01 October 2004
Algorithm efficiency in a crude context
Hog fuel is a mix of bark, sawdust, and planer shavings.
By John Walker, Laila Naidu, and Daniel Smith
With oil prices approaching $50 per barrel and natural gas prices near $6 per million British thermal units (Btus) (MMBtu), the need to reduce energy costs in paper mills is more important than ever.
Acting on any opportunity to improve the efficiency of existing process equipment, with little or no capital investment, is essential. For hog fuel boilers, improved control of the combustion process can happen by reducing the level of excess oxygen, which directly translates into improved boiler efficiency. Here are the operating issues.
Closed loop challenge
Hog fuel boilers are increasingly important in mill power plant operations. There are a number of boiler combustion zone configurations, such as moving grate, fluidized bed, and pile burning, to name a few.
Black liquor the liquid material remaining from pulpwood cooking in the paper-making process
Biomass plant materials and animal waste used especially as a source of fuel
Model-based predictive control (MPC) a method of process control that goes beyond the abilities of the traditional PID (proportional-integral-derivative) type. Similar to the ways humans learn, MPC correlates training (or operating image [internal model]), target (or reference trajectory), action (computation of the structured manipulated variable), and comparison of the actual versus the expected (modeling error compensator).
The boilers typically provide three main functions to the operation: steam production, waste incineration, and electricity generation.
Meeting the steam header pressure demand is of primary concern. Depending on the heating value and water content of the hog fuel, it may be necessary to burn supplemental fuel to maintain the boiler in energy balance with the steam production demand.
Furthermore, some hog fuel boilers operate in "batch" mode, as they require daily hog feed outages to permit ash removal from the combustion chamber. Supplemental fuel is necessary during these outages as well. Optimal closed-loop control of the boiler throughout these wide operating modes is a challenge.
In Kraft mill operations, burning black liquor in the recovery boiler generates green liquor and a significant quantity of steam. Available liquor inventories can often dictate the steam production from the recovery boiler.
Package boilers fired by conventional hydrocarbon fuels such as gas and oil typically operate in a "swing" mode or on a demand basis to maintain the steam header at the desired operating pressure.
Advanced control can improve the ability to swing the hog fuel boiler and thereby reduce fuel costs associated with the package boilers. Beyond boiler control, additional major energy savings opportunities exist in the power and recovery area of mills.
Variability in the heat released by the combustion of the hog fuel often makes control of the hog fuel boiler and the entire steam header very challenging for the plant operators.
Hog fuel is a mixture of bark and biomass or sludge from water treatment operations. The heating value of hog fuel can change dramatically, depending upon the water content and the proportion of bark to sludge.
Northern mills can experience quite severe changes in the hog fuel quality during winter months of operation. Stabilizing the bark boiler operation and improving the overall boiler efficiency can by achieved by improved controls.
Matrix of dynamic relations
The overall control objective is to recover as much heat as possible from the flue gas in the face of continuously changing hog fuel feed quality. Achieving this objective results in greater steam production.
A properly designed advanced control scheme can manipulate the split of under-grate airflow to over-grate airflow to minimize the excess oxygen for changing hog feed quality. This excess oxygen minimization can take place while maintaining steam production at a desired target.
The simplified hog fuel boiler schematic has four handles, or manipulated variables, for control: the hog fuel feeders, the over-fire airflow, the under-grate airflow, and load burner or "under-grate air preheat" fuel flow.
Steam and stack gas
Combustion—the rapid reaction of a fuel with oxygen—is perhaps more important than any other class of industrial chemical reactions. This in spite of the fact that the products of the reaction—CO2, H2O, CO, SO2, and some others depending on what is burning—are worth less than the fuels burned to obtain them.
The significance of these reactions lies in the tremendous quantities of energy they release—energy that boils water to produce steam, which drives the turbines that generate most of the world's electricity.
When a fuel burns—in this case biomass and hydrocarbons—carbon in the fuel reacts to form either CO2 or CO. A combustion reaction in which CO forms is partial or incomplete combustion. The product gas that leaves a combustion furnace is the stack gas or flue gas. Analyzing the stack gas helps determine combustion efficiency.
Actual hog fuel boilers are fitted with multiple hog fuel feeder addition points as well as multiple air injection points, for both the over-fire and under-grate airflows. Depending on local environmental rules, other control loops may be present to maintain compliance.
This matrix shows the process relationships between the controlled objectives (rows) and the manipulated variables (columns), in an "open loop" or fully manual state of operation.
A plus (+) sign indicates a positive relationship between a pair of variables, and a minus (–) sign denotes a negative relationship. For example, a unit increase in the hog fuel feed rate will increase steam production at steady state, reduce excess oxygen, and increase the combustion bed level.
The heart of a model predictive control (MPC) application is a matrix of dynamic process relationships, which we learn through controlled plant testing, conducted in close collaboration with plant operations. A teamwork philosophy with operations during plant testing and control system commissioning leads to a better understanding of the control objective and ultimately better acceptance by the end user.
The economic opportunity for an MPC strategy is to maximize the efficiency of the boiler operation for changing hog fuel quality and load demand on the header. In practice, this happens by manipulating the split of over-fire air to under-grate air to continuously minimize the excess oxygen, subject to a high carbon monoxide (CO) limit.
At all times the advanced controller will maximize hog fuel feed while simultaneously minimizing the fuel to the load burners, subject to steam production requirements and other process or environmental constraints.
The cost of a well-engineered and executed advanced project usually pays for itself through operations savings in less than one year. A typical energy efficiency benefit for MPC on a hog fuel boiler is in the range of 5–15% as illustrated in the heat balance table.
The third column "MPC" example shows a boiler efficiency improvement with MPC in operation. For hog fuel boilers, a 1% reduction in excess oxygen improves the overall boiler efficiency by about 3% at constant feed quality and rate. For illustration purposes we'll assume the excess oxygen can reduce by 2%. Actual benefits for any application are, of course, case dependent.
Behind the byline
John Walker (firstname.lastname@example.org) has worked in advanced control implementation and has serviced the pulp and paper, mineral processing, and petroleum industries for more than twenty years. With bachelor's and master's degrees in mechanical engineering, he is a licensed professional engineer in Ontario. Laila Naidu is an advanced control engineer and has eight years progressive experience in the pulp and paper and petrochemical industries. She has two degrees in chemical engineering and is a licensed professional engineer in Ontario. Daniel Smith (email@example.com) has twenty years experience in process control in North America and Scandinavia. He has two degrees in paper science and a Ph.D. in chemical engineering. All work at Capstone Technology.
Resultant loss in steam production
Here is a heat balance that illustrates the sensitivity of the hog fuel boiler steam production to the water and sludge content of the feed.
The "bone dry hog fuel" and the "water content of the hog fuel" combine to give the total mass fed to the boiler (41.3 klb/hr). For purposes of this example, we used a hog fuel higher heating value (HHV) of 7860 Btu/lb.
Bark fuels average around 8500 Btu/lb. Sludge or biomass typically has lower HHVs than bark, so as the proportion of sludge or biomass in the hog fuel increases, the HHV decreases. With increasing sludge content, the hog fuel heat release and steam production decrease.
Economically, increased sludge processing may reduce landfill or other alternative disposal costs. However, one must weigh this benefit and compare it to the resultant loss in steam production and any electricity generation credit.
As the water content of the hog fuel increases (rainstorms or winter conditions), more energy is required for drying, resulting in less energy available for steam generation.
Another negative effect of increased hog fuel water content is a reduction in boiler efficiency. This results from increased stack losses due to improved heat transfer with increased moisture levels.
For high water content situations, it may be necessary to burn supplemental fuel to maintain the boiler in energy balance to meet the mill steam production demand.
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