In several process industries (oil and gas, pulp and paper, minerals, etc.) production requires both steam and electrical power. In these cases, plant operators often build in an in-situ utility unit to satisfy these needs. These are not mainstream power plants like those normally built for power generation. Indeed, not only is the steam needed at different, very specific pressures and temperatures, but its consumption rate is also highly variable due to the variability of the process conditions, trips and/or starts of steam consumers, etc. It follows that steam network stability and reliable power output are difficult to attain.
In some cases,steam is generated also via energy recovery from other units eg, furnaces in a steam cracker or from byproduct gas usage eg, blast furnace gas for a steel manufacturing plant
. This introduces further disturbances to the steam network as energy and fuel recovery is subject to upstream unit’s availability and cycles.
Further complexity is added by energy market variables, prices, and local rules for energy markets. In addition to that, internal incremental production price depend on variable fuel block prices. It follows that in many cases the optimal power output is very different between peak and low energy prices and thus repositioning of the power production is needed. For instance, when the real time energy price is below the internal production price the tie line import is maximized. But when the real time market price is higher than the internal price, the tie line is minimized.
From an APC point of view, typical actuators are boiler rates, steam turbine inlet / extraction rates, gas turbine MW targets, attemperators, pressure control valves, steam flows to users and vent valves.
There are multiple constraints and couplings, delivering a text book case where APC can outperform classical control schemes, typically based on cascades of PIDs and separate PIDs operating with staggered set-points.