The co-combustion of hard coal with raw and torrefied biomasses (willow (Salix viminalis), olive oil residue and waste wood from furniture manufacturing)

High-temperature pretreatment of coal-based mineral matter in an oxidizing environment significantly enhances the leaching characteristics of rare earth elements (REEs). A research study has found that the temperatures used in fluidized bed combustion (FBC) of coal to produce electricity are near optimum for pretreating the associated mineral matter prior to leaching to maximize the recovery of critical materials. Tests were performed on representative samples collected from preparation plants treating West Kentucky No. 13, Illinois No. 6, and Fire Clay coal seam sources as well as fly ash and bed ash samples from two FBC power plants. Acid leaching tests using 1.2 M HCl at 75 °C were performed on both the coal and the FBC ash samples. Prior to leaching, the coal samples were pretreated at temperatures of 600, 750, and 900 °C in an oxidizing environment to study the effect on leaching characteristics. The results showed that pretreatment at 600 °C for 2 h resulted in a significant increase in REE recovery...

Acid Leaching of Rare Earth Elements from Coal and Coal Ash: Implications for Using Fluidized Bed Combustion To Assist in the Recovery of Critical Materials

The impact of additives addition on the combustion behavior of biomass with high content in ash-forming species is evaluated in this review. Their influence on both emissions (particulate matter and gaseous emissions) and deposits formation (fouling and slagging) are here considered. The uncertainty in the availability, under current sustainability criteria, of good-quality woody biomass (i.e. woody biomass with low content in ash-forming species, mainly derived from stem wood), along with the growing demand for biomass fuels, has caused the pellet industry to attempt to diversify the sources of raw material. Other types of biomass such as bark, non-woody biomass (cereals and herbaceous materials), and residues from the agricultural industry are also potentially useful as raw materials due to the large volumes available. These fuels present however some challenges. They vary strongly in composition, impacting significantly their combustion behaviour. The high content in ash-forming species, such as alkali and alkaline-earth species, chlorine, phosphorous, nitrogen, and silicon can lead to an increase in gaseous (e.g. sulfur oxides and nitrogen oxides) and particulate matter (PM) emissions. They can also lead to operational problems, such as fouling, corrosion, slagging, and agglomeration during the combustion process. There are several routes to mitigate these ash-related problems, whose applicability depends on the technology and scale. In the present review, the use of additives to reduce emissions and deposits formation, as well as further operational problems these may trigger, is evaluated. However, the high heterogeneity in biomass composition and varying nature of the aforementioned combustion issues hinders the possibility to use a “one-size-fits-all” additive, resulting in the need for developing further understanding on the impact of different additives according to biomass composition and combustion conditions. For example, aluminosicates have proven to be effective to reduce fine particulate matter emissions, but they increase the gaseous emissions of HCl and SOx. The impact of Ca-based additives on PM and alkali-induced slagging is inconclusive, although they can capture gaseous emissions such as HCl and SOx. The additive application method and combustion conditions play on top a very significant role. For all this, reaching conclusions on the type of additive, amount, and application method is very challenging. With this work the authors aim at providing an overview of the state-of-art on the use of additives in biomass combustion, as well as to offer a framework to advance in the regulation on the use of additives.

Review of the use of additives to mitigate operational problems associated with the combustion of biomass with high content in ash-forming species

Despite the relatively low emissions in fluidized-bed combustion, NOx emission for biomass combustion is still a major concern because of increasingly stricter regulations. To realize NOx emission behavior in fluidized beds comprehensively, the effects of bed temperature, excess oxygen, staged combustion, and flue gas recirculation (FGR) are investigated in this study. In particular, three different types of operation are applied in staged combustion to find out the key parameter. The results indicate that NOx emissions increase with both bed temperature and excess oxygen, in which the influence of excess oxygen is greater than the other. Lowering bed temperature by water addition seems to be able to simultaneously reduce NOx emission and agglomerate formation, especially for fuels with high nitrogen content, but the pros and cons should be considered. The results in staged combustion infer that the residence time is much more critical than the stoichiometry in the bed. As for FGR, its impact appears to depend on the type of fuel. The correlation between NOx emission behavior and fuel characteristics is also scrutinized; it is concluded that the fuel-N conversion to NOx is essentially related to some features of fuels.

A comprehensive study on NOx emission and fuel nitrogen conversion of solid biomass in bubbling fluidized beds under staged combustion

Study on NOx emission during corn straw/sewage sludge co-combustion: Experiments and modelling

An experimental study of the effect of nitrogen origin on the formation and reduction of NOx in fluidized-bed combustion

Comparison of the thermal behaviors and pollutant emissions of pelletized bamboo combustion in a fluidized bed combustor at different secondary gas injection modes

The vortexing effect on reducing pollutant emission was verified via the combustion of three groups of sawdust with different diameters in a vortexing fluidized bed combustor (VFBC). The temperatures and pollutant emission distributions along the combustor were measured simultaneously. The combustion fraction in four zones of the VFBC were also calculated on the basis of the oxygen consumption. Additionally, the effects of operating variables, such as the in-bed stoichiometric oxygen ratio and the excess oxygen ratio, on the pollutant emissions were also investigated. The results show that the combustion fractions in the bed and freeboard zones increase with larger particle sizes. However, an inverse trend occurred in other zones. The CO emissions from the three sawdust combustions decrease with an increase in the particle size, excess oxygen ratio, and in-bed stoichiometric oxygen ratio, whereas the NOx emissions exhibit an inverse trend. The vortexing effect, which is caused by secondary air injection, ...

Investigation of the Vortexing Effect on Sawdust Combustion in a Fluidized Bed Combustor

Jatropha curcas is considered a promising energy crop. The response surface methodology is introduced to study the combustion performance of pelletized Jatropha seed residue in a vortexing fluidized-bed combustor, and regression models for the bed temperature (Tb), the combustion fraction (Yb), and the NOx emissions are provided. The result shows that the combustion efficiency is above 99.5%, indicating that JSR pellets burn very well in the combustor. CO can be eliminated by stage combustion accompanied by a controlled freeboard temperature, but NOx increases correspondingly. Tb and Yb have similar trends, showing the interaction between them. The variation in the NOx emissions can be attributed to the shift in volatile-N combustion. In summary, the optimum operating conditions lie slightly above the line of Tb equal to 800 °C.

Pin-Wei Li

Papers

A comprehensive study on NOx emission and fuel nitrogen conversion of solid biomass in bubbling fluidized beds under staged combustion

An experimental study of the effect of nitrogen origin on the formation and reduction of NOx in fluidized-bed combustion

Comparison of the thermal behaviors and pollutant emissions of pelletized bamboo combustion in a fluidized bed combustor at different secondary gas injection modes

Investigation of the Vortexing Effect on Sawdust Combustion in a Fluidized Bed Combustor

The Combustion Performance of Pelletized Jatropha Seed Residue in a Vortexing Fluidized-Bed Combustor