part 4
In one biomass cofiring study at large coal power plant, it was quickly discovered that if the utility must pay to have its ash landfilled, as opposed to selling nearly all of its ash, the differential cost to the plant was a loss of more than $5 million per year.
Emissions Controls. At biomass cofiring levels of 10% or less by heat input, most electrostatic precipitators (ESPs) can successfully collect biomass ash. However, some biomass produces ash that has either a very high or very low resistivity, thus making it difficult to collect.
This is often the case in studies at power plants designed for high-sulfur coals, and significant levels of biomass cofiring may require either ESP upgrades or chemical injection. Furthermore, unless the furnace combustion and milling systems are well tuned for both the coal and biomass, boiler fly ash unburned carbon can greatly increase. This is a problem for two reasons. First, high levels of unburned carbon can change the ash resistivity and increase ash re-entrainment after being collected in the ESP. Second, biomass ash with high amounts of unburned carbon can accumulate in hoppers or in fabric filter baghouses, leading to sometimes destructive fires.
Although biomass combustion often reduces NOx emissions, selective catalytic reduction (SCR) catalysts can be very sensitive to heavy metals and alkaline earth elements contained in biomass. If the biomass in question includes a significant amount of post-industrial waste, it can sometimes contain arsenic, lead, cadmium, potassium, and other items harmful to catalyst life. Where high levels of ammonia slip are encountered with either an SCR or a selective noncatalytic reduction system, biomass ash can agglomerate and form tenacious deposits on air heaters downstream (Figure 6).
6. Air heater fouling. High levels of ammonia slip can agglomerate biomass ash on air heater baskets. Controlling ammonia distribution to the furnace or selective catalytic reduction system is critical for avoiding this problem. Source: Black & Veatch
Flue gas desulfurization scrubbers are not normally sensitive to biomass ash, unless the ash contains excessive amounts of chlorine. Some herbaceous biomass ash can contain more than 0.5% chlorine, whereas many scrubbers have a fuel-based chlorine limit of less than 0.2%.
Greenhouse Gas Emissions. Biomass fuel is rarely 100% carbon neutral, and calculating the life-cycle GHG emissions from biomass cofiring or conversion can be quite difficult, especially when one considers all of the possible feedstocks.
The planting and growing cycle, fertilizer use, harvesting, transportation, milling and processing, and transportation to the power plant all contain numerous assumptions that will differ from study to study. A review of 12 studies published in the past three years regarding the net change in GHG emissions at cofiring facilities found that the carbon neutrality of biomass fuels ranged from 40% to 96.5%. Clearly, a life-cycle assessment is required for any proposed biomass cofiring or conversion scheme.
Engineered Biomass Fuels
Most biomass fuels suffer from three fuel properties that limit their use at existing coal-fired power plants: low energy density, high moisture content, and poor grindability. All of these limitations can be addressed via the use of engineered biomass fuels. Engineered biomass fuels can be created by many different processes—including torrefaction, steam exploding, and processing with waste heat—but the net results are very similar.
Typically, a large amount of moisture is driven off the fuel, significantly increasing the heating value, while the chemical structure of the fuel itself is altered such that the biomass becomes more friable and easy to grind in a coal mill. Some processes even promise sulfur, chlorine, and heavy metal removal as part of the upgrading.
The primary drawbacks to engineered biomass fuels are their price and availability. Availability is a function of the market being in its infancy, and price is a result of both handling the fuel twice and the energy and materials used in the upgrade process. Unfortunately, in most cases, engineered biomass products cannot compete with coal without some special legal, environmental, or political incentive at work.
EPRI’s O’Connor is bullish on upgraded biomass. “Torrefied, steam-exploded, and other upgraded biomass products can usually be burned at existing coal-fired power plants with relatively minor modifications. The problem is operating cost—finding an upgraded biomass which can meet environmental goals economically.” But, O’Connor cautions, “ensuring sustainable biomass is the critical thing. A lot of the pellet production is from waste wood or wood unsuitable for other uses. Biomass resource planning needs a sound strategy for sustainability to keep the process as carbon-neutral as possible.”
Can Biomass Blossom?
Although several potential pitfalls have been discussed in this article, few should be considered fatal flaws in a plan for biomass cofiring or biomass conversion. Though it is true that cheap natural gas currently dampens the enthusiasm for biomass, many power plants have no suitable gas supply options. Furthermore, many power plants located in remote regions or on island nations operate in circumstances where even engineered biomass fuel is less expensive than imported fossil fuels. Some power plants are located near plentiful biomass sources, and others have the ability to produce their own fuel.
So, despite the pitfalls, the net effect is that biomass use is increasing for electrical generation, even in the U.S. According to FERC, 219 MW of biomass capacity were added in 2013. Should a GHG emissions or carbon tax be levied in the future, or the price of gas increase significantly, then using biomass may be an option a utility can’t ignore. ¦
— Una Nowling, PE (nowlinguc@bv.com) is a project manager and technology lead for fuels at Black & Veatch. She has worked on fuels-related issues and analyses at more than 550 different units over 20 years, specializing in coal, natural gas, and biofuels. She is also an adjunct professor of mechanical engineering at University of Missouri-Kansas City.