Faced with continued pressure from the EPA, many small to medium sized coal-fired generating stations are facing economic uncertainty. Do they invest in the high-capital-cost systems needed to comply with NOx and SO2 limits, do they operate on a limited basis when demand allows or do they close altogether.
In the middle of this environmental compliance dilemma sits natural gas. Lauded by environmental interest groups as the low cost, low emissions alternative to coal, gas certainly has its appeal. However, as recent activity has shown, gas pricing can be volatile, easily reaching two, or even three, times the price of coal on a per unit basis. Complete conversion from coal to gas could be disastrous if gas prices continue to rise.
Dual Orifice Cofiring, the key component in Breen’s Enhanced Gas Cofiring process, 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 Minimum Unit Load, load control and emissions per mmBTU can be realized.
It isn’t just the addition of variable gas volume that makes Dual Orifice Cofiring effective. Managing the combination of coal, igniter and cofire-gas fuel streams to create the optimum environmental and commercial result is a non-trivial task. But the creation and control of a viable, Enhanced Gas Cofiring system will move your coal-fired asset into a truly Fuel-Flexible plant.
Enhanced gas cofiring removes many of the economic and environmental problems that face coalfired generating assets
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% – 7% of the Unit heat input as gas in the upper furnace at provides NOx reduction and SCR benefits,
2. Placing 25% – 30% of the Unit MCR heat input as gas at the burner level provides dramatic flexibility in unit turn-down,
3. Any amount of 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 2. An NFPA driven need to maintain two pulverizers in service.
Cofiring substantial quantities of gas at low load can relieve both of these drivers. Replacing up to half of the total heat input with gas reduces the amount of SO3 by a base of 50%, essentially eliminating the SO3 factor in the MOT calculation.
From an NFPA standpoint, replacing half of the heat input with gas essentially replicates the presence of the heat input of one mill, eliminating the need for two.
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% redction 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 rebirth of plentiful, and relatively inexpensive natural gas, many plant operators want to add more gas to mix.
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.
Dual Fuel Outlets
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