The $60-million project is part of the commitment the company made under former President Clinton's Global Initiative to fight climate change. It is the first of four such boilers that Pratt plans.
The $60 million project is part of Pratt’s $1 billion commitmment to fight climate change
The boiler is really the "baby" of project engineer Steve Morgan who was in charge of construction and startup, under the overall guidance of John Cosher, project manager. The aim of the project was to reduce energy costs and the amount of material Pratt was sending to landfill. Rejects are recognized as an energy source, so why pay to landfill a fuel?" Morgan says." And, it is estimated that the unit displaces approximately 310 tonnes/day of greenhouse gases.
Approved in 2007, construction began in 2008 and on July 23, 2009, Pratt was able to fire up the boiler using natural gas. The gasification of wood and other waste followed soon after on September 12. Its capacity is 9.3 MW but it only needs to produce enough power to supply the mill's steam and electrical needs, about 8.3 MW. Pratt does not sell electricity back to the Georgia grid. On average, the unit can produce 200,000 lb/hr of steam.
Although open to the elements here, work is underway to enclose the boiler
Able to convert contaminants
The wood waste comes from sawmill residuals, land clearing, road widening, as well as construction and demolition (C&D) sites. It is sourced within a 100-mi radius of the mill. The C&D material is the most valued. "It burns cleaner and has a high Btu value," Morgan explains. "We want as much of this as possible."
The mill also gasifies the plastic contaminants that are removed from the recovered fiber that Pratt uses as furnish for the paper machine. Taking this a step further, it also converts the plastic contaminants from its Staten Island, NY, paperboard mill. The wood and plastic are mixed in a 70:30 ratio before being fed to the boiler. No natural gas is needed.
Pratt does have another natural gas boiler that is used as a backup if needed. However, Morgan points out that the bio-fuel boiler converts cleaner than the natural gas unit.
Upon arrival at the mill, the wood waste is either dumped on a storage pile or fed directly into the process. The material passes through a scalping screen that removes oversize pieces of wood such as stumps as well as any rocks and metal that may be in the load. Pratt pays by the truckload and will do spot checks. It will turn away a load if the moisture content is too high.
After the scalping screen, the material passes to a pile where a chain reclaimer feeds it into a mixing area where the plastic is added.
The plastic contaminants from the mills pass first by a magnet where ferrous metals are removed. Then, there is an eddy current to remove aluminum. Finally, an optical sorter removes any PVC in the material. Pratt is able to recycle the aluminum and ferrous metals.
The boiler bed is sand, which comes from Atlanta Sand. There are specs the supplier must meet: grain size, silica content and contaminant content. Pratt is studying the use of a larger grain to help reduce carryover.
There are two startup burners and the operating temperature of the bed is 1,400°F, which gasifies the fuel. Morgan estimates fuel yield to be 95-97%. Primary air is used to control the bed temperature. If it goes too low, e.g., too much moisture, the boiler operators can add more air.
There is a flue gas recirculation system to help control temperature and emissions. Post-combustion temperature is 1,600-1,700°F and 1,500°F.
The steam drum appears quite small relative to the size of the boiler but it can produce 230,000 lb/hr. It goes through four superheater sections before entering the turbine. Out of the boiler, steam pressure is 930 psi at 860°F. Low-pressure steam is exhausted from the turbine at 165 psi; high-pressure steam is extracted at 265 psi. The steam is sent to the mill, mainly for use in the dryer section but is used anywhere else needed such as the starch cooker. Approximately 70% of the steam is returned to the boiler as condensate.
|Austrian Energy (AEE) supplied the boiler. It is the second such unit it has installed in North America. TPI and Jeffrey Rader supplied the woodyard. Canadian-based Machinex supplied the fuel prep system.
Process Baron supplied the fans. Continental Conveyor did the fuel delivery systems, e.g., silos. Finally, Rockwell Automation was responsible for the control system, both hardware and software. There are more than 1,500 analog and discrete points. Morgan notes that the unit is more complicated to control than a conventional coal or trash-fired boiler. Most of the difference is in the air systems on the bed and controlling temperature.
Training for the operators was done in-house. Most of the operators were at the mill during the entire construction period and were able to do a lot of hands-on from the start of the project.
Pratt reports to the State of Georgia's Environmental Protection Department (EPD). To sample emissions, there is an in-situ FTIR, which is a "box: inside the stack that tests gases. "We can view much more than we need to report," Morgan says.
There are also velocity and opacity meters as well as an oxygen sensor. The mill needs to report NOx and SO2. Its NOx limit is 0.07 lb/million Btu and for SO2, 0.2 lb/million Btu. Also, volatile organic compounds (VOC) need to be below 10.8 tons over a consecutive 12-month period. "We are well under all these numbers," Morgan adds.
The sulphur dioxide is controlled by sodium bicarbonate. "It reacts faster than lime that a lot of people use," Morgan explains.
Ammonia is used to control NOx. Also, Pratt is adding activated carbon for heavy metal removal. Moisture content of the stack gases runs about 25%.
The boiler goes through about 25 tons or a truckload per week of sand. This replaces what drops into the hopper or is sent to the ash silo. Morgan says that it is important to keep the sand on the bed as clean as possible.
Ash can be landfilled although some local farmers are using it as fertilizer. The droppings in the hopper bins are sent to landfill although the mill is looking at a way to take the metals out so the sand can be reused as fill on construction sites. The mill gets about 20 tons per week from the hopper.
Some of the metals are already being taken out. In the office, Morgan has a bag that must weigh close to five pounds and is full of coins that have gone through the process and fallen into the hopper.
Compared with a conventional power boiler, Morgan says the new unit can produce more steam and this has helped the board mill increase production by up to 10% for some grades.
There is a steam condenser that allows the mill to recycle the steam instead of venting it.
As noted, the boiler is part of Pratt's commitment to fight climate change and to live up to its name as a 100% recycler. The new clean energy unit has allowed Pratt to eliminate the use of natural gas, produce its own electricity and reduce the amount of waste it was sending to landfill. The key to the project, adds Morgan, was that with the mill being in an urban area, the unit had to be low-emission.
The first Millugator
|Conyers is the site of Pratt Industries’ US headquarters. Located just east of Atlanta, Conyers is also home to Pratt’s 360,000-ton/yr paperboard mill. Using from a 100% recycled furnish, the mill produces 214,000 tonnes/yr of recycled linerboard and another 179,000 tonnes/yr of corrugated medium on one machine, a Voith unit that was installed when the mill was built in 1995.
Conyers is home of the world’s first Millugator. That is, it is a combined paper mill/box plant, bringing the corrugated process full circle under one roof. Besides the paper machine, there is one BHS corrugator and seven finishing machines on the Millugator site. Pratt has two other corrugators at the Conyers campus.