Global Cement Magazine visited with key Ash Grove manufacturing executives recently to hear about the Midlothian plant’s history and the transformational changes it has undergone in the past few years.
In mid-May, IEEE-IAS/PCA Conference attendees will have the opportunity to tour Ash Grove’s Midlothian plant, the first kiln in the US to be compliant with the National Emissions Standards for Hazardous Air Pollutants (NESHAP). Ash Grove Cement Company’s Midlothian, Texas, plant modernised its pyroprocess and was NESHAP compliant by September 2014, a whole year ahead of most other US cement manufacturing plants. Plant leaders and staff gained a great deal of experience dealing not only with NESHAP compliance, but also with the commissioning bugs that the plant experienced after its three wet kilns were replaced with one semi-dry preheater/calcincer kiln, as well as a new cooler, low-NOx burner and the country’s first Hotdisc® Reactor.
Global Cement Magazine asked a number of manufacturing executives at the plant about the genesis of the factory, as well as its evolution over the years to where it finds itself today.
According to Michael J Hrizuk, senior vice president of manufacturing, the Midlothian plant was built in phases. “The crushers, first mills, kiln, cement silos and associated belts were built 40 years ago. In 1968 - 69, the second kiln, raw mill, finish mill and several more material silos were built. By 1973, the third and final kiln was operating.”
“The plant has been owned and managed by various entities during its lifetime. Gifford-Hill, which built the plant, remained its owner until the late 1980s, when North Texas Cement Company, LP, acquired and managed it. Ownership of this partnership changed several times until early 2003, when Ash Grove Cement Company became the sole owner. Soon after, the Midlothian plant was conveyed to Ash Grove Texas, L.P., and it is now part of the Ash Grove Cement Company,” explained Hrizuk.
So how, asked Global Cement Magazine, does this plant fit into Ash Grove Cement Company’s overall business plan?
“The Texas market is one of the largest and most important among the states. The current demand for cement in Texas outstrips the supply of in-state producers, so having the capacity (approximately 900,000t) at the Midlothian plant is important for Ash Grove to be able to supply its customers’ demand,” according to Hrizuk. “We have cement distribution terminals nearby in Denton and Fort Worth, which extend our company’s reach among customers throughout North Texas.”
The plant’s primary product is ASTM Type I/II cement, and it also manufactures a speciality product used to manufacture house siding, according to Hrizuk. “The plant’s rail facility was upgraded prior to the modernisation project so we could handle cement during the tie-in of the new equipment.”
Modernisation projects
Ronald J Vidergar, vice president – technical services, mentioned some earlier modernisation projects at the plant: “In 1995, we completed installing the tyre-derived alternative fuel mid-kiln injection systems. Those systems were followed by the selective non-catalytic reduction (SNCR) systems in 2007, which had the effect of reducing the plant’s NOx emissions by more than 60% since 1996.”
So, asked Global Cement Magazine, how did Ash Grove make the decision to modernise its Midlothian plant? “We weighed the challenges the industry faces in North-Central Texas and based on the reserves of rock in our Midlothian quarry and the strong Texas market, we considered alternatives,” said Hrizuk. “As a 134-year old company, our board of directors and my colleagues in manufacturing were acutely aware of the challenges we faced going forward, the most notable of which was compliance with air emissions standards. Our plant is approximately 30 minutes drive south of the Dallas-Fort Worth metropolitan area, where nearly seven million people live and reliance on automobile transportation put the region in a ‘non-attainment status’ for ozone. It was unclear to us what direction federal regulators would take in issuing future federal ozone standards and, combined with the region’s current non-attainment status, the Ash Grove board decided to invest in and modernise the Midlothian plant. The modernisation also ensured that the company would create a solution for meeting the then-newly revised portland cement NESHAP regulations.”
“The Ash Grove board decided to invest US$140m in the modernisation, which included eliminating two of the three existing wet kilns and modifying the remaining kiln, which would further reduce air emissions and plant operating costs,” Hrizuk added.
NESHAP rule a catalyst
“The NESHAP rule was a central part of our thinking as we made the recommendation to the Ash Grove board,” said Curtis D. Lesslie, Ash Grove vice president of environmental affairs, who oversees the company’s environmental compliance. The NESHAP rule governs pollutants including: particulate matter (PM), mercury, dioxin/furans, HCl, organic hazardous air pollutants (HAP) and total hydrocarbons. The rule took effect in September 2015.
“Before we modernised the plant, we were controlling NOx emissions using staged combustion through the use of mid-kiln tyre injection as well as SNCR. We were able to reduce our NOx emissions by 60% through the combination of these technologies. However, we wanted to include these technologies and improve on them with the updated pyroprocess,” Lesslie said. “The Midlothian plant is considered to be a ‘new source’ under NESHAP and New Source Performance Standards, so lower emission rates are required for PM, mercury, NOx and SO2. Our objective with the modernised kiln system was to be the first source in the US to demonstrate compliance with these new source standards and we achieved compliance with them. Overall, PM emissions have been reduced by 90%; NOx emissions were reduced by an additional 60% and SO2 emissions have been reduced by more than 95% with the modernised process,” Lesslie said.
Equipment selection
Global Cement Magazine asked if everything changed with the latest project, or if some things stayed the same. “Good question! There were no changes made to the quarry through to the slurry storage systems during the modernisation, nor to the cement grinding and cement storage areas,” according to Hrizuk.
The Midlothian modernisation project was a design-build contract between Ash Grove Cement Company and FLSmidth. FLS engaged H&M Construction Company to perform the construction and the balance of the plan engineering. The foundation work began in 2012, the preheater tower steel was erected starting in January 2013, and first clinker production was 6 June, 2014.
Given the importance of the project to the company, as well as its size, the correct selection of the equipment in the modernisation project was crucial. “First, we selected a semi-dry process from FLSmidth, which allowed us to re-use equipment from the quarry through to the slurry basins. This is the second such pyroprocess operating in the US” according to Hrizuk. In the modernisation process, the 12-feet in diameter by more than 400 foot long (3.7m x 133m long) wet kiln was shortened to 246 feet (75m), and two of the five support piers were removed. The kiln drive and gear were uprated, as were the kiln support rolls on the drive pier. “The new equipment includes the Hot Disc reactor and dryer crusher, the large process bag filters and a coal grinding and dosing system. A new cement unloading system was installed to support cement sales.”
“While the pyroprocess was designed to produce 2600 short tons of clinker per day (2358t/day), Ash Grove and FLSmidth continue to work to achieve this design capacity,” he added.
Constructing the new equipment around the existing kilns was another challenge. “Two kilns remained in operation during the construction period,” Vidergar noted, “ The combination of construction around an operating manufacturing plant was a challenge to both the plant employees and the construction team.”
Ronald J Vidergar offered some more details about the equipment the company selected: “The semi-dry preheater tower consists of a hammer mill dryer crusher, dedusting cyclone and a single stage cyclone preheater with an in-line calciner. The kiln drive is capable of 5 rpm, and it is the fastest in our system. The Stage 1 cyclone diameter is the largest in our fleet of plants.” He said that Ash Grove now has the latest generation of the FLSmidth Cross-Bartm clinker cooler with a fixed inlet and heavy duty roll breaker.
What are the most unusual aspects to the process at Ash Grove Midlothian? Are there any things the IEEE field trip delegates should not miss?
“Delegates will want to see the Hot Disc® reactor, where we burn tyres in the modernised plant,” said Vidergar: “The Hot Disc reactor is a combustion device designed to burn tyres and provide hot gasses to the in-line calciner. The Midlothian reactor is the largest unit currently operating in the world and the first in the US, with a table diameter of 26 feet (8.66m). It is designed to process 7.7 short tons (7t) per hour and at rated kiln capacity, tyres could represent nearly 50% of the heat input to the system.”
To manage NOx emissions, SNCR using a 19% solution of aqueous ammonia is utilised. For mercury emissions control, the plant uses powdered activated carbon injection. Particulate emissions are controlled with a 12 compartment pulse jet bag filter that FLS Airtech supplied. The filter is designed for a gross air-to-cloth ratio of 2.6 feet (0.86m) per minute.
Delegates will see other new equipment. According to Vidergar, “Coal is ground with an Atox 17.5 three-roller vertical mill with a RAKM type dynamic separator. Pulverised fuel metering in the main kiln burner and in-line calciner is accomplished using Pfister rotary feeders. Hydrocarbon emissions from the coal grinding system are controlled by a regenerative thermal oxidiser (RTO). We use coal and natural gas as our primary fuels and passenger car tyres as an alternate fuel, which we burn in the newly installed Hot Disc reactor,” Vidergar said.
“Clinker alkali and sulphur levels are controlled with a kiln bypass and calcium chloride injection,” he said. “The bypass system has the capacity to remove up to 20% of the kiln process gases. The bypass filter is a single-compartment pulse jet collector with 1078 fibreglass bags that FLS Airtech supplied.”
Another change to the Midlothian plant was the plant process control system, which was converted to FLSmidth’s ECS version 8 control system, Vidergar said. “We installed Allen Bradley programmable logic controllers as well as Dell servers and computers to support the system. To round out the control system, we installed FLS’s PxP high level control and CEMScanner kiln shell scanning systems.”
Delegates interested in emissions monitoring will want to see the Continuous Emissions Monitoring System (CEMS), which was provided by CEM Solutions Inc. This equipment measures and reports emissions of particulate matter, NOx, SO2, mercury, ammonia, carbon monoxide and total hydrocarbons. Stack opacity is measured and reported as well as stack oxygen and carbon dioxide levels. The Data Acquisition System (DAS) is CemLink6 by VIM Technologies. This CEMS/DAS design is very similar to that installed at all of Ash Grove’s PC NESHAP plants and provides commonality of equipment and software across the fleet, Vidergar explained.
Education and training
Ash Grove’s Vice President of Manufacturing, Midwest Division, Stuart E. Tomlinson said that the employees were vital in the modernisation process. “Ash Grove had a stable base of employees who had to be educated concerning the operation and maintenance of the new equipment – Hot Disc reactor, dryer crusher, large process bag filters, ATOX coal mill, new kiln drive – which had not been used in the previous wet kiln process. Training everyone to understand how the equipment was intended to correctly operate was an important part of the modernisation effort.”
With so much new equipment, FLS provided specialised equipment and operations training, according to Tomlinson. “The training programs, held at both the Midlothian plant and at FLS’ United States headquarters, were designed for mechanical, electrical and operations personnel, and the training programs took place as employees continued to operate the existing cement plant,” he said.
“During the commissioning process, plant employees invested a significant amount of time and effort to become familiar with the new equipment and to get the plant to operate successfully. It was a much greater effort than we initially anticipated,” Tomlinson said. “Ash Grove employees from other plants also assisted in equipment check-out and plant start up. They worked in two and three week rotations over a six month period.”
Tomlinson also noted, “Despite challenges with the initial kiln operations and performance, Friday 6 June 2014 will always be remembered among Midlothian employees as the day that the first clinker was made in the modernised plant.”
“To date,” Vidergar said, “the major challenges have been demonstrating compliance with new source emissions limits, which the plant has succeeded in accomplishing. The thing that is now our main focus is to determine what is necessary to achieve the plant’s rated kiln capacity.”
IEEE tour
Local Chair Scott Nielson, Ash Grove’s corporate environmental manager, explained, “All of us at Ash Grove are looking forward to hosting the tour on Thursday 19 May. The tour of the modernised Midlothian plant will be an event that conference attendees will want to stay and participate in that day.”
“The logistics are what you would expect. Busses will be provided for participants’ convenience. During the ride, a brief safety training program will occur. We will provide safety glasses and hard hats at the site. We ask all attendees to wear closed toe shoes and clothing suitable for a cement plant tour. Attendees will interact with plant personnel who will be staged in various locations at the plant to respond to questions and ensure visitor safety. We will provide lunch before guests return to the conference hotel,” Nielson added.
“We are excited to host our industry colleagues and show them our modernised Midlothian, Texas, plant,” Nielson said.
Global Cement Magazine thanks the assembled executives, and Jacqueline K. Clark, Director of Communications and Public Affairs at Ash Grove, for their time and effort on our questions and on this article!