lower-pressure system must be stabilized to provide steam at constant pressure to consumers. On the other hand, the high-pressure header must be stable as it is one of the main variables controlled by power boilers. If that is upset, the boiler load and eventually the fuel consumption is disturbed.
Operator acceptance, sustained results
Implementing the Steam Manager system required some new ways of thinking and changes in operating procedures. Ribeiro recalls, “There was a change in the operating paradigm because the operators were used to doing it their way. Then some new concepts were introduced into operation. Therefore some adjustments were needed. Since the system proved to be reliable, the operators felt comfortable with the changes. It was a new tool and it had everyone’s approval. At first they were a little skeptical about it, but soon they saw it brought benefits.”
Process operator acceptance is one of the key objectives of any process management system since the operators have to work with it day-to-day. When the operators fully understand and believe in the way of operation, the results are sustained. Jairo Ferreira, recovery boiler operator, sums up his thoughts, which suggest a continuing belief in the control concept. “Since I came to the department, a lot of excess steam was being released,” he said. “There was an awareness of it, but more drastic measures had to be taken. The process instability was a limiting factor and this became the most important goal of the Steam Manager project. This solution would help us to do what we could not do by ourselves. When the boiler burning rate increases, the plant gets more stable, and the energy distribution pattern is improved. What you need is to manage all that when the process undergoes changes. In the beginning, we were not sure that steam emissions could be reduced. Then, after a closer look at the system, we understood it better and we improved our process. Today, we have reached an unbelievable steam reduction level that exceeded our expectations.”
Pasi Airikka is a product manager for Metso.