Evaluation of energy efficiency in biofuel drying by means of energy and exergy analyses

doi:10.1016/j.applthermaleng.2005.04.005

Applied Thermal Engineering

Volume 25, Issues 17–18, December 2005, Pages 3115-3128

https://doi.org/10.1016/j.applthermaleng.2005.04.005 Get rights and content

Abstract

The calculation of heat consumption is based on the First Law and it gives quantitative information about the energy used in drying. However, it does not pay any attention to the quality of the energy used in drying. To take into account the quality of the energy, attention must be paid to the Second Law, too. Especially in those cases where the energy used in drying may be converted to mechanical work, it is important to consider the Second Law is. In this paper, the energy efficiency of biofuel drying in a pulp and paper mill is evaluated on the basis of energy and exergy analysis. The evaluation is based on the determination of the heat consumption and the irreversibility rate for energy and exergy analysis, respectively. The evaluation methods are applied to two different drying systems, single-stage-drying with partial recycle of spent air, and multi-stage-drying. Both drying systems are also provided with a heat recovery unit in which the inlet air is pre-heated using the outlet air of the dryer. There are two alternative heat sources available for the drying energy, steam at a pressure of 3 bar and water at a temperature of 80 °C. The results show that the heat consumption is only dependent to a small extent on the heat source type or the drying system. On the other hand, the irreversibility rate depends to a considerable on the heat source and the drying system.

Introduction

In general, the main goal of drying is to decrease the moisture content of solid materials to below a certain limit. Drying is part of several industrial processes for example in the food, chemical, and paper and pulp industries. The main objectives of drying are extended storage life, quality enhancement, and ease of handling and further processing [1].

In this paper, the term biofuel refers to biofuels consumed by the pulp and paper industry. The most important biofuels are bark, forest residues, sawdust, and chips. They are by-products of the main processes and are burnt in fluidised bed boilers to produce heat and electricity for the mill. Power is generated by a steam turbine-generator. The moisture content of these biofuels typically varies between 50% and 60% (water per total mass). One of the main objectives of biofuel drying is to increase the electricity production.

The heat sources with the greatest potential for drying energy in pulp and paper mills are secondary heat flows and steams at different pressures. Secondary heat is, by definition, heat transferred from primary heat sources to process flows—pulp, liquor, and washing waters—and further to circulation water, flue gas, steam, and vent gases [2]. Usually, the technically available secondary heat is hot water in the temperature range 50–90 °C. Steam pressures used in pulp and paper mills typically vary from 3–4 bar (back pressure steam) to 10–12 bar (extraction steam).

The most common way to evaluate the energy efficiency of various drying processes is the specific heat consumption (amount of heat supplied per mass of water evaporated); see e.g. [3], [4], [5], [6]. Because biofuel drying is part of the electricity production process, the specific heat consumption does not necessarily give an exact overview of energy efficiency in drying. In addition to heat consumption, the temperature of the heat source used in drying must be included in the analysis of the drying process. This necessitates Second Law analysis for the drying process. For example, Refs. [7], [8], [9] mention the importance of the Second Law/exergy analysis associated with the drying processes, but it is not used in these papers. In Ref. [10], the exergy method is applied to determine the so-called energy saving ability function.

In this paper, the energy efficiency of the drying is evaluated using two analysis methods: the energy method and the exergy method. The energy analysis is based on the determination of the heat consumption. The exergy analysis is based on the determination of the irreversibility rate. The evaluation methods are applied to two different drying systems, single-stage-drying with partial recycle of spent drying air (SSD) and multi-stage-drying (MSD). Both drying systems are also provided with a heat recovery unit where the inlet air is pre-heated with the outlet air of the dryer.

Section snippets

Single-stage dryer with partial recycle of drying air

Fig. 1 shows a continuous single-stage dryer with partial recycle of spent air including heat recovery unit. As a result of the partial recycle of air and the heat recovery, the air temperature before the heater increases. This decreases the heat consumption of the drying process. Because of the recycle, the fresh drying air has less potential for taking up moisture, which leads to a bigger air mass flow through the drying chamber.

By using the notations in Fig. 1, the energy and mass balance

Determination of irreversibility rate

The dryer is an open steady-state system with two flows of matter (moist material and moist air) and one or several heat inputs and outputs to the system. The irreversibility rate for an open steady-state system is derived in [16], and its general form is $I = T_{o} [\sum m_{i} Δ s_{i} - \sum \frac{Φ_{i}}{T_{ri}}]$ where Φ_i represents the heat inputs and outputs from the system. In this paper, the heat input to the system is assumed to be positive. T_r is the temperature at the point on the boundary of the system where the heat transfer is

Results and discussion

The main objective of these calculations is to theoretically compare the energy efficiency of two different drying processes (SSD and MSD) using two evaluation methods (specific heat consumption and irreversibility rate). Both drying systems are provided with heat recovery units (HRU).

Moist biofuel is dried from an initial moisture of 1.5 kg/kg_dm to a final moisture of 0.3 kg/kg_dm, and the dry mass flow of the fuel is 1 kg_dm/s. Two alternative heat sources are available for the heating: steam at a

Conclusions

Two methods to evaluate the energy efficiency of the biofuel drying have been presented and applied to two different drying systems. Both drying systems are also provided with a heat recovery unit, so as to use the best possible drying systems as example dryers. The traditional evaluation method, which is based on the determination of heat consumption, only gives information about the quantity of the energy used in drying. It does not pay any attention to the quality of the energy. The results

References (21)

J. Berghel et al.
Basic design criteria and corresponding results performance of a pilot-scale fluidized superheated atmospheric steam dryer
Biomass and Bioenergy
(2002)
J.G. Brammer et al.
Drying technologies for an integrated gasification bio-energy plant
Renewable and Sustainable Energy Reviews
(1999)
J.D. Kribs et al.
Drying energy conservation for deep-bed barley-malt kilns
Journal of Agricultural Engineering Research
(1997)
R. Wimmerstedt
Recent advances in biofuel drying
Chemical Engineering and Processing
(1999)
H. Björk et al.
Method for life cycle environmental optimisation of a dynamic process exemplified by an analysis of an energy system with a superheated steam dryer in a local districy heat power plant
Chemical Engineering Journal
(2002)
S. Sieniutycz
A development of the relation between drying energy savings and thermodynamic irreversibility
Chemical Engineering Science
(1984)
P. Perre et al.
Advances in transport phenomena during convective drying with superheated steam and moist air
International Journal of Heat and Mass Transfer
(1993)
S. Sokhansanj et al.
Drying of foodstuff
P. Ahtila, P. Svinhufvud, Heat recovery from secondary heat flows of the UPM-Kymmene Kaukas Mill, European Conference...
C.G.J. Baker
Predicting the energy consumption of continuous well-mixed fluidized bed dryers from drying kinetic data
Drying Technology
(1999)

There are more references available in the full text version of this article.

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Evaluation of energy efficiency in biofuel drying by means of energy and exergy analyses

Abstract

Introduction

Section snippets

Single-stage dryer with partial recycle of drying air

Determination of irreversibility rate

Results and discussion

Conclusions

Biomass and Bioenergy

Renewable and Sustainable Energy Reviews

Journal of Agricultural Engineering Research

Chemical Engineering and Processing

Chemical Engineering Journal

Chemical Engineering Science

International Journal of Heat and Mass Transfer

Drying of foodstuff

Predicting the energy consumption of continuous well-mixed fluidized bed dryers from drying kinetic data

Drying Technology