The taper in the stack performs two functions:
- A tapered section is a lot more structurally resistant to wind loads
This seems counter intuitive at first - after all, a tapered cantilever beam with a fixed load at the end deflects more than a straight beam. However, there is less surface area at the top of the stack for winds to push. Since, in the US, ASCE 07-10 and ASME STS-1 utilize a wind gradient, the loads at the top of the stack are significantly larger than the loads at the base. Due to reduction in surface area, this translates to less loading for the base to counteract, and overall reduces the stress.
Typical stack design of this construction is to size the top for optimal velocity, and make that the wind design. Then factor in several methods for making the stack - a linear reduction is a simple fabrication, and tapering down thickness between sections (since you don't need to resist as much loading near the top as the bottom) makes for material cost reduction that are well worth the engineering effort for the manufacturer.
By reducing the cross sectional area of the flow area at the outlet, additional velocity will increase the plume height and result in better dispersion, which avoids downwash effects.
There are additional thermodynamic implications - for example preventing sulfuric acid condensation. These are likely more a function of whatever acid brick you will use to line the steel stack and the thermal reduction (I should mention that certain types of phenolic resin coating can act as a great acid brick - it's worth investigating).