• Energy Research

Abstract This paper reports on a specific phenomenon, noticed during steam injection experiments on a microturbine. During the considered experiments, measurements indicated an unsteady inlet air temperature of the compressor, resulting in unstable operation of the microturbine. Non-continuous exhaust air recirculation was a possible explanation for the observed behaviour of the microturbine. The aim of this paper is to investigate and demonstrate the effects of exhaust recirculation on a microgasturbine. Depending on wind direction, exhaust air re-entered the engine, resulting in changing inlet conditions which affects the operating regime of the microturbine. For this paper, a series of experiments were performed in the wind tunnel. These series of experiments allowed investigation of the effect of the wind direction on flue gasses flow. Next to the experiments, steady-state simulations of exhaust recirculation were performed in order to study the effect of exhaust recirculation on thermodynamic performance of the microturbine. Dynamic simulations of the non-continuous recirculation revealed the effects of frequency and amplitude on average performance and stability. Results from simulations supported the important impact of exhaust recirculation. Wind tunnel tests demonstrated the influence of the wind direction on recirculation and revealed the necessity to heighten the stack, thus preventing exhaust recirculation.

Abstract Microturbines offer new perspectives in small-scale heat and power production. Non-continuous heat demand however often leads to a reduced number of yearly running hours. This paper proposes an alternative by introducing water or steam injection without significantly increasing the overall cost. Steam injection (STIG®) has been successful to boost performance and efficiency in industrial gas turbine cycles and similar effects are expected in the case of microturbines. Owing to the different way of controlling microturbines at non-constant shaftspeed, the response to steam or water injection differs from current STIG® cycles. The purpose of this study was to examine the effects of steam injection on microturbine behavior by simulating its off-design characteristics in Aspen. The dry behavior of a microgasturbine has first been simulated and validated against a limited number of available measurements. After increasing steam injection up to the surge limit, we concluded a large amount of steam can potentially be injected. Next, the heat required to generate steam was rerouted from the water heater. When CHP mode is disabled and all residual heat after the recuperator is used in a STIG® route, 3.3% water can be injected and electric efficiency rises by 5.1%.

The different routes for power production from biomass often lead to an intermediary product such as a synthesis gas or syngas, which is typically rich in hydrogen and carbon monoxide. The simple design, fuel flexibility and size, which often matches the amount of waste energy available in industrial sites, makes microturbines an attractive solution for on-site, decentralized power generation using a limited range of alternative fuels such as synthetic gas. The properties of the synthetic fuel differ from properties of natural gas and a detailed experimental study with a separated microturbine-like pressurized combustor is therefore necessary. The present article reviews the experimental results obtained by gradually switching the fuel feed from natural gas to wet syngas in a pressurized, slightly modified lean premix microturbine combustor. Temperature profiles, pressure, emissions and flame imaging were closely monitored to detect possible problems in operability of the combustor caused by the strong difference in fuel characteristics. No problems regarding auto-ignition, dynamic or static instability were observed throughout the test-run. Temperature profiles stayed well within allowable limits and did not reveal any significant shift in flame anchoring position. The combustion of syngas during full or part load of the combustor produced remarkably low NOx and CO emissions. The microturbine combustor achieved stable full load combustion of syngas at the end of the test-run.

Micro Gas Turbines (mGTs) have arisen as a promising technology for Combined Heat and Power (CHP) thanks to their overall energy efficiencies of 80% (30% electrical + 50% thermal) and the advantages they offer with respect to internal combustion engines. The main limitation of mGTs lies in their rather low electrical efficiency: whenever there is no heat demand, the exhaust gases are directly blown off and the efficiency of the unit is reduced to 30%. Operation in such conditions is generally not economical and can eventually lead to shutdown of the machine. To address this issue, the mGT cycle can be modified so that in moments of low heat demand the heat in the exhaust gases is used to warm up water which is then re-injected in the cycle, thereby increasing the electrical efficiency. The introduction of a saturation tower allows for water injection in mGTs: the resulting cycle is known as a micro Humid Air Turbine (mHAT). The static performance of the mGT Turbec T100 working as an mHAT has been characterised through previous numerical and experimental work at Vrije Universiteit Brussel (VUB). However, the dynamic behaviour of such a complex system is key to protect the components during transient operation. Thus, we have modelled the Turbec T100 mHAT with the TRANSEO tool in order to simulate how the cycle performs when the demanded power output fluctuates. Steady-state results showed that when operating with water injection, the electrical efficiency of the unit is incremented by 3.4% absolute. The transient analysis revealed that power increase ramps higher than 4.2 kW/s or power decrease ramps lower than 3.5 kW/s (absolute value) lead to oscillations which enter the unstable operation region of the compressor. Since power ramps in the controller of the Turbec T100 mGT are limited to 2kW/s, it should be safe to vary the power output of the T100 mHAT when operating with water injection.

Abstract Emissions from a multi-fuel domestic boiler (40 kW), fired with nine different agro-biomass pellets have been compared. The pellets include apple pomace ( Malus domestica ), reed canary grass ( Phalaris arundinacea ), pectin waste from citrus shells ( Citrus reticulata ), sunflower husk ( Helianthus annuus ), peat, two types of straw pellets and two types of wood pellets. The measurements of emissions comprised carbon monoxide (CO), nitrogen oxides (NO x ), unburned hydrocarbons (C x H y ), sulphur oxides (SO x ) and flue dust mass concentration (by DIN plus and isokinetic sampling methods). Comparison of experimental emission values with relevant quality labels (Blue Angel and Swan Mark) and standard (EN-303-5) showed that the boiler satisfied the emissions requirements of Blue Angel, Swan Mark and EN-303-5 when using wood pellets-1 (except CO emission), reed canary grass and citrus pectin waste pellets as fuel at nominal load. The wood pellets-1 yielded the highest boiler efficiency of 92.4%. Dusts emission varied as a function of fine content and elemental constituent of the pellets and was the highest with sunflower husk. CO and C x H y emissions were maximum with peat pellets. NO x emissions were below the concerned permissible values with all experimental pellets. Emissions of NO x and SO x were found maximum with straw pellets. For agro-pellets, statistical differences in ash contents were significant. High ash contents and low ash melting temperature made straw pellets less suitable for domestic applications. Reed canary grass, citrus pectin and apple pellets were the most suitable agro-pellets for small scale boilers with reasonable less ash contents and less emissions as compared to others.

The control of a nonlinear system such as an underground coal gasification (UCG) process is a challenging task. Several nonlinear design approaches are implemented to improve the tracking performance of the UCG process, however, the nonlinear techniques make implementation complex and computationally inefficient. In this work, a constrained linear model predictive control (MPC) is designed for the UCG process to track the desired trajectory of the heating value, while satisfying actuator constraints pertaining to UCG. The unknown states required for MPC design are reconstructed by using linear adaptive Kalman filter (AKF) and unscented Kalman filter (UKF). The design of MPC and AKF is based on the quasi-linear model of the UCG process. A fair comparison between different control strategies is conducted which include MPC– AKF, MPC– UKF, MPC– gain scheduled modified Utkin observer (GSMUO) and dynamic integral sliding mode control (DISMC)–GSMUO. The quantitative analysis and simulation results show that MPC- AKF outperforms its counterparts by yielding the least tracking error and average control energy. This conclusion holds, even in the presence of an external disturbance, parametric variations, and measurement and process noises. Moreover, MPC- AKF yields 51%, 44% and 46% improvement in absolute relative root-mean-squared error with reference to MPC– UKF, MPC– GSMUO and DISMC–GSMUO, respectively. A quantitative analysis has also been carried for AKF and UKF, which shows that the performance of AKF is more robust against changes in the initial values of measurement and process covariances.

Abstract Despite appearing as a promising technology for decentralised Combined Heat and Power (CHP), the rather low electrical efficiency of micro Gas Turbines (mGTs) prevents them from being attractive for users with a variable heat demand. Hot water injection in mGTs, achieved by transforming the cycle into a micro Humid Air Turbine (mHAT), allows increasing the electrical efficiency of these units in moments of low heat demand—therefore decoupling heat and electricity production. This paper introduces and compares the Sankey (enthalpy flow) and Grassmann (exergy flow) diagrams of an mGT based on the Turbec T100 and the corresponding mHAT cycle. Results show that the electrical efficiency of the T100 increases by 1.4% absolute points with water injection, while the total exergy efficiency decreases by 5.1%. Although in the saturation tower there is an enthalpy gain, exergy actually decreases in this component due to the increase in entropy related to the evaporation of water. The benefits of water injection mostly rely on the increased heat capacity of the air-vapour mixture, the lower fuel consumption, the larger amount of heat recovered in the recuperator and the reduced power required in the compressor.

As a consequence of current energy and climate policies, several regional and federal measures are being implemented in Belgium to encourage the utilization of biofuels for automotive applications. The use of biomass for power and/or heat production also gets growing support through CHP and/or CO2 certificates and most probably through new upcoming European directives. As a result, many investors, policy makers and energy companies are investigating these so-called biomass routes and there is definitely a need to gain more insight in this complex matter. This paper presents an analysis and comparison of the most promising options for utilizing the limited biomass resources in Belgium. To allow for a systematic comparison, a new method called System Perturbation Analysis (SPA) was developed. SPA differs from a classical Life Cycle Analysis (LCA) mainly because it looks to geographical system balances of resources and the resulting effects, rather than comparing well-to-wheel trajectories. Therefore SPA is able to identify the best usage of limited resources such as hectares, wood waste or imports, in terms of fossil energy savings or GHG emissions within a given system (in casu: Belgium). Comparative results of such a SPA assessment are presented and discussed in this paper, including the use of wood for transport, heat and power applications. All the considered biofuel scenarios have positive energy and CO2eq balances. The use of wood appears as a good choice in terms of efficiency, CO2 abatement and surface requirements.