Unlocking Hidden Combustion Properties of Waste Streams

By Jim DeSellem, Advisory Engineer, Babcock & Wilcox

Special to The Digest

Renewable power generation from biomass and waste is an important solution to combat climate change – one that reduces methane and carbon dioxide emissions and supports both sustainability and stewardship of resources. Fluidized bed combustion technology offers the potential to use a wide range of carbon-neutral biomass sources, as well as municipal and industrial waste to produce steam for processes and electrical power. However, biomass and waste fuels are not all created equal, and can produce environmental emissions, non-combustible materials and other less-than-ideal properties which can have negative impacts on reliable boiler operations. Quantifying and predicting these impacts is required to evaluate the potential of a biomass or waste fuel as an energy source.

We currently divide biomass/waste into three general groups – traditional biomass, including wood chips and other virgin wood waste, agricultural waste that contains energy crops and field residue; and biomass or waste residuals or leftovers from another process, such as waste sludge from the pulp and paper making process and bio-waste from the manufacture of cellulosic ethanol.

While much is known about the combustion properties and energy-producing potential of traditional biomass and agricultural waste, the third group still presents technical challenges on many levels. The characteristics and reactivity of a waste stream(s) from processed biomass differ significantly from the parent biomass. Traditional testing methods and empirical data are insufficient for predicting the reactivity of elements and other characteristics necessary for a proper evaluation of the energy potential of a waste stream. However, through study and testing, Babcock & Wilcox researchers have discovered previously unknown combustion properties of many biomass waste streams.

Unlocking the Basic Chemistry

The chemistry of biomass and waste is such that not all of the components are reactive and contribute to combustion challenges. The first step in B&W’s research was to find or develop a testing method that revealed the split between the reactive and unreactive components in a potential energy source. Traditional laboratory ultimate analysis was not sufficient to provide all of the answers sought. Advances in chemical fractionation analysis showed the means to differentiate reactive and unreactive constituents quantitatively, but there was no experience or procedure for biomass or waste. A U.S. university, under contract with B&W, developed a modified version of chemical fractionation analysis specifically for biomass and waste. This method identified the portion of the targeted elements that existed in a reactive versus unreactive form.

Chemical fractionation involves performing several washes of a sample fuel with specific solvents and analyzing what was captured in each wash and the remaining residue. Figure 1 shows a typical chemical fractionation sequence.

Figure 1: Schematic of typical chemical fractionation method

Four samples were chosen to test this new procedure for biomass and waste – wood chips, switch grass, corn stover, and poultry litter. The four samples ranged from benign to problematic for energy conversion by combustion. Figure 2 shows one example of the differences and similarities of the reactive and unreactive components the research team compared. The developed chemical fractionation procedure for biomass and waste quantitatively aligned with what our experience told us for these four samples.

Figure 2: Weight percent of water-soluble and ion-exchangeable elements for the candidate samples

Although the results of the chemical fractionation analysis could significantly aid in predicting combustion challenges, the research team still sought to quantitatively compare combustion and operating conditions to the extent of the agglomeration and fouling potential of biomass and waste and demonstrate a relationship between the chemical fractionation data and predicted operation in a commercial unit.

Bringing the Real World into the Lab

Using a traditional pilot test to gather operational data for the relationship between the chemical fractionation data and predicted operation on multiple biomass/wastes was too expensive and time consuming to be practical. The B&W team aimed to develop a simple and cost-effective lab test that produced real-world operating data without the need for a pilot test.

A new bench test was developed around B&W’s 2-inch bubbling fluidized bed (BFB) reactor shown in Figure 3.

Figure 3: Bench Reactor

To develop testing procedures, field materials (fuel mix and bed material) were taken from an operating commercial B&W BFB unit that combusts a very high agglomeration and fouling biomass/waste fuel mix. B&W had considerable experience and operating data on this unit and its fuel mix, which was used to calibrate the bench reactor. It took many iterations to the testing procedure and setup to get the bench reactor to match the commercial BFB’s operating conditions. When finished, the B&W team was impressed with how accurately the bench reactor predicted actual commercial operating conditions and the biomass/waste fuel mix’s sensitivity to changes in the operating conditions.

B&W’s team theorized that as bed material begins to agglomerate/foul, the characteristic of the frequency, fluctuations in the pressure signal change from a Gaussian distribution to a periodic distribution. Therefore, principles of advanced nonlinear signal analysis based on chaos theory can be used to detect the early onset of agglomeration and fouling. Using two highly modified chaos equations to process the bench reactor’s pressure fluctuations, the B&W team was successful in detecting and verifying this phenomenon. This success permitted the prediction of the operating conditions when the chemistry of the biomass/waste was first beginning to shift to a problematic state. The team could now quantify the operating conditions from “very good” through to the point of failure. Figure 4 is a graphical representation of the processed frequency fluctuations for a biomass/waste in the bench reactor.

Figure 4

Now that the calibrated bench reactor could provide accurate operating conditions for a biomass/waste, the team pondered if it could use this bench reactor to advance the use of a reagent to alter the chemistry and improve the operating conditions for biomass/waste that was previously unacceptable for commercial operation. It took several more iterations, but the B&W team developed a successful procedure for its bench reactor to also evaluate a reagent under actual operating conditions.

Using the bench reactor, along with the chemical fractionation data, to evaluate reagents opened the door to further understanding of why previous field trials of reagents by other researchers succeeded or failed. B&W’s team was also able to blend two reagents that multiplied the positive results (1 + 1 = 3 type of effect).

Step 3, Putting It All to the Test

Working with cellulosic ethanol (gen 2) wastes (filtercake and syrup) provided an opportunity to test B&W’s innovative biomass/waste test protocol. Data collected from the chemical fractionation and bench reactor was used to set operating conditions for a test burn of these waste streams using a fluid bed pilot test facility at a U.S. university.

Figure 5

The typical lab ultimate analysis showed that both waste streams should fail under typical operating conditions in a commercial unit. However, the chemical fractionation and bench reactor testing told us that the filtercake would be an acceptable energy source but that the syrup was more challenging to use. Armed with all the lab data and predicted operating conditions, B&W’s researchers proceeded to the pilot testing phase. The fluid bed pilot unit was operated under conditions that B&W predicted would be successful and conditions predicting failure.

Conclusion

The testing campaign with cellulosic ethanol waste streams was a success that identified combustion properties of potential energy sources.  B&W’s innovative lab testing protocol accurately predicted the results of the various operating conditions. B&W operated the pilot unit on a continuous basis using a mixture of filtercake and syrup that represented the production rates of the two waste streams. By varying the operating conditions, B&W’s lab predictions were confirmed accurate. Since these tests, B&W has applied these innovative testing methods to other biomass/waste fuels with similar accuracy.