Enhancing sulphur self-retention by building-in CaO in straw–bitumen pellets

Combined straw–bitumen pellets have been proposed as an alternative fuel. An interesting finding is the potentiality of straw ash constituents to retain sulphur as bitumen that has relatively high sulphur content. The aim of the present work is to enhance sulphur self-retention to directly meet the environmental regulations by building-in CaO in the pellet instead of feeding sorbent separately. CaO powder has been mixed with the pellet constituents during production processes.     

Preventing chlorine deposition on heat transfer surfaces with aluminium-silicon rich biomass residue and additive

Biomass fuels often contain higher concentrations of easily vaporisable alkalis and chlorine than do coal and peat. The more vaporisable the alkalis or chlorine compounds the higher is the risk for ash-related problems. The presence of certain elements may reduce or remove these problems. This work shows how co-combusting of different biomass fuels in a fluidised bed boiler can result in useful interactions that decrease or totally inhibit Cl deposition and bed agglomeration. In a first set of experiments, fuel 1 contained easily vaporised chlorine that produces Cl-rich deposits on superheaters. Fuel 2 was enriched in aluminium silicate, but contained much ash, resulting in low heating value and high load of fly ash. In a second set of experiments, fuel 1 was enriched in Cl and alkalis, which lead to corrosive deposits, bed agglomeration and fouling. As a result of protecting reactions, the mixtures were free from the problems observed during their separate combustion.     

A Techno-economic assessment of the reduction of carbon dioxide emissions through the use of biomass co-combustion

Using sustainably-grown biomass as the sole fuel, or co-fired with coal, is an effective way of reducing the net CO₂ emissions from a combustion power plant. There may be a reduction in efficiency from the use of biomass, mainly as a result of its relatively high moisture content, and the system economics may also be adversely affected.The economic cost of reducing CO₂ emissions through the replacement of coal with biomass can be identified by analysing the system when fuelled solely by biomass, solely by coal and when a coal-biomass mixture is used.The technical feasibility of burning biomass or certain wastes with pulverised coal in utility boilers has been well established. Cofiring had also been found to have little effect on efficiency or flame stability, and pilot plant studies had shown that cofiring could reduce NO_x and SO_x emissions.Several technologies could be applied to the co-combustion of biomass or waste and coal. The assessment studies here examine the potential for co-combustion of (a) a 600 MWe pulverised fuel (PF) power plant, (i) cofiring coal with straw and sewage sludge and (ii) using straw derived fuel gas as return fuel; (b) a 350 MWe pressurised fluidised bed combustion (PFBC) system cofiring coal with sewage sludge; (c) 250 and 125 MWe circulating fluidised bed combustion (CFBC) plants cofiring coal with straw and sewage sludge; (d) 25 MWe CFBC systems cofiring low and high sulphur content coal with straw, wood and woody matter pressed from olive stones (WPOS); and (e) 12 MWe CFBC cofiring low and high sulphur content coal with straw.The technical, environmental and economic analysis of such technologies, using the ECLIPSE suite of process simulation software, is the subject of this study. System efficiencies for generating electricity are evaluated and compared for the different technologies and system scales. The capital costs of systems are estimated for coal-firing and also any additional costs introduced when biomass is used. The Break-even electricity selling price is calculated for each technology, taking into account the system scale and fuel used.Since net CO₂ emissions are reduced when biomass is used, the effect of the use of biomass on the electricity selling price can be found and the premium required for emissions reduction assessed. Consideration is also given to the level of subvention required, either as a Carbon dioxide Credit or as a Renewable Credit, to make the systems using biomass competitive with those fuelled only with coal.It would appear that a Renewable Credit (RC) is a more transparent and cost-effective mechanism to support the use of biomass in such power plants than a Carbon dioxide Credit (CC).     

Co-combustion of pulverized coal and solid recovered fuel in an entrained flow reactor - General combustion and ash behaviour

Co-combustion of a bituminous coal and a solid recovered fuel (SRF) was carried out in an entrained flow reactor, and the influence of additives such as NaCl, PVC, ammonium sulphate, and kaolinite on co-combustion was investigated. The co-combustion experiments were carried out with SRF shares of 7.9 wt.%, 14.8 wt.% and 25 wt.%, respectively. The effect of additives was evaluated by maintaining the share of secondary fuel (mixture of SRF and additive) at 14.8 wt.%. The experimental results showed that the fuel burnout, NO and SO₂ emission in co-combustion of coal and SRF were decreased with increasing share of SRF. The majority of the additives inhibited the burnout, except for NaCl which seemed to have a promoting effect. The impact of additives on NO emission was mostly insignificant, except for ammonium sulphate which greatly reduced the NO emission. For SO₂ emission, it was found that all of the additives increased the S-retention in ash. Analysis of the bulk composition of fly ash from different experiments indicated that the majority of S and Cl in the fuels were released to gas phase during combustion, whereas the K and Na in the fuels were mainly retained in ash. When co-firing coal and SRF, approximately 99 wt.% of the K and Na in fly ash was present in water insoluble form such as aluminosilicates or silicates. The addition of NaCl, PVC, and ammonium sulphate generally promoted the vaporization of Na and K, resulting in an increased formation of water soluble alkalis such as alkali chlorides or sulphates. The vaporization degree of Na and K was found to be correlated during the experiments, suggesting an interaction between the vaporization of Na and K during pulverized fuel combustion. By collecting deposits on an air-cooled probe during the experiments, it was found that the ash deposition propensity in co-combustion was decreased with increasing share of SRF. The addition of NaCl and PVC significantly increased the ash deposition propensity, whereas the addition of ammonium sulphate or kaolinite showed a slight reducing effect. The chlorine content in the deposits generally implied a low corrosion potential during co-combustion of coal and SRF, except for the experiments with NaCl or PVC addition.     

Hydrogen-diesel fuel co-combustion strategies in light duty and heavy duty CI engines

The co-combustion of diesel fuel with H₂ presents a promising route to reduce the adverse effects of diesel engine exhaust pollutants on the environment and human health. This paper presents the results of H₂-diesel co-combustion experiments carried out on two different research facilities, a light duty and a heavy duty diesel engine. For both engines, H₂ was supplied to the engine intake manifold and aspirated with the intake air. H₂ concentrations of up to 20% vol/vol and 8% vol/vol were tested in the light duty and heavy duty engines respectively. Exhaust gas circulation (EGR) was also utilised for some of the tests to control exhaust NO_x emissions.The results showed NO_x emissions increase with increasing H₂ in the case of the light duty engine, however, in contrast, for the heavy duty engine NO_x emissions were stable/reduced slightly with H₂, attributable to lower in-cylinder gas temperatures during diffusion-controlled combustion. CO and particulate emissions were observed to reduce as the intake H₂ was increased. For the light duty, H₂ was observed to auto-ignite intermittently before diesel fuel injection had started, when the intake H₂ concentration was 20% vol/vol. A similar effect was observed in the heavy duty engine at just over 8% H₂ concentration.     

Controlling the excess heat from oxy-combustion of coal by blending with biomass

Two different biomass species such as sunflower seed shell and hazelnut shell were blended with Soma-Denis lignite to determine the effects of co-combustion on the thermal reactivity and the burnout of the lignite sample. For this purpose, Thermogravimetric Analysis and Differential Scanning Calorimetry techniques were applied from ambient to 900 °C with a heating rate of 40 °C/min under dry air and pure oxygen conditions. It was found that the thermal reactivities of the biomass materials and the lignite are highly different from each other under each oxidizing medium. On the other hand, the presence of biomass in the burning medium led to important influences not only on the burnout levels but also on the heat flows. The heat flow from the burning of lignite increased fivefold when the oxidizing medium was altered from dry air to pure oxygen. But, in case of co-combustion under oxygen, the excess heat arising from combustion of lignite could be reduced and this may be helpful to control the temperature of the combustion chamber. Based on this, co-combustion of coal/biomass blends under oxygen may be suggested as an alternative method to the “Carbon Dioxide Recycle Method” encountered in the oxyfuel combustion systems.     

A technical and environmental analysis of co-combustion of coal and biomass in fluidised bed technologies

The use of biomass, which is considered to produce no net CO₂ emissions in its life cycle, can reduce the effective CO₂ emissions of a coal-fired power generation system, when co-fired with the coal, but may also reduce system efficiency.The technical and environmental analysis of fluidised bed technologies, using the ECLIPSE suite of process simulation software, is the subject of this study. System efficiencies for generating electricity are evaluated and compared for the different technologies and system scales.Several technologies could be applied to the co-combustion of biomass or waste and coal. The assessment studies here examine the potential for co-combustion of (a) a 600 MWe pulverised fuel (PF) power plant (as a reference system), (i) co-firing coal with straw and sewage sludge and (ii) using straw derived fuel gas as return fuel; (b) a 350 MWe pressurised fluidised bed combustion (PFBC) system co-firing coal with sewage sludge; (c) 250 MWe and 125 MWe circulating fluidised bed combustion (CFBC) plants co-firing coal with straw and sewage sludge; (d) 25 MWe CFBC systems co-firing low and high sulphur content coal with straw, wood and woody matter pressed from olive stones (WPOS); (e) 12 MWe CFBC co-firing low and high sulphur content coal with straw or wood; and (f) 12 MWe bubbling fluidised bed combustion (BFBC), also co-firing low and high sulphur content coal with straw or wood.In the large systems the use of both straw and sewage sludge resulted in a small reduction in efficiency (compared with systems using only coal as fuel).In the small-scale systems the high moisture content of the wood chips chosen caused a significant efficiency reduction.Net CO₂ emissions are reduced when biomass is used, and these are compared for the different types and scales of fluidised bed technologies. NO_x emissions were affected by a number of factors, such as bed temperature, amount of sorbent used for SO₂ capture and HCl emitted.     

Effects of biomass co-firing with coal on ash properties. Part II: Leaching, toxicity and radiological behaviour

In this paper, the leaching, toxicity behaviour and the radioactivity content of solid residues coming from the co-combustion of biomass with coal were studied. A variety of samples collected from semi-industrial scale tests were analysed for their leaching and toxicity properties. Natural radioactivity and radon exhalation rate were also measured in samples collected from tests performed in a pilot facility in Germany (IVD, University of Stuttgart) and large-scale power plants. The high toxicity levels detected in the ash samples of olive kernel could be attributed to the relatively increased concentrations of Zn, Ni, Mn, Co, Cd. The effect of biomass co-combustion on the radioactivity content of fly ash was dependent on the fuel mixture used as well as the ash sampling location along the flue gas pathway. Activity concentrations of most nuclides of interest, namely ²³⁸U, ²²⁶Ra, ²¹⁰Pb and ²³²Th are comparable to those of fly ash produced when burning pure coal, while increased concentrations where observed for ⁴⁰K. In some cases the artificial radionuclide ¹³⁷Cs was also detected.