Steady and transient state behavior of a gasification process under fixed-bed downdraft configuration.

Gasification is a thermochemical process that has gained significant interest in the field of biomass energy conversion. Despite the level of technological maturity of the process, the dynamic variation of the process as a result of changes in both the properties of the gasifying agent and biomass has not been analysed in sufficient depth. Therefore, the present study characterizes the process dynamically as a function of step-type changes in rice husk biomass moisture content and gasifying airflow. To identify stability conditions and the range for inducing disturbances, steady-state tests were carried out using a 32-factorial design. The experimental results demonstrate that within the tested range of airflow, the gasification process operates in the oxygen-limited zone. Despite increasing the airflow from 20 to 40 standard liters per minute (SLPM) and driving the reaction towards the combustion zone, the high temperatures achieved resulted in the gas reaching a peak Lower Heating Value (LHV) of 2.6 MJ/Nm3 and a gas power of 2.6 kW, with a Cold Gas Efficiency (CGE) of 62%. In contrast, the effect of biomass moisture content was negligible due to the thermal inertia of the reactor and the natural variation of the process. Dynamic evaluation revealed that the oxidation temperature and gas concentration were the variables that took the longest to return to stability after air disturbances. It took approximately 1200 s for the hydrogen (H2) concentration to stabilize, while the gas power required about 300 s. No clear results were observed regarding the impact of the dynamic disturbance in moisture content, which varied between 12.3% w.t and 21.5% w.t.

Comparative Exergy and Environmental Assessment of the Residual Biomass Gasification Routes for Hydrogen and Ammonia Production.

The need to reduce the dependency of chemicals on fossil fuels has recently motivated the adoption of renewable energies in those sectors. In addition, due to a growing population, the treatment and disposition of residual biomass from agricultural processes, such as sugar cane and orange bagasse, or even from human waste, such as sewage sludge, will be a challenge for the next generation. These residual biomasses can be an attractive alternative for the production of environmentally friendly fuels and make the economy more circular and efficient. However, these raw materials have been hitherto widely used as fuel for boilers or disposed of in sanitary landfills, losing their capacity to generate other by-products in addition to contributing to the emissions of gases that promote global warming. For this reason, this work analyzes and optimizes the biomass-based routes of biochemical production (namely, hydrogen and ammonia) using the gasification of residual biomasses. Moreover, the capture of biogenic CO2 aims to reduce the environmental burden, leading to negative emissions in the overall energy system. In this context, the chemical plants were designed, modeled, and simulated using Aspen plus™ software. The energy integration and optimization were performed using the OSMOSE Lua Platform. The exergy destruction, exergy efficiency, and general balance of the CO2 emissions were evaluated. As a result, the irreversibility generated by the gasification unit has a relevant influence on the exergy efficiency of the entire plant. On the other hand, an overall negative emission balance of -5.95 kgCO2/kgH2 in the hydrogen production route and -1.615 kgCO2/kgNH3 in the ammonia production route can be achieved, thus removing from the atmosphere 0.901 tCO2/tbiomass and 1.096 tCO2/tbiomass, respectively.

Study of biorefineries based on experimental data: production of bioethanol, biogas, syngas, and electricity using coffee-cut stems as raw material.

Energy-driven biorefineries can be designed considering biotechnological and thermochemical conversion pathways. Nevertheless, energy and environmental comparisons are necessary to establish the best way to upgrade lignocellulosic biomass and set the requirements of these processes in different scenarios. This paper aims to evaluate experimentally a biorefinery producing energy vectors using coffee-cut stems (CCS) as feedstock. The obtained yields were the basis for energy and environmental analysis, in two different biorefinery scenarios: (i) production of bioethanol and biogas and (ii) production of syngas and electricity. The energy results indicated that the overall energy efficiency calculated in the first scenario was only 9.15%. Meanwhile, the second biorefinery configuration based on thermochemical routes presented an energy efficiency value of 70.89%. This difference was attributed to the higher consumption of utilities in the biorefinery based on biotechnological routes. The environmental results showed that the impact category of climate change for the first biorefinery (i.e., 0.0193 kg CO2 eq./MJ) had a lower value than that of the second process (i.e., 0.2377 kg CO2 eq./MJ). Thus, the biorefinery based on the biotechnological route presented a better environmental performance. Additionally, the results for both biorefineries allowed concluding that the inclusion of by-products and co-products in the calculation of the environmental analysis can dramatically affect the results.

Evaluation of biogas and syngas as energy vectors for heat and power generation using lignocellulosic biomass as raw material

The use of nonrenewable energy sources to provide the worldwide energy needs has caused different problems such as global warming, water pollution, and smog production. In this sense, lignocellulosic biomass has been postulated as a renewable energy source able to produce energy carriers that can cover this energy demand. Biogas and syngas are two energy vectors that have been suggested to generate heat and power through their use in cogeneration systems. Therefore, the aim of this review is to develop a comparison between these energy vectors considering their main features based on literature reports. In addition, a techno-economic and energy assessment of the heat and power generation using these vectors as energy sources is performed. If lignocellulosic biomass is used as raw material, biogas is more commonly used for cogeneration purposes than syngas. However, syngas from biomass gasification has a great potential to be employed as a chemical platform in the production of value-added products. Moreover, the investment costs to generate heat and power from lignocellulosic materials using the anaerobic digestion technology are higher than those using the gasification technology. As a conclusion, it was evidenced that upgraded biogas has a higher potential to produce heat and power than syngas. Nevertheless, the implementation of both energy vectors into the energy market is important to cover the increasing worldwide energy demand.

Thermogravimetric characterization and gasification of pecan nut shells.

This study focuses on the evaluation of pecan nut shells as an alternative source of energy through pyrolysis and gasification. The physicochemical characteristics of the selected biomass that can influence the process efficiency, consumption rates, and the product yield, as well as create operational problems, were determined. In addition, the thermal decomposition kinetics necessary for prediction of consumption rates and yields were determined. Finally, the performance of a downdraft gasifier fed with pecan nut shells was analyzed in terms of process efficiency and exit gas characteristics. It was found that the pyrolytic decomposition of the nut shells can be modeled adequately using a single equation considering two independent parallel reactions. The performance of the gasification process can be influenced by the particle size and air flow rate, requiring a proper combination of these parameters for reliable operation and production of a valuable syngas.