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The selected plants, the common reed (Phragmites australis), common cattail (Typha latifolia), and water hyacinth (Eichhornia crassipes), were collected from various water systems in late autumn. Sampling was in autumn with the objective of recovering absorbed nutrients before they were returned to the water body by die-back processes. At the same time, mowing of emergent plants such as reeds and cattails at the right time forces the growth of new shoots (Karunaratne et al., 2004). Harvested biomass samples were divided into shoots and roots (root were also collected about 30 cm below ground to research its metal content), and sun-dried. Each part was then cut more finely, oven-dried, and crushed to pass through a 2-mm-mesh sieve. The habitat soil and water were also sampled and the samples refrigerated until element analysis. Wheat and rice straw were selected as the conventional agricultural biomass for comparison with aquatic plant biomass. Wheat straw was harvested in Chongqing, China, and rice straw was sampled at a ricefield site near that at which the aquatic plants were harvested. These straw samples were prepared in the same way as the aquatic plant samples.

The selected coal sample, bituminous Foo-yong coal mined at Chongqing, China, was crushed mechanically and screened through a 2-mm-mesh sieve. Slaked lime (reagent-grade calcium hydroxide) was used as the sulfur fixation agent.

Test samples of coal-biomass briquettes (BBs) were prepared by mixing the pulverized coal and each type of biomass at the mass ratio of 3 : 1, adding slaked lime to the mixture at the molar ratio Ca/S=2, and compacting the mixture with a hydraulic press-type high-pressure jack (model J-15; Iuchi, now As One, Osaka, Japan) to small tablets (7×12 mm, about 1 g each), using a pressure of 4 t cm^-2.

Each biomass and coal sample was analyzed in accordance with JIS M 8812 (JIS, 1993) procedures for the proximate analysis, and by the Eschka method (JIS M 8813; JIS, 1994) for total sulfur determination. Calorific value was measured with an adiabatic bomb calorimeter (IKA C7000, Staufen, Germany). Carbon content was measured with a CHN recorder (Yanaco MT-3, Kyoto, Japan).

Prepared BB samples were subjected to pressure tests investigate whether this solid fuel had enough compressive strength to bear intracontinental transport in China. This experiment was done using an unconfined compression apparatus (S-56A MARUTO Testing Machine Company, Tokyo, Japan). Lignin contents of all biomass types were also measured in accordance with JIS P 8008 procedure (JIS, 1976).

To check that the sulfur dioxide emission-reduction capacity of the BBs was not altered when aquatic plants were used as the biomass instead of the conventional biomass sources, the desulfurization effect was investigated by a combustion experiment with test BB samples. The experimental setup is shown in Fig. 2. Synthetic air and nitrogen gas were used for the co-combustion gas and dilution gas, respectively. A twostep dilution was performed using acrylic buffer tanks with a capacity of about 1.3 L, and the dilution ratio was determined by monitoring the O₂ level with an infrared gas analyzer (CGT-7000, Shimadzu, Kyoto, Japan). The furnace temperature profile was maintained at 873 K. Emitted sulfur dioxide was monitored using the gas analyzer (SOA-7000, Shimadzu, Kyoto, Japan). The end point of combustion was judged by the return of a reduced CO₂ concentration peak, monitored with another infrared gas analyzer (CGT-7000). The desulfurization ratio was calculated by using equation (1):

Fig. 2. 
Combustion experimental apparatus for measurement of sulfur fixation ratio.

where DR (%) is the desulfurization ratio, S_coal (g/g) is the amount of emitted sulfur dioxide per unit weight of coal during the coal combustion process, and S_BB (g/g) is that emitted during the BB combustion process.

very important. We consider appropriate plants to be those (1) for which the maintenance of a stable supply for use as an energy resource is anticipated, (2) that contain much lignin, for BB strength (Gao et al., 2001b), and (3) that contain many plant nutrient salts (nitrogen, phosphorus, and potassium) so that the ash will act as a fertilizer when amended to soil.

For there to be a stable supply, an aquatic plant should be fast-growing. In a database of aquatic plants available for water purification, constructed by Fujita et al. (2001), the fastest growing species is common reed, followed by water hyacinth and common cattail. These plants are all perennial macrophytes that grow about 30 g-dw m^-2 day^-1, so a stable supply can be anticipated. Reeds and cattails are both poaceous emergent plants that were expected to contain a large quantity of lignin because of their fibrous characteristics. On the other hand, the floating water hyacinth (family Pontederiaceae) contains less lignin than reeds or cattails, but because it accumulates more nitrogen and phosphorus, it is often used for water purification. For these reasons, these three aquatic plant species were selected for this study.

Results of the proximate analysis of the coal and various biomass types are shown in Table 1. The fixed carbon value of Foo-yong coal was about 61% (air-dried basis). On a dry, mineral-matter-free basis, it corresponds to about 82%. This coal is classified as a low-volatility bituminous coal, as defined by the coal classification system proposed by American Society for Testing and Materials (ASTM, 1992). However, this coal is low grade because of its ash and sulfur content, 23.97% and 4.07%, respectively; on the basis of these values, it is categorized as a high-ash (total ash >15%) and high-sulfur coal (total sulfur >3%).

The chemical energy content, that is, the calorific value, of the biomass is one of the most important parameters in biomass selection. The relationship between carbon content and calorific value for the selected coal and the various types of biomass are shown in Fig. 3.

Fig. 3. 
Carbon content and calorific value of selected coal and aquatic biomasses.

All of the biomass types had a HHV of 15-20 MJ kg^-1 on a dry weight basis. The aquatic plants exhibited carbon contents and calorific values equal to or greater than those of wheat straw, an agricultural waste biomass. Among the aquatic plants, the emergent plants, common reed and common cattail, had a higher capacity than the floating plant, water hyacinth. The high calorific value of all selected hydrophytes indicates that those biomass sources are in no way inferior to the conventional biomass sources for energy use.

On the basis of the calorific values measured in this experiment, we estimated the greenhouse gas reduction effect that would result from the adoption of BBs for fuel. The use of a carbon-neutral biomass instead of coal would lead to a reduction in net CO₂ emissions corresponding to the reduction of coal mass. For example, to produce 1 GJ energy from BBs instead of coal, 8-10 kg of coal is replaced by 12 kg of biomass, which is an 18-22% reduction in the amount of coal used; thus, the use of BB fuel can reduce net CO₂ emissions effectively.

The compressive strength of BBs is known to depend on the binding effect, called the “lignin-induced strength effect,” derived from the softening of lignin materials in the biomass (Gao et al., 2001b).

When the lignin contents of the selected hydrophytes and wheat straw were compared with the compressive strength of the BBs prepared with each type of biomass (Fig. 4), the lignin content and the compressive strength of the BBs were found to be positively correlated. This result supports the suggestion that lignin content is a major factor controlling the strength of BBs prepared with plant biomass. Previously, Gao et al. (2001b) reported that the critical strength of BB solid fuel necessary to bear intracontinental transport in China is 40 kgf (≈392 N). BBs prepared with water hyacinth biomass were at about this threshold level, but the BBs prepared with the other two hydrophytes were well above this threshold. Although there are little differences among lignin content value for each biomass (RSD: 1-2%, n=4), as the lignin content increased, the variation in strength became larger, owing to differences in the fibrousness of the different biomass sources. These results show that emergent plants are suitable as a raw materials for preparation of BB fuel. However, water hyacinth is also suitable if used in combination with emergent plants having a high lignin content such as reed and cattail.

Fig. 4. 
Lignin content of selected biomasses and compressive strength of coal-biomass briquettes prepared using these biomasses; dotted line in this figure is threshold value of compressive strength which is bearable for vibration of intracontinental transportation.

The desulfurization ratios achieved during BB combustion using hydrophyte biomass were slightly lower than those achieved with conventional types of biomass, but the ratios were still 80% or higher (Fig. 5). This result demonstrates that the new BBs also have a high sulfur-fixation effect. In contrast to the compressive strength data, there was little repetitive variation (~1.5%). The relatively lower desulfurization ratio of BBs made with common reed was probably due to its being more fibrous than the other biomass types, which may have affected the accessibility of the desulfurization agent to the coal. When examined in detail, the difference in sulfur emission among the different biomass types is negligible, on the order of 0.01-0.001 wt%.

Fig. 5. 
Desulfurization ratio of BB prepared using rice straw and three aquatic biomasses (n=2, the error span marker on each bar means max-minimum range).