• Energy Research

Abstract To provide preferable operating conditions for biomass carbonization, the carbonization properties of maize straw, cotton stalk and poplar wood were investigated in nitrogen atmosphere by segmented heating process at different final temperatures of 300–800 °C for 1 h. Segmented heating carbonization contributes to higher ratio of mass and energy yields than a constant temperature/linear heating process with final temperature of around 300 °C. The fuel properties and characteristics of raw and carbonized biomass were analysed systematically. Increasing carbonization temperature, the fuel ratios of the obtained chars rise exponentially, and the profile of atomic H/C ratio versus atomic O/C ratio approximately decreases linearly. There are small changes of sulphur and nitrogen content in the chars. To provide an estimation method for higher heating value for torrefied/carbonized biomass, a new correlation is proposed based on ultimate analysis. The correlation is H H V = 19.9579 + 0.1284 C + 0.3355 H − 0.1209 O − 1.3836 N − 5.4680 S , which is validated through experimental data. The correlation has a low level of errors, that the correlation efficient, mean absolute error, average absolute error and average bias error are 0.971, − 6.75 × 10 − 6 MJ, 1.2% and 0.02% respectively. This work will make a contribution to the biomass pretreatment process.

The influence of the briquetting process on SO2 and NO release characteristics, combustion properties and kinetic characteristics during biomass combustion was investigated. Two biomass (Wheat straw and Tree bulk) and two obtained briquettes were analysed. The briquetting process helps to prevent the release of SO2 and NO. The experimental results show that once the biomass is made into a briquette, when the reaction temperature is 900 ∘C, the sulphur release ratio for TB was reduced from 34.7% to 4.3% and for WS was reduced from 12.4% to 1.6%. When the reaction temperature increases to 1000 ∘C, the sulphur release ratio for TB was reduced from 73.4% to 30.4%, for WS it was reduced from 58.4% to 10.2%. SEM micrographs show that the compact structure of the TB-Briquette and WS-Briquette reduce the rate of SO2 and NO release during combustion. The thermogravimetry confirmed that the combustion performance of WS-Briquette is the best, while the TB-Briquette is the worst. According to the Coats-Redfern method, the fitting was performed at segments of 250 ∘C to 550 ∘C, and the correlation coefficient of the fitting degree was above 0.99. The effective collision rate of WS-Briquette is much higher than that of other briquettes. Compared to BR-1 and BR-2, trying to mix TB with WS to make a compound biomass briquette can enhance the combustion performance of TB-Briquette. The results may guide the upgrading of biomass briquettes technology and benefit the efficient application of biomass briquettes.

Recently, the supercritical carbon dioxide (SCO 2 ) power cycle has become a hotspot in the field of energy-efficient utilization. The utilization of additives in the power cycle has been proven to be an effective way to improve the SCO 2 power cycle efficiency. As one of the core components of the system, the influence of CO 2 -based mixtures on turbine performance needs to be further explored. In this study, the preliminary design and three-dimensional numerical simulation of a 500 kW radial-inflow turbine (RIT) for small-scale SCO 2 power systems were carried out. Furthermore, the design and off-design performance of high Reynolds number and small size turbine under the change of the CO 2 -based binary mixture compositions and mixing ratios were studied. Increasing the amount of nitrogen, oxygen, or helium into CO 2 has a negative effect on the RIT performance, and the appropriate amount of xenon or krypton can improve the turbine efficiency. Moreover, mixtures with higher krypton additions adapt to higher heat source conditions. The loss of the turbine stage passage shows that a large amount of helium greatly reduces the working fluid density, and the high amount of xenon has a great influence on the dynamic viscosity, which all makes the RIT operation deviate from the steady state. Therefore, the CFD model simulation fails indicating that RIT designed based on pure CO 2 may not run smoothly and continuously. The losses in the stage with pure CO 2 and CO 2 –Kr mixture were investigated. The results indicate that the losses originated from the stator cannot be ignored and that the improvement of efficiency is mainly owed to the reduction in clearance losses. There is no doubt that the viewpoints proposed in this paper have significant reference value for the practical application of the SCO 2 power cycle using mixtures. ; Green Open Access added to TU Delft Institutional Repository 'You share, we take care!' - Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the ...

During the operation of supercritical CO $ _2 $ (sCO $ _2 $ ) turbomachineries, large pressure ratios, supersonic conditions and non-ideal gas fluid dynamic phenomenon may happen, which will decrease the whole cycle efficiencies. Hence non-standard designs for the turbomachineries geometry are needed. Successfully simulations in capturing non-ideal gas fluid dynamics and coupled with optimization algorithm are hard and currently not available in any commercial CFD software. Therefore, the problem of optimizing sCO $ _2 $ turbomachinery is difficult to solve, which has become a major obstacle to obtain compact and high-efficiency sCO $ _2 $ turbomachineries. In this study, a modularized geometry optimizer is developed to obtain the non-standard geometric designs for sCO $ _2 $ turbomachineries. Multiple techniques are applied to this optimizer, include the Nelder–Mead algorithm, Mahalanobis distance and stochastic algorithm. The newly developed optimizer can successfully find the optimum satisfying the objective function under given weighting factors. The computational cost can be reduced through a stochastic algorithm. To validate the optimizer, a convergent–divergent nozzle for air with a target Mach number equal to 2.4 is optimized. Different starting points and combinations of weighting factors are used to create a Pareto front. Adjust the weighting factors for different terms of the objective function leading the optimizer to go to different directions in n-dimensional spaces. Three optimized cases, one is Mach number optimized, one is outlet flow uniformity optimized and the other is compromised case, are picked out and analyzed. The results show that the optimizer can successfully find optimized geometry than the reference case and potentially save computational cost. Due to the modularized characteristics, the components of this optimizer can be replaced with any available techniques, which mean the optimizer can be applied to solve different types of optimization problems.

Associated data for the project: influence of ammonium dihydrogen phosphate (NH4H2PO4) on the potassium (K) retention ability in the ash phase and the ash melting characteristics during biomass combustion was investigated. Experiments were performed in a tube furnace experimental system and an ash melting point detector.

Abstract Low energy density and ash fusion problem were problems that limit biomass widespread utilization. To avoid these, maize straw (MS) was blended with two additive –NH4H2PO4 (ADP) and Ca(H2PO4)2 (CPM)-respectively, and the production chain of mixing additive-briquetting-pyrolysis was put forward to produce MS char briquettes (MSC, MSC-ADP and MSC-CPM). Ash fusion characteristics of MS char briquettes after combustion was investigated by XRF, XRD, SEM-EDS and simulation. Results show that the softening temperature increased with the enhanced ratio of P2O5/K2O until 0.2, and CaO/K2O until 0.9. The higher amount of CaO and P2O5, the lower amount of K2O, the better ash fusion characteristics. The P2O5 had larger influence on softening temperature. The surfaces of MSC ashes formed at 1000 °C were fusion and smooth in micro observation, while surfaces of MSC-ADP and MSC-CPM ashes were coarse and the particles were much looser. Both SEM-EDS and XRD results illustrated that phosphorus in additives combined with potassium to produce high melting temperature potassium phosphates rather than low melting temperature potassium silicates. The potassium fixation ability of ADP was better than that of CPM. The calcium silicates and calcium phosphates produced by CPM could improve ash characteristics.

Abstract In the current world, the large consumption of fossil fuel leads to some global environmental problems, using biomass energy is one of the good solutions. However, due to the more strict emission policy, the sulphur releasing during biomass combustion cannot be ignored. In order to study the self-sulphur retention characteristics, the briquetting process of biomass is carried out. In this study, a batch of biomass briquettes combustion experiments are conducted to investigate self-sulphur retention characteristics. Five kinds of biomass powder, biomass briquettes, and briquettes with additive samples are used to carry out the combustion experiments, which are performed in a lab-scale tube furnace system. The influence of the briquetting process, reaction temperature, added coal and additives are studied comprehensively. The briquetting process gives remarkable enhancements on sulphur retention characteristics compare to the raw biomass powder, which is mainly due to the effect of the structural transformation. The enhancement are 35.00%, 76.68%, 36.15%, 27.34% and 31.36% compared to the original biomass samples. The reaction temperature behaves severely on the release of sulphur, that when the reaction temperature reaches 900 ∘ C , the sulphur release ratio starts to sharply increase. Adding coal into the biomass samples can increase the sulphur retention ability and the caloric value. The effects of different additives on sulphur retention ratios are also studied, exhibiting opposite trends as temperature increased. The organic Ca-based additives show better performance to enhance the self-sulphur retention ability. What's more, the potassium-based additives have a higher improvement than the calcium-based ones. The mechanism on sulphur retention of biomass briquettes with or without additives is also discussed. This study develops insightful understand on biomass self-sulphur retention ability, and its behaviours under the effect of the briquetting process, reaction temperature, added coal or additives, which could use as a guild for biomass briquetting combustion.