In recent years, the increase in global energy consumption has led to rising pollution levels.

Fossil fuels such as coal and oil significantly emit NOx, SOx and CO₂ upon combustion, which subsequently cause acid rain, ozone depletion, global warming and their after effects such as rising sea levels [1]. Thus, it is necessary to find a renewable resource for reducing the gaseous emissions from fossil fuels thereby bringing down the costs of mitigating these pollutants.

Biomass is a major renewable, sustainable and environmental-friendly energy source, which can convert carbon to solid, liquid and gaseous products through different processes such as gasification. It is ranked fourth among the primary energy sources such as coal, oil and natural gas and currently caters for about 14% of the world’s total energy consumption [2]. Biomass is constituted of a broad range of organic materials derived from plants and recently living organisms.

The general chemical formula for biomass is CxHyOz [3]. Biomass can generate electricity, heat, vehicle fuel and can also produce the feedstock for several chemical industries. Generally, there are four types of biomass: woody plant, herbaceous plants, aquatic plants and manures [3]-[5]. Most of the researchers have studied the woody and herbaceous plants because they have lower moisture content than other species. The key differences in the properties of biomass in comparison with coal and coke are [1][4][6][7]:

1. Greater moisture content.
2. Lower heating value.
3. Higher volatile matter.
4. Higher reactivity of both biomass and its resulting char, due to holocellulose and lignin contents of biomass with weaker bonds.
5. Lower ash.
6. Higher alkali content (especially the herbaceous type), which plays a key role in thermo chemical processes.
7. Lower bulk density thus making the transportation and storage costs higher.
8. Greater oxygen and hydrogen content.
9. Lower sulfur and nitrogen content.

Also, biomass energy utilization has gained particular interest in recent years due to the progressive depletion of conventional fossil fuels, that calls for an increased use of renewable energy sources. Moreover, the moderate sulphur and greenhouse gas emissions associated with the use of biomass for energy production respond to the growing pressure of government policies about achievement of better environmental sustainability of power generation processes in terms of air pollution control [8]. Biomass may be used to meet a wide variety of energy needs, including generating electricity, providing process heat for industrial facilities, heating homes and fuelling vehicles. The conversion of biomass to such useful forms of energy, also called bio-energy, can be achieved using a number of different technological solutions that can be separated into two basic categories, namely thermochemical processes and biochemical/biological processes.

Figure 1 shows biomass gasification process. In this process, waste heat is generated from gasification, refining equipment etc. Thermal energy storage system is required in order to store waste heat.

The stored energy by thermal energy storage system is utilized for heating of residential and industrial buildings, etc.

Some studies [9]-[13] on thermo-physical properties as PCMs are concentrated in finding and developing the materials of latent heat thermal energy storage system (LHTES).

The present paper is concerned with PCMs of organic material at temperature region of biomass gasification process waste heat source about 373~523K, which are investigated the melting and solidification characteristics.

The goal of this study is selecting the PCMs at temperature region of biomass gasification process waste heat source (373~523K) by clarify the possibility of optimum mixture ratios

Figure 1 
Schematic diagram of biomass gasification process

The middle temperature LHTES approach can reduce the energy storage container, since the energy density is large. We have to choose the PCMs which can be utilized in temperature range of biomass gasification process waste heat about 373~523 K.

In order to find the best material in the experiment, we screened the relationship between latent heat and melting point.

The distribution of materials with melting point in the range 373~523 K are shown in Figure 2, which describes the latent heat and melting point for molten salts, organic materials and inorganic material [14]. Molten salt have big corrosiveness with other material. If it is increased the heat to hydrate, it is separated from water.

Figure 2 
Latent heat of melting point of PCMs

Figure 3 
Schematic diagram of experimental appartus

From Figure 2, it is clear that the optimum material is generally known as erythritol and mannitol with melting point range 373~523 K. Among phase change materials of organic material, erythritol and mannitol of sugar alcohol have specially high latent heat. In addition, erythritol and mannitol are environmentally friendly materials, non-toxic and non-flammable. The selected thermo-physical properities of the PCMs are given in Table 1.

In this study, erythritol with purity of more than 99.5% (Mitsubishi Chemical Corp.) and mannitol with purity of more than 99.0% (Tokyo chemical industry Corp.) were used as PCM samples.

In this study, two measuring equipments were used to investigate the melting and solidification behavior of erythritol, mannitol and their compounds as phase change materials (PCMs). One of two measuring equipment is the differential scanning calorimetry (DSC; Reigaku, Thermo Plus2/DSC8270) which was used to measure the melting point and latent heat of these PCMs. Other equipment is schematically shown in Figure 2. This apparatus mainly consists of a constant temperature oil bath (TOMAS, T-301), glass tubes (25ø ×200 mm), a digital camera, and a data logger.

The melting process was prepared as the initial condition of oil bath at 373 K. To melt these PCMs in the glasstubes, the oil was heated until its temperature increased to 453K (4 K/min) using an electric heater with an impeller. The solidification period started directly after the completion of the melting period. The electric heater of oil bath was tuned off and the oil bath was naturally cooling (0.13 K/min) until these PCMs in the glass tube were solidified. During this period, temperature variation of these PCMs and surrounding oil bath were measured by thermocouples of K-type (0.1 mm). The measured temperature data were recorded by a data logger in 13-sec intervals. And these PCMs set in the glass tubes were observed using a digital camera.

In the present study is proposed to use erythritol, mannitol and their compounds of organic material at middle temperature about 373~573 K, which are investigated from the melting and solidification characteristics. When selected PCMs mixed, these compounds are also investigated optimum mixture ratio. The acquired main conclusions are summarized as follows.

Through the comparison of the heat storage temperature and energy density of PCMs, sugar alcohol system PCMs was indicated effective.

At mannitol contents from 50 to 90 mass%, the melting points of these compounds have two melting points.

If the mixture of erythritol & 30 mass% of mannitol may control the super-cooling phenomenon, it can be used as PCM in LHTES, which has relatively low degrees of super-cooling. In this study, mannitol, erythritol and their compounds as PCMs had the phenomenon of super-cooling, which will be studied for relaxation of super-cooling in the subject of next investigations.