Eva Thorin

Co-digestion of cultivated microalgae and sewage sludge from municipal waste water treatment

In this study two wet microalgae cultures and one dried microalgae culture were co-digested in different proportions with sewage sludge in mesophilic and thermophilic conditions. The aim was to evaluate if the co-digestion could lead to an increased efficiency of methane production compared to digestion of sewage sludge alone. The results showed that co-digestion with both wet and dried microalgae, in certain proportions, increased the biochemical methane potential (BMP) compared with digestion of sewage sludge alone in mesophilic conditions. The BMP was significantly higher than the calculated BMP in many of the mixtures. This synergetic effect was statistically significant in a mixture containing 63% (w/w VS based) undigested sewage sludge and 37% (w/w VS based) wet algae slurry, which produced 23% more methane than observed with undigested sewage sludge alone. The trend was that thermophilic co-digestion of microalgae and undigested sewage sludge did not give the same synergy.

Performance evaluation of adding ethanol production into an existing combined heat and power plant

In this paper, the configuration and performance of a polygeneration system are studied by modelling the integration of a lignocellulosic wood-to-ethanol process with an existing combined heat and power (CHP) plant. Data from actual plants are applied to validate the simulation models. The integrated polygeneration system reaches a total efficiency of 50%, meeting the heating load in the district heating system. Excess heat from the ethanol production plant supplies 7.9 MW to the district heating system, accounting for 17.5% of the heat supply at full heating load. The simulation results show that the production of ethanol from woody biomass is more efficient when integrated with a CHP plant compared to a stand-alone production plant. The total biomass consumption is reduced by 13.9% while producing the same amounts of heat, electricity and ethanol fuel as in the stand-alone configurations. The results showed that another feature of the integrated polygeneration system is the longer annual operating period compared to existing cogeneration. Thus, the renewable electricity production is increased by 2.7% per year.

Thermodynamic Properties of Ammonia–Water Mixtures for Power Cycles

Power cycles with ammonia–water mixtures as working fluids have been shown to reach higher thermal efficiencies than the traditional steam turbine (Rankine) cycle with water as the working fluid. Different correlations for the thermo-dynamic properties of ammonia–water mixtures have been used in studies of ammonia–water mixture cycles described in the literature. Four of these correlations are compared in this paper. The differences in thermal efficiencies for a bottoming Kalina cycle when these four property correlations are used are in the range 0.5 to 3.3%. The properties for saturated liquid and vapor according to three of the correlations and available experimental data are also compared at high pressures and temperatures [up to 20 MPa and 337°C (610 K)]. The difference in saturation temperature for the different correlations is up to 20%, and the difference in saturation enthalpy is as high as 100% when the pressure is 20 MPa.

Ammonia-water power cycles for direct-fired cogeneration applications

Abstract There is a great interest in increasing the efficiency of power generation. In many applications it has been shown that using an ammonia–water mixture as working fluid increases the power output 1 , 2 , 3 (1. Kalina, A., in Second Law Analysis—Industrial and Environmental Applications, ASME AES, 1991, 191, 41.; 2. Kalina, A., in Proceedings of the American Power Conference, Vol. 55-I, 55th Annual Meeting, Chicago, 1993, p. 191; 3. Olsson, E., Thorin, E., Deijors, C. and Svedberg, G., Flowers ’94, Florence, Italy, 6–8 July 1994.). The Kalina cycle is the best known power cycle that uses ammonia–water mixtures as working fluid. In Sweden, the new power plants being built are direct-fired biomass-fueled steam turbine cycles. They generate power and produce heat for district heating. The plants are quite small, around 100 MWfuel. The aim of the study reported here was to investigate whether there are any thermodynamic advantages of using ammonia–water mixture cycles in small direct-fired biomass fueled cogeneration plants. The main interest is to achieve a higher net power output. The fuel input rate has been assumed to correspond to 80 MW. The district heating network supply temperatures are 90, 100 and 110°C respectively and the return temperature is 50°C. Different configurations of the ammonia–water mixture cycle were compared to a Rankine steam cycle with a five-pressure turbine and three preheaters. Conventional condensing power applications were also studied. Of the three different supply temperatures to the district heating network, the Rankine steam cycle has the highest net power generation. The ammonia–water cycle approaches the Rankine steam cycle when a high supply temperature is desired. For a cogeneration plant without reheat, the difference in net power generation is between 4 and 11%. With condensing power application, the ammonia–water cycle reaches higher power generation than the Rankine steam cycle.

The effects of different mixing intensities during anaerobic digestion of the organic fraction of municipal solid waste

Mixing inside an anaerobic digester is often continuous and is not actively controlled. The selected mixing regime can however affect both gas production and the energy efficiency of the biogas plant. This study aims to evaluate these effects and compare three different mixing regimes, 150 RPM and 25 RPM continuous mixing and minimally intermittent mixing for both digestion of fresh substrate and post-digestion of the organic fraction of municipal solid waste. The results show that a lower mixing intensity leads to a higher biogas production rate and higher total biogas production in both cases. 25 RPM continuous mixing and minimally intermittent mixing resulted in similar biogas production after process stabilization, while 150 RPM continuous mixing resulted in lower production throughout the experiment. The lower gas production at 150 RPM could not be explained by the inhibition of volatile fatty acids. Cumulative biogas production until day 31 was 295 ± 2.9, 317 ± 1.9 and 304 ± 2.8N ml/g VS added during digestion of fresh feed and 113 ± 1.3, 134 ± 1.1 and 130 ± 2.3N ml/g VS added during post digestion for the 150 RPM, 25 RPM and minimally mixed intensities respectively. As well as increasing gas production, optimal mixing can improve the energy efficiency of the anaerobic digestion process.

Thermophysical Properties of Ammonia–Water Mixtures for Prediction of Heat Transfer Areas in Power Cycles

In power cycles using ammonia–water mixtures as the working fluid, several heat exchangers are used. The influence of different correlations for predicting thermophysical properties on the calculations of the size of the heat exchangers is presented. Different correlations for predicting both the thermodynamic and the transport properties are included. The use of different correlations for the thermodynamic properties gives a difference in the total heat exchanger area of 7%, but for individual heat exchangers, the difference is up to 24%. Different correlations for the mixture transport properties give differences in the predicted heat exchanger areas that are, at most, about 10% for the individual heat exchangers. The influence on the total heat exchanger area is not larger than 3%. A difference in the total heat exchanger area of 7% would probably correspond to less than 2% of the total cost for the process equipment. Experimental data and correlations developed for the ammonia–water mixture transport properties are very scarce. The evaporation and condensation processes involving ammonia–water mixtures are also not fully understood.

Influence of Temperature in Radio Frequency Measurements of Moisture Content in Biofuel

A method that permits the determination of moisture content in biofuel in a fast and representative way is under development. The method uses radio frequency waves within the range of 310 MHz to 800 MHz and measures the reflection coefficient in samples with volume of about 0.1 m3. The influence of sample temperature in the measurements is shown in this study. Two biofuel types were used, with moisture content varying between 31% and 63% and temperature varying between 1degC and 63degC. The data was evaluated with multivariate data analysis. Results show that it is not possible to identify the sample temperature as a principal component in a principal component analysis and partial least squares regression shows no correlation between temperature and the radio frequency data. For the frequency interval and the temperature range studied, it was not possible to detect any influence of sample temperature on moisture content prediction with the radio frequency method

Annual performance analysis and comparison of pellet production integrated with an existing combined heat and power plant

Three optional pellet production processes integrated with an existing biomass-based CHP plant using different raw materials (wood chips and solid hydrolysis residues) are studied. The year is divided into 12 periods, and the integrated biorefinery systems are modeled and simulated for each period. The annual economic performance of three integrated biorefinery systems is analyzed based on the simulation results. The option of pellet production integrated with the existing CHP plant with the exhaust flue gas and superheated steam as drying mediums has the lowest specific pellet production cost of 105 €/t(pellet), the shortest payback time of less than 2 years and the greatest CO(2) reduction of the three options. An advantage in common among the three options is a dramatic increase of the total annual power production and significant CO(2) reduction in spite of a small decrease of power efficiency.

The Use of Automatic Meter Readings for a Demand-Based Tariff

A determining factor for a successful implementation of a demand-based pricing model or control strategy in electricity markets is not only the effects of peak load management, but also the economical consequences for the utility operator and the end customer. In this economical modeling a subset of 460 residential customers has been implemented in a software tool analyzing the economical outcome of three different tariffs. Two demand-based tariffs were investigated and compared with a traditional energy-based tariff. The demand-based tariffs transform the flat income curve into a more complex, due to a stronger economical dependency to the system peak loads. The demand-based tariffs move the revenues to the high-peak period, November-March, and the utility operator gains a good matching between system peaks and distribution of incomes

Comparison of correlations for predicting thermodynamic properties of ammonia-water mixtures

Tillner-Roth and Friend have presented a new correlation for the thermodynamic properties of ammonia–water mixtures. In this study, the new correlation has been compared to other correlations used in simulations of power cycles using ammonia–water mixtures as working fluids. The saturation properties for mixtures, calculated with the different correlations, have been examined at different temperatures and pressures. Available experimental data have been included in the comparison. The variation of the enthalpy with temperature at different pressures for a mixture has also been compared. The correlations have been examined for use in power cycle simulations as well. The comparison reveals that the new correlation shows a more reasonable behavior when the critical point of the mixture is approached. At lower temperatures and pressures, the compared correlations give very similar results. The differences in the results from the cycle simulations, using different correlations, are small but they tend to increase with increasing maximum pressure in the cycle.