Plants all over the world are facing tightening regulations with regard to emissions. In many cases, ageing boilers must be replaced in order to achieve compliance. This was the case at a Tate and Lyle processing plant in Tennessee, USA. Along with regulatory demands to slash harmful emissions, it faced rising fuel costs.
“With new regulations setting far more stringent emissions limits, and our coal and electricity costs rising sharply, it was not looking good for our future,” said Jim Lang, Technical Engineering Manager at the Loudon Plant, near Knoxville, Tennessee, part of Tate and Lyle Global Operations.
Management considered two different approaches to resolve the situation:
- The first was a boiler upgrade supported by Selective Catalytic Reduction (SCR) and Flue-Gas Desulfurization (FGD) technology to reduce pollution.
- The second possibility was the construction of a new natural gas-fired generating plant that worked together with waste heat boiler technology to boost efficiency, lower emissions and satisfy steam requirements. This proved to be the most attractive option.
The company invested $60 million into a natural gas-fired combined heat and power (CHP) system consisting of two Siemens Energy SGT-700 gas turbines and two Siemens SGen-100A generators as well as Heat Recovery Steam Generators (HRSGs) with supplemental duct firing from Rentech Boiler Systems of Abilene, TX. By spending a little more to upgrade the entire facility, the Tate & Lyle were able to produce its own steam, as well as generate its own power rather than buying power from the local utility.
The plant processes U.S. grown corn into both speciality sweeteners and bulk ingredients which are distributed globally. Many of the processes utilize boilers, tanks and pumps as well as plenty of steam. In the past, this steam came from two coal boilers as well as a package boiler. But the price of coal grew steadily, and the plant had to comply with the Boiler Maximum Achievable Control Technology (MACT) rule as part of the National Emission Standard for Hazardous Air Pollutants. It targets pollutants such as mercury, carbon monoxide, particulate matter and sulfur dioxide (SO2).
Each of the Siemens SGT-700s can produce 33 MW of electrical capacity. These twin-shaft gas turbines have an electrical efficiency of 37.2%. They offer easy on-site maintenance and can operate with a wide range of gaseous and liquid fuels.
Tate & Lyle liked the idea of CHP as it reduces fuel consumption by making use of the heat normally emitted as waste. This means that the same electrical and thermal outputs can be achieved at much lower costs, while emissions are minimized.
“By generating electricity on-site and recovering heat that would typically be wasted, Tate & Lyle not only increase energy efficiency but also substantially reduces energy costs and carbon emissions,” said John Gibson, Head of Sales North America, Siemens Power & Gas. “The entire Rentech HRSG package was more detailed and more streamlined than the others. Technically it was what we were looking for, as well as being one of the lower-cost bids we received.”
The Rentech HRSG is a heat exchanger which uses waste heat boilers to recover energy from the exhaust stream of a turbine. It uses heat from the exhaust to produce steam for use in facility processes. Major components of an HRSG are a water preheater, economizer, evaporate and superheater.
The HRSG has a modular, cross-flow design. Together with the steam turbines, the new plant provides 400,000 lbs per hour of steam. Rentech is also providing supplemental firing duct burners with each HRSG. This feature doubles steam production when used by raising the gas turbine exhaust temperature, thereby improving the amount of steam produced. The supplemental duct burners are often used to meet high steam demands.
Due to the many elements involved in construction and the fact that the facility had to continue production processes, the delivery and construction process had to be timed correctly. The mechanical contractor Icon Industrial Services needed the elements of the boiler package to arrive at different times to avoid cluttering up the site with parts that were not due to be installed until a few weeks later. The HRSG was delivered in stages to fit avoid cluttering up the site.
The old boilers are being retained in order to increase overall steam output. In addition, the plant now has the capability to take one or more boilers or an HRSG down for maintenance without crippling plant output. In the past, a boiler going down could severely curtail production. Now, however, maintenance can be regularly scheduled and carried out without having the plant suffer. The coal boilers will sit cold unless there is an outage, in which case they will be fired up for steam production.
For Boiler MACT, the plant estimates a reduction in its SO2 output per year by 2200 tons and PM10 (particulate matter consisting of very small liquid and solid particles floating in the air of less than 10 microns in diameter) by 35 tons per year. As well as producing steam, the facility realized big savings by switching from grid power to generating its own electricity. Greenhouse gas emissions were cut by nearly 50% and Tate & Lyle’s CO2 emissions by around 10%.
“On a good day, our coal boilers might reach 75% efficiency while our Rentech HRSGs are 84% efficient,” said Lang. “The payback from our new plant will be relatively fast due to the savings in terms of bills from the local utility, and lower fuel costs.”
Gerardo Lara , VP-RENTECH Boiler Systems USA