All co-firing experiments were conducted using a 30 kW (100,000 BTU/h, approximately 15 lb or 6.80 kg of coal/h) small-scale furnace capable of firing most types of ground fuels. A schematic of the furnace is shown in Figure 3.20. Propane and natural gas are used to heat the furnace to the operating temperature of 1100°C (2000°F). Type K (shielded, ungrounded) thermocouples are used to measure the temperature along the axial length of the furnace. A solid fuel hopper feeds coal and coal/biomass blends during experiments. Primary air (6m3/h, 15-20% of total air) is necessary to propel the finely ground solid fuel through the fuel line and to the furnace. Prior to ventilation, all exhaust gases pass through a water-cooling spray to significantly lower the temperature of the gases. A sump pump pumps this water out of the furnace. More details are provided in Frazitta et al. (1999), Arumugam et al. (2006c), Annamalai et al. (2005), Lawrence et al. (2009) and in Thien et al. (2012).

The secondary air (75-80% of total air) heater was run for an hour before the experiment was started. The secondary air is swirled (Swirl number = 0.7) prior to entry into the combustion chamber. Once the secondary air reached a steady temperature, approximately 500 K (440.33°F), the propane torches were ignited. Natural gas and propane were used to preheat the furnace to operating temperatures. Once the furnace reached 1366 K (2000°F), the natural gas was turned off and the natural gas line was closed. The solid feeder line was opened and the solid feeder was turned on and set to the desired fuel flow rate. The primary and secondary air lines were set to the appropriate flow rates to obtain the desired equivalence ratio. The furnace was allowed to run for 30 minutes before the first readings were taken. The measurement was taken at the last sampling port just before the quenching water sprays and the wet flue gases were ducted to the atmosphere. After taking a measurement at this equivalence ratio, the secondary air could be adjusted to a different equivalence ratio. After taking measurements at all desired equivalence ratios, the furnace was turned off.

Fuel properties played a significant impact on the burnt fraction and the emissions created by combustion. The results from the co-firing experiments performed are discussed and their role in evaluating the combustion performance of the fuels is explained. The performance was evaluated by measuring combustion efficiency (burnt fraction) and the emissions levels of pollutants that include NOX and CO. In addition, overall fuel nitrogen conversion efficiency to NOX was also determined. The mercury emissions are presented elsewhere (Udayasarathy, 2007).

The co-firing involves a mix of finely pulverized biomass and coal. The size distribution was obtained using an ASTM sieve shaker. Sauter mean diameter (commonly abbreviated as SMD or d32) is commonly used for estimating the average size of solid fuel particles. The SMD is defined as the diameter of a sphere that has the same ratio of volume to surface area. It is represented as the following equation:

n

J2dl ■ n

SMD or d32 = -=————-

^ di ■ ni

i=1

where di is the diameter of particles and ni is the number of the particles of diameter di. According to the Rosin Rammler fit, the cumulative mass fraction CMF or drops (or particles) with a dimension lesser than dp is given as (Annamalai and Puri, 2007):

CMF = 1 — exp(-bdp)

where b: size constant, n: distribution constant and is a measure of spread of drop size. In terms

of the dp, charac size:

CMF = 1 — exp

where dp, charac denotes the characteristic drop or particle size for which CMF = 1 — exp(-1) = 63.2% and

1

dn

p, charac

The fraction R having size greater than dp is:

The plot of ln{R} vs. dp must be linear and the slope yields “n” and dp, charac is determined from the plot at R = 0.368. The values of “n”, “b” and SMD are presented in Table 3.9 for several fuels. Note that the coals had a larger SMD than those of DB fuels. The dirt (or mineral matter) that got collected with the DB fuels passed through all of the sieves and collected in the pan. This caused the DBs to have a smaller SMD.

Table 3.9. Size distribution parameters, adopted from Lawrence (2007).

Size distribution parameters

TXL WYO LA-PC-DB-SepS HA-PC-DB-SoilS

n 1.2991 1.4369 1.0934 1.2612

b 0.000934 0.00042 0.0024 0.0013

SMD (microns) 396 396 96.7 91.6