The large-scale implementation of bioenergy demands solid biofuels which can be transported, stored and used efficiently. Torrefaction as a form of pyrolysis converts biomass into biofuels with ...according improved properties such as energy density, grindability and hydrophobicity. Several initiatives advanced this development. The first pilot-scale and demonstration plants displayed the maturity and potential of the technology.
The European research project SECTOR intended to shorten the time-to-market. Within the project 158 Mg of biomass were torrefied through different technologies (rotary drum, toroidal reactor, moving bed). Their production led to process optimization of combined torrefaction-densification steps for various feedstocks through analysing changes in structure and composition. The torrefied pellets and briquettes were subjected to logistic tests (handling and storage) as well as to tests in small- and large-scale end-uses. This led to further improvement of the torrefied product meeting logistics/end-use requirements, e.g. durability, grindability, hydrophobicity, biodegradation and energy density. Durability exceeds now 95%.
With these test results also international standards of advanced solid biofuels were initiated (ISO standards) as a prerequisite for global trade of torrefied material. Accompanying economic and environmental assessment identified a broad range of scenarios in which torrefied biomass perform better in these areas than traditional solid biofuels (e.g. white pellets), depending e.g. on feedstock, plant size, transport distances, integration of torrefaction in existing industries and end use. The implementation of industrial plants is the next step for the technology development. Different end user markets within and outside Europe can open opportunities here.
•The SECTOR project advanced torrefaction along the complete value chain, thus shortening the time to market.•Three torrefaction technologies incl. densification have been improved and 158 Mg of torrefied material was produced.•Tests in storage and end use applications from small to large scale revealed no major obstacles.•The test results served in the initiation of new ISO standards, which are in draft stage.•Environmental and economic assessment identified beneficial applications from a multitude of scenarios.
Several approaches are established to analyse the fouling and slagging propensities of coal ashes, but the same cannot be said of solid recovered fuel (SRF) ashes. This work has been conducted by ...using some fouling and slagging indicators, which are commonly applicable to coal ashes, on SRF ashes to ascertain their applicability.
In this work, laboratory prepared ashes derived from municipal solid waste (MSW), sewage sludge, demolition wood, shredded rubber tyres, and plastic/paper fluff are analysed for their fusibility leading to fouling and slagging using three approaches; the ash fusibility temperatures (AFT), ternary phase diagrams, and fouling/slagging indices. The results from each approach are examined to determine the inclination of the ashes toward fouling and slagging. A subsequent inter-comparison of the methods was conducted to validate the methods which are in agreement and are applicable to SRF ashes. The study showed that ternary equilibrium phase diagram SiO
2–CaO–Al
2O
3, various fouling and slagging indices, and AFT can be used to complement each other to predict ash fusion properties, fouling and slagging propensities of SRF ashes.
The quality of fly ash from wood dust combustion with an alumina-silicate additive in a laboratory scale pulverized fuel reactor is assessed. Fly ash analyses include XRD, SEM-EDX, micro-Raman ...spectroscopy, ICP-OES, and mobility of elements. XRD results show that the addition of the additive change the fly ash to a pozzolanic nature, improving its quality for applications that follow strict material provisions. SEM analyses showed the morphology of fly ash particles changed to having more spherical smaller particles that involves much fewer larger particles covered by sticky fly ash particles on the surface too. The reduction of irregular particles and the increase in spherical particles improve the fly ash quality in concrete use. Mobility results showed a decrease in the amount of some potentially hazardous elements leached from the fly ash generated with the additive. Results from micro-Raman spectroscopy had the added value of identifying mineral phases for particles associated with a scale size, giving rise to determination of mineral phases for the different fly ash size fractions or particles. The study shows that an expansion of EN 450-1 is warranted for consideration to include an additive with combustion of higher amounts of a greenwood, or only with a greenwood, or with co-firing lower amounts of other biomass types in order to acquire fly ash in compliance with the standard.
•Aluminate silicate additives may improve wood dust fly ash for concrete utilization.•Wood fly ash is transformed from crystalline dominated to a pozzolanic domination.•The mobility of some heavy metals increase in the ash due to use of the additive.•Wood dust combustion with an alumina-silicate additive may be new fuel for EN450-1.
The need for flexible power generation is growing worldwide as the energy transition is altering the operational regimes of thermal power plants. Plasma ignition systems, as an alternative technology ...to the conventional start-up method with natural gas or oil firing, offer a cost- and energy-efficient start-up process in pulverized fuel power stations. The application of plasma ignition systems for cold start-ups using different qualities of pre-dried lignite is investigated in a pilot-scale combustion facility. A plasma integrated swirl burner is developed and validated using highly ignitable lignite dust. Eight pre-dried lignite qualities with a moisture content of up to 30% and a broad particle size distribution are investigated for this application to determine the applicability and limitations of the plasma ignition system with regard to the fuel quality. The performance of lignites for cold start-up in the plasma ignition system are categorized based on their ignition and combustion performance. All lignite qualities were ignited under the cold-start-up condition with a plasma power of 4 kW to 7 kW. Lignite qualities with a moisture content of up to 20% and a median particle size of below 450 μm form a self-sustained flame with short-time plasma-supported combustion, while flame blow-out is observed for lignites with lower qualities.
Several investigations have shown that the differences between deposits obtained in oxy-firing and air-firing of coal mainly are due to differences in the flame temperature. Consequently, deposit ...rate predictions not taking the in-flight history into account are unlikely to be successful. In this paper, a model for predicting the deposit formation propensity of pulverized coal in oxy-fuel and air combustion due to the inertial impaction mechanism is developed and tested. The model builds on the use of viscosity as an indicator of the sticking probability. The composition and amount of the amorphous slag phase in the coal ash are calculated assuming thermodynamic equilibrium. Further, it is assumed that the maximum temperature the ash particle has experienced will control the composition and amount of the amorphous slag phase. As the ash particle impacts the probability to stick is estimated using the viscosity of this melt composition, but with the temperature of particle temperature at the moment of impaction. In the equilibrium calculation no material exchange with the gas phase is assumed. This assumption is based on X-ray diffraction (XRD) investigations of coal ash samples produced in a lab-scale burner simulating oxy-fuel and air combustion. The XRD showed that there was no significant impact on the mineralogy of the coal ash caused by the gas atmosphere. The probability of an ash particle to stick as a function of maximum experienced temperature and impact temperature was evaluated for three coals. For one of the coals a CFD study on particle deposit is done for a 300kWth test facility.
•A temperature-history based coal ash particle impaction criteria•Computationally low cost model suitable for CFD•Ash properties from oxy-firing and conventional firing experiments
The lignite boilers are designed for lower quality fuels, and often the ash is not utilized. This work assessed the impact of combustion of an herbaceous biomass with a low-quality Greek lignite on ...the quality of the resulting fly ash. Test results were compared with those of fly ash samples from an industrial facility using the same fuel qualities. Inductively coupled plasma-optical (ICP) emission spectroscopy, X-ray powder diffraction (XRD), and scanning electron microscope (SEM) analyses were performed on the collected samples. Despite the significantly higher contents of K, Na and S in the biomass, at a 50% co-firing thermal share, the major and minor oxides in the fly ash were comparable to the lignite fly ash quality. This is attributed to the high ash content of the lignite, the low ash content of the biomass, and the much higher heating value of the biomass. There were improvements in fly ash performance characteristics with the herbaceous biomass in the fuel blend. The initial setting time and volume stability evaluations were improved with the biomass in the fuel blend. The work supports efforts of good practices in ash management, social responsibility, a circular economy, power plant renewable energy operations, and co-firing herbaceous biomass fuels in lignite power plants.
The ignition characteristics of coal dust is of high importance for the flame stability in coal-fired power plants. We investigate the ignitability of six lignite dust qualities and one hard coal ...using dust explosion tests and an ignitability characteristic number. The paper aims to identify the degree of impact of the properties of coals, such as the moisture content, the ash content etc., on the ignition characteristics and ultimately to compare the identified relevant ignition parameters to the ignition performance of the dust qualities in an industrially relevant environment. The minimum cloud ignition temperature (MCIT), the maximum rate of pressure rise ((dp/dt)max), the maximum explosion pressure (pmax), the deflagration index (Kst-value) and the modified ignitability characteristic number (ZWZmod.) were determined and were attributed to the moisture content, the ash content and the median particle size. The MCIT was largely influenced by the volatile content, whereas the variations of moisture and ash contents within the range of 10% to 20% did not have a significant impact on the MCIT. The maximum explosion pressure did not differ considerably and stayed in a narrow range among the tested dust qualities. The deflagration index showed a higher sensitivity to the dust properties. The deflagration index and the modified ignitability characteristics number dropped as the moisture content increased and the volatile content reduced. The Kst and ZWZmod. values showed the highest susceptibility to the coal dust properties. Hence, they were used as representative parameters for further comparison with the ignition performance of coal dust in a pilot-scale testing. The results showed that both parameters predicted the ignition performance relatively well and can be used as indicators for the prediction of the ignition performance.
•Excess oxygen is used for flame cooling and maintaining momentum at the burner.•Oxygen-to-fuel equivalence ratios of 2, 3, 3.4 and 4 are studied.•Higher burnout efficiency but also higher NO in ...over-stoichiometric tests.•Flue gas rich in oxygen is suitable for further use as an oxidizer in the calciner where NO reburning is possible.
Cement production is a highly CO2-intensive process, where two-thirds of the emissions are unavoidable process emissions and the rest come from the combustion process. Oxyfuel technology is an innovative way to apply carbon capture and allow the industry to reduce CO2 emissions, even reaching net zero CO2 emissions. The oxyfuel combustion process can be retrofitted to existing plants and designed for new-build plants, where the plant can be designed with no flue gas recirculation (FGR). A down-scaled kiln burner is tested in oxyfuel conditions with different oxygen-to-fuel ratios in technical and pilot-scale facilities at the University of Stuttgart. In the technical-scale facility, experiments are conducted to compare a case with synthetic FGR at near-stoichiometric conditions (OXY32) and over-stoichiometric conditions (with λ*3.4), λ* being the oxygen-to-fuel ratio. Experiments in the pilot-scale facility are conducted at varying stoichiometric conditions, λ*2, λ*3 and λ*4. In both facilities, a reference case with air combustion is conducted. The highest measured temperature in the air, λ*3 and λ*4 cases were 1020 °C, 1321 °C and 1116 °C, respectively. In the oxyfuel cases, after the peak temperature is reached, the temperature profiles stabilize to similar temperatures as measured in the air case. The inlet oxidizer gas concentration and stoichiometry highly affect the CO and NO formation. For all oxyfuel cases, the CO emission rate in the flue gas measurements is below 20 mg/MJ indicating high burnout efficiency. In the technical-scale tests, the NO emission rate at 2.5 m from the burner is lower in the OXY32 case compared to the air case, with 138 and 247 mg/MJ, respectively. The NO emission rate of the λ*3.4 case is 461 mg/MJ, a consequence of no reducing or reburning zone. The experiments show that the increased over-stoichiometric conditions have a desirable effect on the temperature profile and oxygen can be used as a suitable diluent in comparison to N2 and CO2, but NO formation is increased. In the cement production process, this can be solved by designing a reducing zone in the calciner to properly reduce NO.
Biomass boiler issues regarding slagging, fouling and corrosion are related to alkali species present in fuels. These alkali species are released as gaseous alkali chlorides, hydroxides and/or ...sulfates during combustion. Alkali chlorides/sulfates later condense on cold boiler surfaces enhancing fouling and corrosion. Subsequent deposition of silica-rich ashes leads to the formation of low temperature melting eutectics, especially alkali/earth alkali-silicates (K/Ca-Si), and creates compact and strong deposits. Condensing chlorides/sulfates and low melting eutectics both reduce boiler performance and its availability.
Two different woody biomass fuels and their mixture with two different kinds of aluminosilicate base additives were combusted during this study. Deposits were sampled using two different kinds of probes; cooled deposit probe and un-cooled deposit probe. The macroscopic and microscopic observations and chemical composition of deposit samples are compared using electron microscopic image analysis (EMPA) for both deposit samples. The additive significantly changes the morphology and composition of the deposit samples. The difference in morphology and composition can be explained by the change in potassium chemistry between two cases.
•Deposition mechanism is driven by a release of potassium species and their interaction with SiO2.•Molten deposits are basically related to the formation of low melting potassium silicates.•Additive changes the chemistry of potassium towards less problematic deposits.•Additive reduces the formation of molten deposit and gas phase potassium chloride.
Oxy-fuel combustion stands as a promising carbon capture technology to significantly reduce CO sub(2) emissions from industrial combustion processes. Due to a different process layout compared to ...power industry as well as different boundary conditions further investigations and demonstration activities are required to develop the oxy-fuel cement process to maturity. This paper presents an overview on research activities and current state-of-the-art on the development of oxy-fuel combustion applied to the cement process. Oxy-firing concepts for cement plants are introduced under two process configurations. Modifications regarding plant lay-out and key components as well as operational implications are discussed. Relevant research projects focusing on the application of the oxy-fuel technology in the cement industry are presented and finally fields, in which further research is required, are identified.
PDF
