Pekka Ahtila

Drying of Activated Sludge Under Partial Vacuum Conditions—An Experimental Study

Abstract A typical wastewater treatment system in a pulp and paper mill in Finland treats wastewater both mechanically and biologically. Sludges resulting from these processes have to be disposed of. One possible way of doing this is to incinerate them with solid fuel in the power plant of the mill. To minimize the amount of sludge and to make the use of the sludge energy efficient, it has to be dried before incineration. Mechanical drying of the sludge from biological wastewater treatment is difficult to carry out. Using secondary energies may provide a competitive way of arranging drying: a method of doing this by using partial vacuum evaporation to utilize the low temperature secondary heat in sludge drying is under development. A laboratory study to examine the behavior of activated sludge under partial vacuum evaporation conditions was carried out using a laboratory rotating evaporator to analyze the drying of activated sludge from three mills at 40–80°C boiling temperatures. This article presents the results from the tests. These are promising; it was possible to reach high dry solids content. Also, no boiling point temperature rise was detected, fouling of the evaporator seemed low and easily avoidable, condensate from the evaporator weas relatively clean and returnable to the processes of the mill.

Determination of the Real Loss of Power for a Condensing and a Backpressure Turbine by Means of Second Law Analysis

All real processes generate entropy and the power/exergy loss is usually determined by means of the Gouy-Stodola law. If the system only exchanges heat at the environmental temperature, the Gouy-Stodola law gives the correct loss of power. However, most industrial processes exchange heat at higher or lower temperatures than the actual environmental temperature. When calculating the real loss of power in these cases, the Gouy-Stodola law does not give the correct loss if the actual environmental temperature is used. The first aim of this paper is to show through simple steam turbine examples that the previous statement is true. The second aim of the paper is to define the effective temperature to calculate the real power loss of the system with the Gouy-Stodola law, and to apply it to turbine examples. Example calculations also show that the correct power loss can be defined if the effective temperature is used instead of the real environmental temperature.

Reduction of organic emissions by using a multistage drying system for wood-based biomasses

Abstract Organic emissions during the thermal drying process are strongly dependent on the drying temperature. In the traditional single stage drying system, the inlet temperature of the drying air has to be relatively high in order to keep the airflow for drying small. In the multistage drying system, the drying airflow is heated up again after the first drying stage with higher moisture content, and then again after the second, and subsequent drying stages. In this method, the drying temperatures are limited in all stages to acceptable low levels, and only the moisture content of the drying air is increasing from one stage to another. As a result the multistage drying system has a lower drying temperature. We have studied the dependence of the organic emissions on the drying temperature, and present the results from drying units operating at temperatures of 100–200°C and below 100°C. The results are compared to previous measurements found in the literature. The estimates for the emissions at higher drying temperatures are derived from the literature values.

Improving the power-to-heat ratio in CHP plants by means of a biofuel multistage drying system

In biofuel drying in industrial power plants, a new multistage drying system (MSDS) can now be applied before combustion. By reducing the drying air mass flow, MSDS makes it possible to use a proportion of the combustion air in drying, and to lead that moist drying air into the combustion stages of the power plant boiler. MSDS increases the power-to-heat ratio and capacity of the power plant compared to power plants without drying systems. Previous increases came from biofuel homogenization and increased energy efficiency. Biofuel homogenization also reduces unburned organic emissions as a result of more complete combustion.

Experimental Study on Drying of Bark in Fixed Beds

Bark has a heterogeneous particle size distribution and an irregular particle shape. For this reason, drying models usually give only rough estimations of the drying rate, and drying tests are needed to define the drying rate precisely. In this study, normal drying curves and a characteristic drying curve are experimentally determined for bark that has been dried in thick fixed beds. The study evaluates the shape of the characteristic drying curve as well as the accuracy of the characteristic drying curve method. In drying tests the influence of drying air temperature and bed height on drying time and the shape of the characteristic drying curve has been studied. Four different drying air temperatures (50, 70, 90, and 110°C) and three bed heights (50, 150, and 250 mm) have been used in the tests. Results indicate that a linear characteristic drying rate curve can be used in bark drying to extrapolate drying data from one set of drying conditions to another, when the bed height is high (in this study 150–250 mm) and the inlet air temperature is below 100°C. For thin beds (in this study 50 mm) the characteristic drying curve method cannot be used over such a wide range on drying temperatures as in the case of higher beds.

Wood Chip Drying in Fixed Beds: Drying Kinetics and Economics of Drying at a Municipal Combined Heat and Power Plant Site

Drying of wood chips at a power plant site increases the lower heating value and decreases composting and dry matter losses in long-term storage. In this study, experimentally measured drying curves for wood chips are produced to study the drying kinetics of the chips. Drying curves have been used to assess whether or not wood chip drying in a batch-type fixed bed dryer at a municipal Combined Heat and Power (CHP) plant site is profitable when using external excess heat or solar energy as a heat source in drying, and what could be the reasonable drying parameters for that. The curves have been determined by changing bed heights (100, 300, 500 mm), temperature of inlet air (30, 50, 70, 90°C) and air velocities (0.3 m/s, 0.5 m/s, 0.7 m/s) in the tests. Air has been used as a drying gas. The results show that drying time decreases considerably when the temperature of the drying air increases from 50°C to 70°C. The influence of drying air temperature on the drying time is no longer so remarkable as the temperature increases from 70°C to 90°C. This indicates that the inlet air temperature should be at least 70°C. The results also indicate that the air velocity should be at least 0.5 m/s in order to achieve reasonable drying times. The economics of drying have been evaluated in the study by calculating the payback period for the dryer. According to the results, drying could be profitable if investment costs are appropriate (not much higher than 1,500 €/m2) and the price of the wood chips is sufficiently high (higher than ∼15–20 €/MWh). It is, however, important to consider that drying at a municipal CHP plant site is always case-dependent.

Simulation Model for the Model-Based Control of a Biofuel Dryer at an Industrial Combined Heat and Power Plant

The primary aim of this article is to present a simulation model for a bark dryer integrated into a combined heat and power plant. The same model can be used for the model-based control of the dryer. The secondary aim is to evaluate how useful the control is from an economic point of view. Results show that the final fuel moisture content can be stabilised by controlling the drying temperature(s). On the other hand, the deviation in final bark moisture decreases even when the dryer has no control at all. Net incomes resulting from drying increase in most cases compared to dryers without control. The need for control cannot be justified on economic grounds.

Can bacterial biofiltration be replaced by autotrophic organisms in recirculating fresh water aquaculture

In recirculating aquaculture, a bacterial biofilter is applied to convert ammonium, excreted by the fish, to the non-toxic nitrate. Unfortunately, nitrifying bacteria produce off-flavor compounds that lower fish quality. We investigated, by calculations and estimations, possibilities to replace the biofilter by autotrophic organisms that incorporate ammonium in biomass, consume other mineral nutrients and produce marketable biomass and oxygen. The capacity of microalgae, macroalgae, duckweed, strawberry, and tomato to assimilate ammonium was calculated, using data from an existing Finnish fresh water fish farm. Microalgae were found to be the most effective for ammonium removal, and they would be able to consume the ammonium produced by a fish farm if the algae were grown in a facility with approximately twice the area of the fish farm itself. Macroalgae and duckweed appeared to be the second best option for ammonium removal, and strawberry and tomato were predicted to have a somewhat smaller capacity for ammonium removal. Due to low ammonium content, microalgae cannot be cultivated in the recirculating water, but rather the nutrients should be allowed to diffuse through a semipermeable membrane to microalgae.

Increasing energy efficiency in passenger ships by novel energy conservation measures

ABSTRACT To achieve increasing emission requirements, the cruise ship industry is working to develop higher efficiency ships. Cruise ships are different from other ship types in their relatively higher consumption of electrical power, steam and hot water. Several novel high-efficiency system concepts are possible for on-board electrical power generation and other utility services, each with differing impacts and first costs. Low-emission concepts novel to the cruise ship industry include combinations of exhaust gas heat recovery, heat pumps, steam turbines and organic Rankine cycles (ORCs). Yet, evaluation of these concepts is difficult given the different operating modes of cruises, and overall efficiency is dependent on the dynamic operational sequences. In this paper, we compare alternative energy efficiency concepts for cruise ships through simulation studies of the ship operations when equipped with different novel power generation systems. We find that the dual pressure steam systems and ORC offer the greatest potential for energy efficiency improvements in the cruise ship industry. We also find that relatively conventional technologies enable cruise ships to comply with planned upcoming higher ship energy efficiency requirements.

Techno-economic evaluation of biomass drying in moving beds: The effect of drying kinetics on drying costs

Abstract Drying woody biomass holds the potential to improve the energy efficiency of certain processes, such as in CHP plants. Drying can also be a necessary unit process in several energy conversion processes (e.g. in biomass gasification). Belt dryers are typically used for drying when low temperature air (100–110 °C) is used. This article aims to produce new knowledge about the influence of the main design parameters on the drying costs of a low temperature belt dryer when three different materials (forest residue, bark, as well as sawdust and soot sludge mixture) are dried using it. The influence is analyzed by changing the following parameters: bed height, air temperature, air velocity and initial/final moisture contents of the material. The study aims to evaluate which of these parameters has an actual effect on drying costs. Results indicate that the lowest costs are achieved with the highest air temperature if the heat price is the same for every air temperature level. However, an optimal bed height depends on the material. Increasing the air velocity does not necessarily decrease the costs. In the sensitivity analysis, to factor in the influence of the temperature on the heat price, the price was changed for every drying air temperature (1, 5, 10 and 15 €/MWh). This analysis showed that the lowest drying costs are achieved by the lowest air temperature in all cases, thus indicating that the price of the heat has a remarkable influence on the economics of drying. Furthermore, the results support the use of low temperature heat sources in drying if they are clearly less expensive than higher temperature heat sources. However, if the prices for lower and higher air temperatures are of the same magnitude, the higher air temperatures are preferable. In general, this paper shows that it is important to pay attention to the main design parameters to optimize total drying costs. For example, if an overly low bed height is used in woodchips or bark drying, the total drying costs might be dozens of per cent higher than in the most economic case.