Any real application of pipeline transport of biomass from a field location (as opposed to mill residue) will normally require an initial truck haul to get the biomass to the pipeline inlet. This means that the fixed costs associated with both truck and pipeline transport are incurred. Thus, e. g., truck hauling of 2 million dry t/yr of biomass to a pipeline inlet at an average haul distance of 35 km (1), as might occur in a whole-forest harvest operation, with further transport of biomass by one — or two-way pipeline would have cost curves as shown in Fig. 3. The alternative of transport by truck alone is shown by the dashed line in Fig. 3.

Since by inspection of Fig. 1 all pipelines with a capacity of <0.5 million dry t/yr (one-way) or 1.25 million dry t/yr (two-way) have a higher incre­mental cost (slope) per kilometer than the alternative of hauling by truck, it is clear that pipelines below this capacity cannot compete with the alter­native of leaving the biomass on the truck for the extra distance. In the example illustrated in Fig. 3, at 2 million dry t/yr the minimum pipeline distance to recover the fixed costs of the pipeline as compared to truck haul are 75 km for a one-way pipeline (in addition to the initial 35-km truck haul to the pipeline inlet), and 470 km for a two-way pipeline (again in addition to the initial truck haul); pipeline distances shorter than this are less eco­nomic than continued hauling by truck. Hence, pipelining of truck-deliv­ered biomass at a concentration of 30% is only feasible at both large capacity and medium to long distances.

Immersion time (hours)

Fig. 4. Carrier fluid content of biomass after different hours of immersion in carrier fluid.

Absorption of Carrier Fluid by Biomass

We performed a series of simple experiments to explore the uptake of carrier fluid by biomass. Fresh wood chips, both hardwood (aspen) and softwood (spruce), were kept sealed and cool until immersion in room temperature water or oil; they were drained and dried to determine mois­ture level. Water drainage was brief, about 1 min., although one test of a longer drainage period showed a negligible impact of longer drainage times. The oil used in our study is a heavy gas oil fraction from Syncrude Canada, with a nominal boiling range of approx 325—550°C and a viscos­ity of 1.3 Pa s at 20°C. This type of oil is typical of an industrial-grade furnace oil. Wood chips were drained of oil for 1 hr before weighing. Figure 4 shows the carrier fluid content of biomass after exposure to car­rier fluid for varying periods of time. Note that immersion time can be related to pipeline distance because at a typical slurry velocity of 1.5 m/s, the slurry would travel 5.4 km/h.

The choice of an oil carrier requires a tradeoff between the viscosity of the carrier, which drops with lower boiling range of the oil fraction, and the value of the carrier, which increases with lower boiling range. At one extreme, a diesel fraction would have low viscosity but has such a high value as a transportation fuel that its use as a thermal fuel would be cost prohibitive. At the other extreme, a residuum fraction would have low value but such a high viscosity that transport of the slurry would likely be prohibitive in operating (pumping) cost. In the present study, we have arbitrarily selected a heavy gas oil as the balance between these competing considerations.

During water immersion, 1 kg of mixed spruce and aspen wood chips at an average 50% water content would pick up an additional 0.51 kg of water and reach a terminal moisture level of about 67%. Water uptake is quick; even after immersion for 3 h moisture levels exceed 63%. This is similar to the findings of Brebner (4) and Wasp et al. (6), who reported saturated wood values of 65%. We conducted two experiments with straw and found that moisture level rose from 14% as received to >80% after exposure of 3 h. This is similar to the findings of Jenkins et al. (9) for rice straw from California.

Absorption of water has serious implications for any process such as direct combustion that converts absorbed liquid water in the fuel into emit­ted water vapor in the flue gas, in that it reduces the lower heating value (LHV) of the biomass and requires more biomass per unit of heat released by combustion, an effect also noted by Yoshida et al. (10). Figure 5 shows the loss in LHV and the corresponding increase in biomass that must be delivered to a direct combustion-based biomass operation at 67% mois­ture level. Werther et al. (11) note some other problems with increasing moisture in the direct combustion of biomass: reduced combustion tem­perature, delayed release of volatiles, poor ignition, and higher volumes of flue gas. These secondary impacts on efficiency and operability of a direct combustion unit are not considered in Fig. 5.

One can conceptually break down biomass utilization into three com­ponent cost categories: (1) field harvest of biomass, (2) transportation from the field to the biomass processing site, (3) cost of processing/conversion. For direct combustion of truck-transported biomass from harvesting of the whole forest in western Canada at or near optimum scale, the percentage and cost per megawatt-hour are as follows: category 1: 33.4%, 15.77$/MWh; category 2: 14.3%, 6.74$/MWh; and category 3: 52.3%, 24.65$/MWh (1).

Since, as shown in Fig. 5, changing the moisture level of wood chips from 50% to 67% increases the requirement for field biomass in direct combus­tion by 78% for a given output of heat and power, it is evident that water — based pipelining of wood chips cannot be economical for direct combustion, because the increase in field harvest cost associated with the higher biom­ass requirement is larger than any possible transportation cost saving. For straw, so much water is taken up that the LHV is effectively zero; pipeline transport of straw to a direct combustion application would destroy the heating value of the fuel.

This impact is not true for a fuel process such as supercritical water gasification of biomass (12—14) that does not produce water vapor from absorbed water, since the higher heating value (HHV) value of the biomass is effectively realized by countercurrent exchange of heat between prod­ucts and feed that results in condensation of produced water. The impact of absorbed water is also not an issue for fermentation of biomass, since this is a water-based process. Pipelining of biomass to fermentation processes offers the promise of larger-scale, more economic processing of ethanol, chemicals, and byproducts such as lignin. However, the pipeline design would require more detailed assessment since saccharification in the pipe­line would be a logical processing alternative, and this would require tem­perature control during pipeline transport. This more detailed assessment is the subject of future study.

During oil immersion for 48 h, 1 kg of mixed conifer and aspen wood chips at an average 50% water content would pick up an additional 0.45 kg of oil and reach an oil level of 31%. Comparable figures for 124 hours are an uptake of 0.52 kg to reach a oil level of 34%. Direct combusting wood chips delivered in a heavy gas oil can be thought of as cofiring a mix of about two/ thirds oil and one/third wood on a thermal basis. Pipeline cost would increase because of additional pumping; the increase would depend on the viscosity of the oil fraction that was selected as the transport carrier fluid.

Discussion

Pipeline transport of oil and natural gas is clearly far more economical than truck transport, even in relatively small pipelines. Three factors com­bine to make the transport of energy in the form of biomass far less economic:

1. The density of energy in the pipeline is far lower for biomass than for oil. The present work is based on 30% biomass by volume in a carrier liquid. Wasp et al. (6) based their work on 22% biomass. Brebner (4) and Elliott (5) indicated that at about 47% concentration by volume a slurry of wood chips and water cannot flow. Given the low heat content of wood per unit volume relative to oil and the low concen­tration of wood chips in water, the energy density in a 30% wood chip slurry is about 8% compared to oil, even based on HHV, and hence far larger pipelines are required to transport the same amount of energy.

2. The pressure drop in the pipeline is high for suspended solids in a carrier fluid. For example, Wasp et al. (6) indicate that at 30% con­centration of wood and a velocity of 1.4 m/s, a wood chip slurry in a 214-mm-diameter pipeline has a pressure drop that is three times larger than for water alone.

3. Recycle of the carrier fluid will often be required in biomass trans­port by pipeline, both because large quantities of water will not be available at the inlet end and because discharge of water that has carried the biomass will, in some jurisdictions, be prohibited. This requires that a second pipeline and set of pumping stations be con­structed.

In addition to these cost elements, transport of biomass for a direct combustion application by water creates a prohibitive drop in the LHV of the fuel because of absorbed water. These issues limit the application of pipeline transport of biomass to large applications that use oil as a carrier medium, or that supply a process for which the heat content of the fuel is not degraded by the requirement to remove absorbed water as vapor, such as a supercritical water gasification process.

Transport of wood chips by oil precludes firing a high percentage of biomass owing to high oil uptake by wood chips. We consider it unlikely that a two-thirds oil and one-third wood fuel mixture would have high interest today as a power plant fuel, since even a heavy gas oil fraction has too high a value as a transportation fuel precursor to be diverted into power generation.