A feedstock preheater is the second most important part of an SCW gasifier system. The heat required to preheat the feedstock (water and biomass) is a significant fraction of the potential heating value of the product gas. Without efficient recovery of heat from the product gas, the external energy needed for gasification may exceed the energy produced, making the gasifier a net energy consumer. The feedstock should therefore obtain as much of its enthalpy as possible from the sensible heat of the product. This is one of the most important aspects of SCW plant design.

Figure 7.10 compares the capital costs of different components of an SCWG plant. We can see that the heat-recovery exchanger represents 50 to 60% of the total capital cost of the plant, which makes it a critical component.

Efficient heat exchange between the feed and the product is the primary goal of an SCWG heat-recovery system. However, for supercritical water intended for hazardous waste reduction (SCWO) or synthesis reaction (SCWS), it may not be all that important since the primary purpose of these systems is the production of chemicals, not energy as in a supercritical gasifier.

The heat-exchange efficiency, h, defines how much of the available heat in the product stream can be picked up by the feed stream.

h и

product-out product-in

n h — H V /

feed-in product-in

where H is the enthalpy, and the subscripts define the liquid it refers to.

Theoretically, the heat-exchange efficiency can be 100% if no heat of vapor­ization is required to heat the feed and an infinite heat-exchange surface area is available. Of course, these conditions are not possible. Figure 7.11 shows variations in heat-exchange efficiency with changes in tube surface area and water pressure.

The specific heat of water rises sharply close to its critical point and then drops equally sharply as the temperature increases (Figure 7.2). Thus, around

Amos Amos (PSA) FZK Matsumura Gen. Atomics

□ purification ed reactor № heat exchanger ■ feed preparation

20 [6] [7]

the critical point we may expect a modest temperature rise along the heat — exchanger length.

Thermal conductivity in SCW is lower than that in subcritical water because SCW’s intermolecular space is greater than that in liquid. A slight increase in conductivity is noticed as the fluid approaches the critical point. This increase is due to an increase in the agitation of molecules when the change from a liquidlike to a gaslike state (SCW) takes place. Above the critical point, thermal conductivity decreases rapidly with temperature.

The heat-transfer coefficient varies with temperature near its pseudo-critical value (see Section 7.2) because of variations in the thermophysical properties of water. As the temperature approaches the pseudo-critical value, conductivity and viscosity decrease but specific heat increases. The drop in viscosity and the peak of specific heat at the pseudo-critical temperature overcome the effect of decreased thermal conductivity so as to increase the overall heat-transfer rate.

As the temperature further increases, beyond the pseudo-critical point, the specific heat decreases sharply; the drop in thermal conductivity continues as well, and therefore the heat-transfer coefficient reduces. For a given heat flux, the wall temperature rises for the drop in heat-transfer coefficient. Generally, for high heat flux and low mass flux, the heat transfer deteriorates, leading to hot spots in the tube.