• Energy Research

Abstract Co-firing of biofuels and coal in power plants is considered by the Danish utilities as a potential tool in reducing CO 2 emissions. To test this, full-scale measurements were carried out for 1 week on a 250 MW e pulverized coal fired unit using 10–20% straw (thermal basis). With an increased fraction of straw in the fuel, a net decrease in NO x and SO 2 emissions was measured. The SO 2 emission decreased partly due to the lower sulfur content of the fuel per MJ, but also due to higher sulfur retention in the ash. The NO emission decreased solely due to lower conversion of fuel-N. An increased fraction of straw in the fuel blend resulted in a higher potassium content, but no significant increase in slagging or fouling was observed. Only small amounts of deposit at the lower part of the radiant superheater and little slagging at the furnace walls were observed as a result of co-firing straw and coal. No significant effects on the performance of the desulfurization plant were detected, which may be due to the short test period, probably not allowing the desulfurization process to reach steady-state operation.

Phosphorus (P) from biomass can cause operational problems in thermal conversion processes. In order to explore the release mechanism of P to the gas phase, carbothermic reduction of meta-, pyro-, and orthophosphates of ash elements commonly found in biomass; sodium, potassium, magnesium, and calcium was investigated. Mixtures of each phosphate and activated carbon were heated to 1135 °C in a laboratory-scale reactor, with the CO and CO2 evolving from the sample monitored, and the chemical composition of selected residues analyzed to quantify the release of P. Thermal gravimetric analysis was also performed on selected samples. The alkaline earth phosphates were reduced in steps, following the sequence meta → pyro → ortho → alkaline earth oxide. However, the alkali metaphosphates appear to be reduced in one step, in which both alkali and P are released. Alkali pyro- and orthophosphate appear to undergo a two-step process. In the first step, mainly alkali is released and in the second step both alkali and P. An intermediate is produced in the first step, which has a K:P:O atomic ratio of about 2:1:2.7, indicating it might be a phosphite with the overall stoichiometry; K4P2O5. The reduction of alkaline earth phosphates could be interpreted using available thermodynamic data, whereas thermodynamic equilibrium calculations for the alkali phosphates did not correspond well to the experimental observations. Kinetics were derived for the different reduction reactions, and can be used to compare the reactivity of the phosphates. The work suggests that carbothermic reduction reactions are important for the release of P in the temperature range 850–1135 °C and relevant for biomass combustion, pyrolysis and gasification.

In this study, the applicability of a "fed-batch" strategy, that is, sequential loading of substrate or substrate plus enzymes during enzymatic hydrolysis was evaluated for hydrolysis of steam-pretreated barley straw. The specific aims were to achieve hydrolysis of high substrate levels, low viscosity during hydrolysis, and high glucose concentrations. An enzyme system comprising Celluclast and Novozyme 188, a commercial cellulase product derived from Trichoderma reesei and a beta-glucosidase derived from Aspergillus niger, respectively, was used for the enzymatic hydrolysis. The highest final glucose concentration, 78 g/l, after 72 h of reaction, was obtained with an initial, full substrate loading of 15% dry matter weight/weight (w/w DM). Conversely, the glucose yields, in grams per gram of DM, were highest at lower substrate concentrations, with the highest glucose yield being 0.53 g/g DM for the reaction with a substrate loading of 5% w/w DM after 72 h. The reactions subjected to gradual loading of substrate or substrate plus enzymes to increase the substrate levels from 5 to 15% w/w DM, consistently provided lower concentrations of glucose after 72 h of reaction; however, the initial rates of conversion varied in the different reactions. Rapid cellulose degradation was accompanied by rapid decreases in viscosity before addition of extra substrate, but when extra substrate or substrate plus enzymes were added, the viscosities of the slurries increased and the hydrolytic efficiencies decreased temporarily.

Fast pyrolysis of lignin from an ethanol plant was investigated on a lab scale pyrolysis centrifuge reactor (PCR) with respect to pyrolysis temperature, reactor gas residence time, and feed rate. A...

Abstract The properties of slurries made of pyrolysis oil mixed with wood, char or ground char were investigated with respect to phase transitions, rheological properties, elemental compositions, and energy density. Also the pumping properties of the slurries were investigated at temperatures of 25, 40 and 60 °C and solid loadings from 0 to 20 wt%. The phase transitions of the wood slurry samples were observed at lower solid loadings compared to the char slurry samples. The apparent viscosity of the slurry samples was found to be considerably impacted by solid loading (0–20 wt%) and temperature (25–60 °C), especially in the phase transition region. The slurry viscosities with 20 wt% char loading, 20 wt% ground char loading and 15 wt% wood loading (at a shear rate of 100 s −1 ) are 0.7, 1.0 and 1.7 Pa.s, respectively at 60 °C and these values increases 1.2–1.4 times at 40 °C and 3–4 times at 25 °C. The wood, char and ground char slurry samples with 5–20 wt% solid loading obtain a volumetric energy density of 21–23 GJ/m 3 . The slurry sample with 20 wt% ground char having a d 80 of 118 μm was pumped successfully into a pressurized chamber (0–6 bar) while plugging appeared when the slurry samples with 15 wt% char having a d 80 of 276 μm was pumped into the pressurized chamber.

Agglomeration may cause severe operational problems in fluidized beds for biomass combustion and gasification. Application of external forces is a common way for improving fluidization quality, which may act as a potential countermeasure to mitigate the agglomeration problem. In this work, a pulsed flow is applied to a fluidized bed reactor to examine its effect on mitigating agglomeration in combustion and gasification of wheat straw. The influence of the pulsation parameters, including flow ratio of pulsation/total flow, pulsation frequency and pulsation duty cycle, on the agglomeration tendency were studied in a lab-scale bubbling fluidized bed reactor using silica sand as bed material. The results reveal that application of a pulsed flow to the reactor can reduce the agglomeration tendency in fluidized bed combustor and gasifier. The maximum reduction of agglomeration tendency has been achieved under a condition with pulsation flow ratio of 0.4, pulsation frequency of 1.5 Hz and pulsation duty cycle of 25/75. Among the parameters studied, pulsation duty cycle exhibits the most significant effect on reducing agglomeration tendency, while pulsation frequency in the studied range shows the smallest impact. Moreover, the results show that the combustion performance is not affected by application of the pulsed flow.

The influence of the air combustion mixing into the main flow, operating conditions and fuel type on the level of NO emission in the in-line low-NO x calciners was investigated. The analysis was carried out using a heterogeneous mathematical model based on non-isothermal diffusion-reaction models for char combustion and limestone calcination. The mixing in the oxidizing zone was modeled using a modified Zwietering approach, which supposes that only oxygen or both oxygen and nitrogen are gradually mixed into the sub-stoichiometric suspension leaving the reducing zone. The analysis shows that the difference in NO emission due to the mixing rate of the oxygen into the main flow is the consequence of the different local oxygen concentration in the oxidizing zone and different char-N oxidation rates. The change of limestone feed position in the oxidizing zone modifies greatly the temperature in the region up to the feed level and thereby the NO emission. In the case of bituminous coal, changing the limestone feed ratio in the reducing zone corresponds to an insignificant modification of the NO concentration. The increase of the air combustion temperature has a negative influence on NO emission.

Fast pyrolysis may be used for sewage sludge treatment with the advantages of a significant reduction of solid waste volume and production of a bio-oil that can be used as fuel. A study of the infl...

Converting existing coal fired power plants to biomass is a readily implemented strategy to increase the share of renewable energy. However, changing from one fuel to another is not straightforward: Experience shows that wood pellets ignite more readily than coal in power plant mills or storages. This is not very well explained by apply-ing conventional thermal ignition theory. An experimental study at lab scale, using pinewood as an example fuel, was conducted to examine self-heating and self-ignition. Supplemental experiments were performed with bituminous coal. Instead of characterizing ignition temperature in terms of sample volume, mass-scaling seems more physically correct for the self-ignition of solids. Findings also suggest that the transition between self-heating and self-ignition is controlled both by the availability of reactive material and temperature. Comparison of experiments at 20% oxygen with those under inert atmosphere revealed two distinct pathways, pyrolysis and exothermic heterogeneous oxidation. At low temperatures and sufficient oxygen availability, heterogeneous oxidation of the solid seems to be favored over pyrolysis for wood, but not for coal. Current ignition models do not reflect the existence of these different pathways, which may be the reason behind the discrepancy between theory and observations. Proceedings of the 25th European Biomass Conference and Exhibition, 12-15 June 2017, Stockholm, Sweden, pp. 332-337