In today’s tough economic environment aging coal-fired generating stations are faced with a tough dilema. Do they invest in the high-capital-cost systems needed to comply with particulate, NOx and SO2 limits, or do they operate on a derated basis.
Cofiring with natural gas offers a solution to these environmental compliance issues. The increased availability of natural gas and competitive costs in recent years have made this a viable option for utilities with ready access to natural gas.
Dual Orifice Cofiring, is the key component in Breen’s Enhanced Gas Cofiring process. It provides a reliable NFPA Class 1 Igniter coupled with a high volume, annular, secondary gas supply. With the ability to control natural gas heat input up to 35% of Maximum Unit Continuous Rating, significant improvements in operations and environmental compliance can be realized.
Operational benefits can be realized in a variety of areas – improved efficiency through lower excess air, reduced LOI. Lower operating costs in many areas from reduced coal usage – reduced erosion of boiler tubes, lesser slagging, reduced flyash disposal costs, lower pulverizer operation & maintenance costs.
Environmental compliance benefits include reduced flyash allowing particulate control equipment to perform at higher efficiencies and lower emissions. Gas Cofiring also leads to reduced NOx, SO2 and CO2 emissions.
Enhanced gas cofiring (EGC) removes many of the economic and environmental problems that face coalfired generating assets face today.
Environmental and Commercial Impact
A coal plant needs to burn coal to operate correctly. The emissivity of the
coal particles as they burn is assumed in the heat transfer surface design.
However, some portion of the coal can be replaced with cleaner burning
natural gas to capture the cost/enironmental benefits of both fuels.
The goal of EGC is to replace 35% of the coal with gas as shown in this diagram.
1. Placing 5% – 10% of the Unit heat input as gas in the upper furnace for NOx reduction as part of Fuel Lean Gas Reburn (FLGR) solutions,
2. Placing 25% – 30% of the Unit MCR heat input as gas at the burner level provides dramatic flexibility in unit turn-down,
3. The coal replaced with natural gas provides a reduction in SO2/SO3, Hg, HCl, particulate and CO2 emissions
Minimum Load Reduction
The ability of the unit to operate at its absolute minimum is critical in today’s operating environment. Coal-fired assets must be able to rapidly adjust to changes in customer demand and to changing availability of wind and solar power.
Minimum load is generally driven by two major variables:
1. The SCRs Minimum Operating Temperature (MOT) 2. An NFPA driven need to maintain two pulverizers in service.
Cofiring substantial quantities of gas at low load can relieve both of these limitations.
Replacing up to half of the total heat input with gas reduces the amount of SO3 by half, essentially eliminating the SO3 factor in the MOT (lower SO3 allows lower MOT).
From an NFPA standpoint, replacing half of the heat input with gas essentially makes one of two coal mills redundant.
Breen provides a variety of solutions for Acid Gas Management and Fuel Flexibility. Our Enhanced Gas CoFiring solution may be your key to low-cost compliance with today’s environmental and economic demands.
Environmental Operating Co-Benefits
According to a 2010 study, the annual cost of operating wet FGD and SCR equipment amounted to $135/Ton and $410/Ton respectively. A similar cost for Hg is estimated at $5,000/Lb and SO3 requires almost $2,500/Ton to control.
For fuel cost conditions where natural gas is less than $1.00/mmBTU more expensive than coal, these environmental operating co-benefits actually offset the added fuel cost.
Co-benefit Areas Include: FGD Limestone Use, SCR Ammonia Use, Dry Sorbent Use for SO3, Activated Carbon Use for Mercury control and reduced Byproduct disposal costs for Scrubber sludge and flyash. Maintenance cost improvements
due to lower equipment loading and extended catalyst operating times can also be considered.
Replacing 35% of the coal input with natural gas yields:
1 – 20% reduction in CO2
2 – 35% reduction in SO2/SO3
3 – 45% reduction in NOx
4 – 35% reduction in Particulate
5 – 35% reduction in Mercury
Besides the positive environmental effects, Enhanced Gas Cofiring has two major commercial benefits:
1. Reduction in Unit Minimum Load
2. Reduction in Environmental Operating Costs
The Science Behind Dual Orifice Cofiring:
The concept of adding natural gas to a coal fired boiler is nothing new. Gas igniters have been used for many years as both ignition and coal-flame stabilization devices. But, with the availability of natural gas at competitive pricing makes it attractive to introduce more gas and achieve fuel flexibility.
The problem has historically been how and where to put this variable gas input. Placing it at the igniter location has value since gas needs to be present there, anyway. But placing 35% of the unit MCR heat input through a device designed for 10% creates significant competition between the gas flame and the coal flame for available oxygen.
Additionally, doubling, or tripling the gas throughput also places the resulting flame near the center of the boiler, minimizing waterwall steam creation and increasing superheat/reheat steam temperatures.
Breen Enhanced Gas Cofiring
An NFPA Class 1 Core Igniter An annular gas outlet surrounding the core igniter
The core igniter is a fixed flow device sized for the burner it supports. The second, annular, gas outlet is controlled separately and can be fixed or variable depending on the application.
The ratio between the two fuel outlets is site specific to provide optimum flame stabilization from the core and variable, annular, flow to keep cofire heat input near the walls for better steam production and minimal coal flame disruption