This section discusses the experimental equipment and methods used for the torrefaction and hydrothermal pretreatment of rice husks, which are a form of low-grade biomass.
The purpose of the experiment was to study the processing capacity and performance of the two pretreatment methods on rice husks and then find a treatment method that can be used for Myanmar rice husks. The experimental equipment is located in Japan, but due to transportation restrictions, rice husks produced in Myanmar could not be used. Understanding the impact of regional differences in production is a future issue. All the rice husks used for the experiments in this study were produced in Nagano Prefecture, Japan.
2.1. Experimental Equipment and Method of Torrefaction
Figure 3a–c and
Figure 4 show the superheated steam torrefaction tester that was used in the experiments and an overview of that tester. The equipment consists of two layers of heating zones and one layer of a cooling zone. The raw material is fed from the upper chute through the hopper, and is moved inside by a screw feeder. The equipment is heated by electric heaters at the bottom of each layer that can be set to approximately 300–600 °C and superheated steam injection systems, one of which is located in the upper layer and two are located in the middle layer. The central section of the upper and lower layers and the chutes that connect each layer have a thermocouple that measures the temperature. The exhaust gas that is generated in the equipment is cooled, and some of it is recovered as drainage that contains tar. The rest is released into the atmosphere after passing through a deodorizer.
Table 1 shows the specifications of rice husk torrefaction equipment.
The objective of this torrefaction experiment was to obtain a high heat retention ratio with the goal of improving transportation efficiency, so 250 °C was set, which has a high mass residual rate, as a target condition. For comparison, torrefaction experiments at 150 and 180 °C were also conducted.
Regarding the above-mentioned mass residual ratio (
) and heat retention ratio (
), they can be calculated as follows.
where
and
are the weight before and after preprocessing.
where
and
are the heat before and after preprocessing.
2.3. Evaluation of Transportation Cost Using a Resource Island Model
To evaluate the efficiency of transportation of products by torrefaction and hydrothermal treatment, the effect of reducing transportation costs was estimated by introducing the resource island model [
19], imagining the transportation of rice husks from the Ayeyarwady district in Myanmar, where there is a large surplus of rice husks, to a region where the supply of rice husks is low.
The evaluation of the transportation efficiency of biomass necessitates an estimation model for transportation costs. There are two main methods for collecting rice husks [
20]. If the transportation distance is short, the farmer can collect the agricultural residues and directly transport them to the power plant. If the transportation distance is long, the agricultural residues in a certain region are generally collected by a broker and then transported to the power plant. The general distribution of agricultural biomass resources is fairly dispersed, and the resource production regions are relatively isolated such that, overall, it could be considered as a resource island distribution.
Figure 7 shows an overview of the resource island model. Based on the resource island distribution model, the following were assumed regarding the rice husk collection process.
The production cycle unit for biomass resources is one year.
Biomass resources are evenly distributed in the same resource island, and differences in yield of the same crop due to different conditions are ignored.
The differences in parameters between the biomass of different types are also ignored.
The transportation cost of biomass mainly comprises collection within the island and transport outside the island. If a compression process is introduced, we envision a flow wherein the collection process is one in which the rice husks are collected at the central region of the island and then compressed, and from the central region of the island the products are transported to power plants and processing facilities.
Referring to
Figure 8,
is the distance between the central area of each resource island to the transportation destination, where there are power plants and processing facilities.
is the radius of the island.
is the biomass density.
is the quantity of biomass collected at each island. The biomass collection quantity
can be calculated as follows.
where
n is the number of resource islands in the collection area.
The purchasing cost
can be calculated as follows.
where
is the price of biomass.
The labor cost
can be calculated as follows.
where
is the labor cost of unit biomass.
From the above, the transportation cost within the resource island
can be calculated as follows:
where
β is the twist factor. The twist factor is to correct the distance calculation in the formula. Considering that the collection distance is not a straight line, a twist factor correction is introduced. In this experiment,
β was set to 1.5. Pt as the biomass transportation cost per unit distance.
On the other hand, the transportation cost from the resource island to the power plant
can be calculated as follows.
There are pretreatment and compression processes, so it is necessary to include equipment costs and operational costs in the calculation.
The total labor cost of pretreatment and compression processes
can be calculated as follows.
where
is the labor cost of unit biomass pretreatment and compression.
Q is the collected biomass.
The total costs of pretreatment and compression are considered as initial investment , operational cost and labor cost .
The total costs of pretreatment and compression cost
can be calculated as follows.
where
r is the productivity index, and it changes according to the treatment technology applied.
is the equipment depreciation cost,
is the maintenance cost adjusted according to the production index, and
is the maintenance cost without adjustments.
From the above, the total raw material cost for one resource island
C can be calculated as follows.
From the above formula, the major factors that affect cost are biomass purchasing cost , biomass transportation cost per unit distance , and transportation distance Li.
A resource island model was used based on the transport of rice husks from Myanmar’s Ayeyarwady region, which has an abundance of rice husks, to an area with a small supply of rice husks, and then we calculated the transportation cost reduction effects.
In terms of transportation, we calculated three parameters in the pretreatment process: mass residual ratio
, energy density improvement ratio
, and energy residual ratio
. Considering the entire energy balance, the income of the power plant is proportional to the energy residual ratio
. The biomass accumulated at each island that is corrected through pretreatment
can be calculated as follows.
The correction of the cost to transport from an island to the power plant
can be calculated as follows.
The simplified decrease ratio in transportation cost after pretreatment
is expressed by Formula (14).
Figure 8 shows the difference in raw material cost between when pretreatment exists and when direct combustion is applied. By improving the energy density of the raw materials through pretreatment, the transportation cost from collection to power generation is significantly reduced. The longer the transportation distance, the more it becomes necessary to perform pretreatment on agricultural biomass.