Coal
and biomass co-combustion in existing utility boilers is a
promising option of mitigating the fossil energy crisis and reducing
the gaseous emissions of NO
x
, SO
x
, and CO2. However, ash-related problems,
including fouling, slagging, and corrosion cause damage to the heat
exchange tube and reduce boiler efficiency. In an attempt to give
better insights into the slagging behavior during coal/biomass combustion,
an experimental investigation was conducted to study the growth of
slag when coal was co-fired with wood and corn stalk in a 300 kW pilot-scale
furnace. For comparison, combustion of pure coal was also conducted.
During the experiments, biomass proportions of 5 and 10% by weight
were examined. Slags formed on an oil-cooled deposition probe were
collected, sampled, and analyzed using scanning electron microscopy
and X-ray diffraction (XRD). The change in slag thickness with time
was obtained by a charge-coupled device monitoring system. With two
thermocouples in the probe, the heat flux through the slag could be
measured. The slag from pure coal combustion showed a layered structure
with different levels of compactness and hardness. The heat flux decreased
by 31.7% as the slag grew to 5.19 mm. The results showed that co-firing
wood significantly inhibited the slagging behavior. Especially in
the 10% wood case, hardly any slag was collected from the probe. Nevertheless,
co-firing corn stalk resulted in severe slagging, with a slag thickness
of 5.5 and 6.1 mm for two blend ratios. The formation of bubbles in
the deposits together with greater deposit thickness caused heat transfer
deterioration. XRD results revealed that the influence of co-firing
biomass and corn stalk caused quite different changes to mineral species
from wood. It was observed that fly ash under different biomass co-firing
conditions differed little on mineral compositions.
This
paper presents an experimental study on bubble formation in the process
of sintering of Zhundong coal and corn stalk (CS) ash blends in a
horizontal chamber furnace. The effect of the blend ratio of biomass
ash was investigated. Bubble parameters, such as number, area, and
porosity, were measured on the basis of a metalloscope equipped with
a charge-coupled device camera and digital image processing technique.
After sintering experiments, ash and condensed matter in the bubbles
were sampled and analyzed by X-ray diffraction. In addition, a chemical
equilibrium calculation was conducted to reveal the influence of biomass
ash on the formation of bubbles. The experimental results show that
a 50% CS blend has the greatest melting degree and the formation time
of bubbles is earlier than other cases, while a low ratio of CS ash
has limited influence on bubble formation and mineral composition.
The formation mechanism of bubbles is proposed on the basis of the
results. The condensed matter in the bubbles mainly contains NaCl,
CaSO4, and KCl. The results indicate that chlorine promotes
the transformation of alkali metals to gaseous phase. The chemical
equilibrium calculation verified the experimental results.
Experiments
were conducted in a pilot-scale test furnace to investigate the influence
of CaO additive on the growth of ash deposits and the characteristics
of fly ash. The amounts of CaO additive mixed with Datong coal were
0 and 3.5 wt %, and the operating temperature of the furnace was 1573
K. An oil-cooled probe was inserted in the furnace, and a charge-coupled
device (CCD) monitoring system was equipped with a water-cooling system
to obtain the variation of thickness of the deposits with time. This
system was used to evaluate the buildup of the deposits. Online monitoring
of flue gas was carried out for the emissions of NO
x
and SO2 using the flue gas analyzer. The fly ash
was collected from the bottom of the cyclone separator, and its characteristics
were determined using a range of analytical techniques, including
polarization microscopy, physisorption analysis, X-ray diffraction
(XRD), and X-ray fluorescence (XRF). Frequent shedding of ash deposits
was observed for pure coal. Nevertheless, under the condition with
additive, serious slagging with a thickness of the deposits up to
66 mm occurred. The addition of CaO resulted in a reduction of SO2 but an increase of NO
x
emission.
The fly ash produced by burning pure coal had a greater specific surface
area (SSA). The XRD and XRF analysis results revealed that the CaO
additive had a significant impact on the mineral and chemical composition
of the fly ash.
The United States of America and the People's Republic of China are responsible for over 40% of the world's CO 2 emissions annually and they will be able to effectively reduce global emissions if they fulfil their commitments jointly in climate change mitigation. Here we briefly summarize past climate collaborations between the two countries and compare their national climate policies. The major problems are the mutual distrust between the two countries and the priority of economic development over climate change mitigation within each of them. As atmospheric CO 2 levels are still increasing at an accelerating rate, it is essential for the largest two emitters to turn ongoing bilateral dialogue into prompt mitigation action and maintain long-term joint efforts in reducing emissions. We suggest that the two countries should recognize and take advantage of their differences in socioeconomic, political, and technological conditions. Furthermore, the two countries need to share their experiences and technologies for safely utilizing relatively clean energy resources, especially shale gas. The success in climate cooperation between the USA and China is critical to sustainable development around the world.
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