In general, the plants must run below 75 mg/Nm3 about 96% of the time, and never exceed 300 mg/Nm3. However, generating capacity shortages, maintenance shutdowns, varying coal qualities and high load factors make this a difficult job, even though emissions are centrally monitored at Eskom's Integrated Generation Control Centre at Megawatt Park.
As a result, Bytes recently reorganized Eskom's existing process control infrastructure, which included Invensys' ArchestrA system platform, data historian, Active Factory historian clients and Wonderware's InTouch SCADA/HMI, Information Server and Alarm Provider software. Also, the integrator developed one KPI dashboard for each thermal power station, showing hourly emission values, their limits and length of time normal limits were exceeded. Any exemptions to the limits are also shown, so the system can adapt.
"During start-up after shutdown, a unit's emissions will be higher than during normal operation, so it's important to know when a unit is about to come on line, how long it was off and how long the higher emission level lasts," says Machiel Engelbrecht, Bytes' system engineer. "This helps us apply for the necessary exemptions. Also, the supplied information puts Megawatt Park in a good position to initiate preventive measures, and ensure optimal load distribution in the event of a shutdown due to excessive emissions."
Consequently, early warnings are raised when emission values get within 20% of acceptable limits, which helps prevent penalties and unit shutdowns. "Because of the scalability and versatility of our new software, it was possible to deliver a control system for a process that previously had to be done manually due to its complexity," adds Engelbrecht.
Sun to Steam for Oil
While parallels between sustainability and process control are growing clearer, green manufacturing's diversity is already creating some strange bedfellows, For instance, in what has to be one of the most ironic sustainability efforts, Berry Petroleum in Denver, Colo., cooperated with system integrator T.J. Cross Engineers Inc. in Bakersfield, Calif., and Glass Point Solar to use the sun's power to help produce steam for injection into oil field reservoirs, where it heats heavy crude to reduce viscosity and aid extraction. This enhanced oil recovery (EOR) process is automated by AutomationDirect's controls, which work with GlassPoint's solar steam generators to preheat water and then introduce it into Berry's natural gas-fired steam generator, which greatly reduces fuel consumption and operating costs.
"GlassPoint's solar water heating design uses greenhouses to house its entire solar concentrator mirror and water heating system," says David Llewellyn, senior engineer in T.J. Cross's process controls group. "For this project, the single 7000-sq ft greenhouse is made of galvanized steel and aluminum with a tempered 4-mm glass roof and walls. The mirrors inside are made of a reflective, anodized-aluminum material that's widely used in commercial lighting and is durable indoors, but isn't usable outdoors. These mirrors are light enough to be easily supported by the greenhouse and can be positioned using only the torque generated by eight 40-watt stepper motors and drives. The mirrors focus on GlassPoint's thermal-conversion device, which is a just a carbon steel pipe, and illuminate it with 85 times more intensity than direct sunlight."
AutomationDirect's Model Productivity3000 programmable automation controller (PAC) works with GlassPoint's proprietary controls to track the sun, secure the most power, maintain heated water temperature at about 190 °F and regulate flow. Data logging is performed by AutomationDirect's DataWorx software, which translates about 10 parameters from the PAC to a Microsoft Access database every 30 seconds. Operator interface to the PAC is provided by AutomationDirect's 12-in. C-more touchscreen (Figure 2).
"The system maintains desired flow rate and corresponding 190 °F output via a flow control loop," adds Llewellyn. "In the loop, a control valve is installed downstream of a constant speed pump. The PAC receives a 4-20-mA signal proportional to temperature from a transmitter installed at the receiver tube discharge from the greenhouse. This measured process variable is compared to the 190 °F setpoint in a PID loop control executed within the PAC. The PID loop output is a 4-20-mA signal that modulates the control valve to regulate water flow to the receiver pipes."
Deepening Green Process
So where else can sustainability spread? Well, one idea is to turn around and make green its own product and industry. In this case, Polyflow in Akron, Ohio, reports it's developed a practical, cost-effective pyrolysis process, which basically heats dirty waste plastic in a reaction vessel until it breaks down into a fluid similar to crude oil that can be refined into fuels. While pyrolysis has been possible for decades, Polyflow reports it's only been possible to tweak it well enough to handle multi-stream plastic sources for the past few years. As a result, Polyflow recently built and tested 400-pound batches in a semi-trailer-mounted prototype, and is now working with integrator South Shore Controls in Perry, Ohio, to scale it up to an 8.5-foot x 60-foot reactor cylinder that will continuously process 2.5 tons per hour (Figure 3). The plastic waste input will result in 70% liquid product, 13% char and 17% non-condensable gas, which can help run the cylinder. Also, Polyflow's process will need 1.7 million BTUs to operate, but the liquid produced reportedly will be able to produce 27 million BTUs.
Polyflow also uses a three-stage process to sort and extrude its plastic raw material, heat treat it, and then distill and refine its end products. Likewise, Polyflow is using Rockwell Automation's PowerFlex variable-frequency drives and starters, as well as ControlLogix PLCs to control I/O devices monitoring the temperature, pressure and flow of the separation equipment, reactor, distiller and refining processes. This pyrolysis process is networked via EtherNet/IP and managed by Rockwell's FactoryTalk View software platform. "The cylinder has to be made of stainless steel to withstand the mechanical and thermal stresses of this process, as well as the corrosive chemicals it produces," says Rick Stark, South Shore's president. "However, though the reactor uses natural gas to get up and running, it can use its distiller to generate its own fuel, and operate as a self-sustaining system."
Jay Shabel, Polyflow's CEO, adds that his firm's pyrolysis process can potentially divert 30% of the waste that usually goes into landfills and replace 13% of foreign oil sources. "There are seven different process parameters that are unique to this pyrolysis process, which allow it to take multiple sources of polymer waste and turn it into an aromatic-rich liquid," explains Schabel. "However, while our controls and automation already helps manage our pilot process, they'll be even more necessary when this becomes a continuous process, and we have to constantly respond to changes in feedstocks and conditions, or when market data indicates we should switch to making a different product."