Research outline

Use the Energy Hidden in Trees and Grass!

Photo-1: Grass cutting

Photo-2: A heap of waste vegetation

Fig. 1: Moisture content of vegetation taken from road and river maintaining

1. Biomass, an Earth-Friendly Resource
The use of biomass has been drawing a lot of attention as one solution to global warming. Biomass is the resource contained in organic substances such as raw garbage, sewage sludge, livestock feces, and waste vegetation. Because biomass originally comes from recyclable material such as plants, its combustion causes no further accumulation of greenhouse gases in the atmosphere (since plants absorb CO2 and restore the resource), so that there is no contribution to global warming. Biomass remains as troublesome waste if not utilized, but serves as an earth-friendly resource if wisely used.

2. The Large Amount of Energy hidden in Vegetation originating from River and Road Maintaining
A large amount of waste vegetation is taken every year from the weeding and trimming of trees performed as a part of river, road and park maintaining services (Photos 1 and 2). At present, this waste is not fully utilized. However, our survey revealed that it contains a significantly large amount of energy. Our estimates, based on a questionnaire survey and sample analysis, indicates that the quantity of heat contained in the waste vegetation taken in one year from roads and rivers maintained by the national government alone is equivalent to the energy generated from approximately 85,000 ㎥ of heavy oil, i.e. enough oil to fill around thirty-four 50 m-long swimming pools (50 m × 25 m × 2 m).

3. What Energy is Actually Usable?
Whatever the fuel is, it is impossible to make use of all the energy that fuel contains. When burning fuel to get energy, there is thermal loss from the boiler, and large energy losses also occur when generating electricity. When a fuel contains water, thermal energy is expended in turning it into steam (this is known as the "latent heat"), and the energy we can use (the lower heating value) is the heating value inherently contained in the substance (the higher heating value) minus the latent heat. As a result, the percentage of water contained in the biomass has a great impact on its usefulness as fuel.

4. Investigating the Right Application for the Right Resource
Fig. 1 shows the moisture content of vegetation taken from road and river maintaining. In river maintenance, cut grass is often left for natural drying for a few days before being transported away, which reduces the moisture it contains. On occasion, however, the moisture content of some waste goes up to around 60% due to weather conditions or other factors. In road maintaining, as there is generally not sufficient space to temporarily store trimmed branches, it seems that they are transported away immediately after trimming, which results in them having high moisture content. It is therefore very important to keep such waste vegetation as dry as possible, by preventing it from becoming wet during storage, or by drying it under the sun or with waste heat prior to use etc. These active measures make a great difference to energy efficiency. For biomass with high moisture content, it may be more effective to recover energy through the use of chemical processes that do not involve burning, such as methane fermentation. However, as has been proved by combustion tests in a demonstration plant, it is efficient in terms of energy efficiency to use even undried biomass material as an auxiliary fuel when burning sewage sludge (using heavy oil as the main fuel), sewage sludge containing more water than the undried biomass material.
These experimental results reveal that differing appropriate applications can be found for vegetation biomass with different moisture contents, such as methane fermentation or use as auxiliary fuel.

(Contact: Recycling Research Team)

Survey on the Characteristics of Nutrient Release from Sediment in Brackish Lake

Fig. 1 Map showing location of Aburagafuchi

Photo-1: Nutrient release rate test

Fig. 2 Release of nitrogen and phosphorus is lower under 0% salinity aerobic conditions, reducing nutrient loads to Ise Bay

Aburagafuchi, located in the downstream area of the Ise Bay catchment basin, is a brackish lake (lake filled with a mixture of sea water and fresh water) measuring 0.64 ㎢ in area, 6.3 ㎞ in circumference and 3 m in average depth (Fig. 1). It is one of the eutrophic lakes in Japan due to progressive deterioration of water quality caused by urbanization in the catchment area.
For this research, the two following surveys were conducted in order to check the mass balance of nutrient in Aburagafuchi.

(1) Surface water sampled from the Osada and Hanba Rivers, which flow into Aburagafuchi, and from the Takahama River, a major river flowing out from the lake (Fig. 1), was analyzed for nitrogen and phosphorus concentration. The flow rates in these rivers observed.
(2) The sediment of Aburagafuchi was sampled using an acrylic pipe in order to clarify the relationship between nutrient release rate from the sediment and the bottom water quality. The nutrient release rate tests were conducted under aerobic conditions (conditions with saturated dissolved oxygen) and anaerobic conditions (conditions without dissolved oxygen) for each of the three conditions stated below (Photo-1). The water in each acrylic pipe was sampled once every few hours from the start of the experiment and analyzed for nitrogen and phosphorus concentration.
1. Bottom water in Aburagafuchi (1.6% salinity)
2. Mixture of surface water of Aburagafuchi and seawater (1:1) (1.5% salinity)
3. Surface water of Aburagafuchi (0% salinity)

The result of nutrient release rate tests indicate that the nitrogen release from sediment under condition 3 is 10% lower than that of under condition 2 (Fig.2). Phosphorus release from sediment under condition 3 is 30% lower than that of under condition 2. It means that nutrient loads of outflow from 0% salinity/aerobic condition lake is lower than those from 1.5% salinity/anaerobic condition. Nutrient concentrations in Aburagafuchi become lower by keeping bottom water fresh and aerobic conditions. Furthermore, the nutrient loads to Ise Bay, which located downstream of Aburagafuchi, can be reduced.

(Contact: Water Quality Research Team)