(529c) Ultrasound Measurements of Temperature Profile Across Gasifier Refractories

Ultrasound Measurements of Temperature Profile Across Gasifier Refractories

Yunlu Jia and Mikhail Skliar

Department of Chemical Engineering

University
of Utah

Coal and biomass gasification is a process
characterized by extreme operating conditions of high temperatures and pressures,
chemical aggressiveness, and mechanical abrasion. Several technological
challenges impact the reliability and economics of coal gasification. The key
challenge remains the complete lack of sensors that are capable to reliably
perform in harsh environment over an extended period of operation. The
conventional approach of developing hardened conventional insertion sensors has
proven to have limited effectiveness in the aggressive gasifier
environment.  Gasifier operators routinely use ?indirect measurements?,
such as methane concentration in the gasifier product gas (?temperature? is
reported in ppm of methane!), to estimate the average temperature in the
system. Such indirect methods cannot be used to infer temperature if the
observed methane concentration is caused by the presence of a region or regions
inside the gasifier that are running particularly hot. Note that optical
techniques, including combustion specific measurements of temperatures and
reaction composition, are hardly suitable for a pressurized, slagging gasifier
since the flowing slag blocks optical access ports. They are also not suitable
for measuring thickness and other characteristics of non-reflective and
non-transparent containments.

In this presentation, we describe a novel
approach that uses noninvasive ultrasound to measure: (1) spatial distribution
of temperature across the refractory or other containment; (2) temperature of
the hot side of the refractory and (3) refractory thickness. In the simplest
implementation of the temperature measurements using ultrasound (US), the time
of flight (TOF or return delay) of an US pulse introduced on the cold side of
the refractory and reflected from the refractory's hot side is measured. The
temperature dependence of the speed of sound can then be used to determine an
?average? temperature over the refractory thickness. This simple approach fails
when significant thermal gradients are present, as is the case for gasifier
refractories. The central idea, summarized in Fig. 1, which enables us to
measure the temperature distribution when large gradients are present, is to
create an US propagation path inside the refractory with controlled
backscattering at predetermined spatial locations. In Fig. 1, the same
piezoelectric element serves as the transducer and receiver; modification for
the case of a separate receiver is straight forward.  The
transducer-generated pulse propagates through an engineered material which produces
multiple partial echoes (panel B). The time of flight of each echo is measured
and used to calculate the speed of sound (SOS) which changes with the
temperature (panel C) of the corresponding segment of the refractory. By
sequentially estimating the temperature of each segment, the temperature
distribution along the entire path of ultrasound propagation is obtained (panel
D). We will present experimental validation of this approach and outline
options for creating engineered materials that produce partial US reflections
from pre-determined positions. The achievable accuracy and spatial resolution
of the measured temperature profile will be discussed.