Progress Variables to Resolve the Steady State Period in a Batch-Type Fixed Bed Combustor

ABSTRACT

Lab scale combustion of solid fuels, such as biomass, is mostly performed using fixed bed combustors. Steady state data detailing various progress variables (temperature and emissions) are widely available within the literature. However, there is very little information available on how the onset of the steady state operation is identified especially in relation to batch operation, which is subject to depleting fuel bed. This paper uses experiments coupled with a systemic post-processing approach based on MATLAB in order to identify the onset of the steady state operation. The results show that by using both the percentile mean deviation and slope of certain measured progress variables (including the fuel bed temperature, freeboard temperature, fuel mass loss, and emissions) the steady state regime can be accurately predicted. The use of freeboard temperature data alone is not sufficient. Based on the methods developed, radiation error in thermocouple data was found to be significant in both non-staged and staged air combustion modes of fixed bed combustor. Moreover, the radiation error was lower for downstream thermocouples in contrast to upstream thermocouples.

KEYWORDS:

• Fixed bed
• combustion
• methodology
• temperature correction
• steady state
• batch-type

Nomenclature

$C{P}_g$	=	Gas thermal conductivity (J kg−1 K−1)
$d_{T}C$	=	Thermocouple bed diameter (m)
dt	=	Temperature difference
$h_g$	=	Gas convective heat transfer coefficient (W m−2 K−2)
$k_g$	=	Gas thermal conductivity (W m−1 K−1)
L	=	Combustor length (mm)
$M_i$	=	Measured variable
$\bar{M}$	=	Athematic mean of selected variable
$\dot{m}$	=	Slope
$N$	=	Number of samples
$Nu$	=	Nusselt Number
$P_r$	=	Prandtl Number
r	=	Radial distance from combustor wall (mm)
$R_i$	=	Percentile mean deviation
$R_e$	=	Reynolds number
$T_b$	=	Thermocouple bead temperature (K)
$T_g$	=	True gas temperature (K)True gas temperature (K)
$T_w$	=	Near wall temperature (K)
$Qp$	=	Primary air flow rate (l/min)
Qs	=	Secondary flow rate (l/min)
x	=	Axial distance from the top surface of fuel bed at the start of experiment (mm)

Greek letters

ε	=	Thermocouple bead emissivity
εTotal	=	Total error
${\epsilon }_s$	=	Systemic error
${\epsilon }_r$	=	Random error
$\bar{\mathrm{\varnothing }}$	=	Mean of the measured parameter
${\mu }_g$	=	Gas dynamic viscosity (kg m−1 s−1)
${\sigma }_b$	=	Stefen-Boltzmann constant = 5.67 × 10 −8 (W m−2 K−4)

Subscripts

e	=	Emissions
m	=	Mass

Acknowledgments

The first author acknowledges the PhD scholarship support provided by Higher Education Commission (HEC), Pakistan and Edith Cowan University (ECU) Australia. Dr. Babak Rashidian is also thanked for his assistance with the MATLAB code applied.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Supplementary Material

Supplemental data for this article can be accessed online at https://doi.org/10.1080/00102202.2022.2150969.

Notes

¹ The flow rate is equal to 0.358 kg-m⁻²-s⁻¹ by taking into account density of air 1.23 kg-m⁻³ and area of fuel bed as 0.0320 m²..