Pressure and temperature control using the Horner APG OCS300 integrated graphical controller.
Every process plant has a good control system at the heart of its successful operation. When AceFurnace was contracted to produce the full detailed mechanical and process design for a wood gasification system, a suitable controller needed to be chosen.
The developmental nature of the project meant that the client was unable at the beginning to specify the eventual list of features and functions, and so a PLC system with a touch screen HMI has been used to give maximum flexibility for future additions and changes.
The Hormer OCS 300 was chosen on cost grounds. The unit provides a competent PLC at its core, combined with a good LCD colour touch screen. Horner also provides their “Cscape” programming software as a free download, further reducing costs compared with alternative providers.
The system has been well received by the end user; the touch screen is proving robust and effective, and the accuracy and stability of the analogue PID functions has rivalled that of standalone controllers at a considerable cost saving.
Having now been asked to provide the detailed design for a scaled up system, a similar Horner controller will be used, utilising much of the code already written and tested for the OCS300.
The gasification system comprises an externally heated retort heated by flue gases from a combustion furnace burning the charcoal by-product from wood gas production. Fine woodchip is fed to the inside of the retort where it is pyrolysed to produce a gas of 17 to 21 MJ.m-3 and solid charcoal. The charcoal is burned to produce heat, and the gas is scrubbed with water to remove condensable components and acid gases. This gas is either fed to a flare, or to a gas engine to generate power. A proportion of the gas may also be diverted to the charcoal combustion unit for energy balancing.
Several proportional controllers are needed:
A master loop controlling the retort external temperature
Selectable slave loops using either diesel oil for preheat or charcoal for normal operation
A pressure control loop keeping the draught constant in the combustion furnace
A pressure control loop keeping the internal retort pressure close to atmospheric as the gas evolution rate varies.
Various other ancillary temperature controllers.
The external retort temperature is controlled by a thermocouple in the outlet duct from the enclosure surrounding the retort. The temperature measured here tends to lag the temperature in the combustion furnace by a considerable margin. Hence the combustion furnace is provided with its own slave loop that limits the temperature attained at its outlet. The set-point for this slave loop is the control variable or output from the retort master. A facility is provided in the code for switching between the generated or remote set point and a fixed local set point. The first slave loop controls furnace heating using an oil burner having 2 stage (high / low) operation. High fire operation is time proportioned by converting the 0 – 32000 control variable value to a fraction of the settable cycle time. Once the CV value falls to zero (no high fire) an increase in temperature will then control the burner as an on/off controller with dead-band.
The control ladder can be viewed here
Some sample control screens can be viewed here