DIGITALISATION WORLD PULP&PAPER 92 The potential benefits that can be achieved through dynamic control are vast. system (as seen in Figure. 3) is an integral part of pulp mills where the lime mud is calcined into quick lime. It is then further processed into white liquor (to be used in the cooking section) by passing it through a series of slakers and causticisers. The process is endothermic, a great deal of energy is required in the calcination process and is one of the leading contributors to a pulp mill’s energy costs. CHALLENGE Calcination is a highly interactive, multi-variable process, characterised by non-linear dynamics, subprocesses with time scale that vary from seconds to hours and a vastly varying feed that makes the optimisation and efficient control of a lime kiln a challenging problem. Furthermore, the complex operating conditions often result in multiple objectives (environmental, energy and efficiency) that are often conflicting in nature to be managed adequately. Because of the nature of the kiln where operating conditions exceed 1,000 degrees Celsius and significant quantities of dust are produced, most sensors are unreliable and frequently provide false signals that hamper real-time control. Other common disturbances in a lime kiln include varying density of the lime mud feed, varying production demands due to upsets in the cooking section and varying properties of the fuel, which makes designing robust control all the more difficult. Safety is of utmost concern, as poor control could lead to hazardous situations with the production of the dangerous carbon monoxide, and even lead to explosions. OPPORTUNITY Designing robust supervisory control and advanced control logic that coordinate the existing regulatory control system of a lime kiln is demanding yet essential. The potential benefits that can be achieved through dynamic control are vast. First and foremost is the significant increase in fuel savings or energy efficiency, which provides an immediate economic benefit. Stable lime quality will greatly improve the cooking process and result in less off-spec pulp production. The lifetime of the lime kiln will also increase, lowering the maintenance costs associated with refractory repairs, ring formation, and other effects of poor temperature regulation. Finally, the environmental costs of treating flue gas can also be reduced. The lime kiln control architecture is typically built on a set of basic Figure 3. Lime kiln system.