Adding a proportional control valve to an LP gas valve train

I'm building a 200-L fully automated, gas-fired pilot brewery. Here's the problem: I need to be able to remotely modulate the flame from a ~50,000-Btu/hr impinged jet gas burner via a potentiometer mounted on the control panel. The only valves I've been able to find that meet my criteria are the Asco 8202/3 Proportional solenoid valves, but they don't appear to be compataible with combustable gas; and the Asco AH8D series actuators/V710 body, which seem to be too big.

Basically, I need to be able to throttle down the burner as the temperature in the kettle approaches 100*C, because of the instability of the wort as it comes to a boil. (Wort is what beer is called prior to fermentation).

Wort coming to an initial boil has a nasty tendency to violently boil over the top of the kettle if the firing rate is too high at boiling point, until the proteins in the wort have had time to coagulate, and then it settles down for the remainder of the 1-hr boil.

Does anyone have any ideas on how to safely and reliably perform this task, or better yet, a good book? Ultimately, I have to thoroughly understand this system because I am the one who is going to have to build and maintain it.
Check Belimo for an actuator.  they have some small ones... you may have to make a small bracket to hold the actuator from rotating and you can use a coupler/set screw to extend the shaft of the butterball valve if needed.  Powerflame uses this setup on their X4 modulating burner, but with a honeywell actuator.  The belimo is smaller though.  I will look tomorrow and let you know which one I used before.

You can put this valve after everything in the gas train.

At 50MBH, I would use the 1/2" valve.  You aren't going to have "great" control, but the Belimo actuators have "stops" you can use to limit the stroke.  That shouldn't be an issue.  You probably end up around 50-70% of full rate???  Is that your target?

Hopefully you have access to a combustion analyzer so you can check the CO.

Also, I don't have a problem helping you.  If you are making beer... that is a good thing.
I assume you are just putting this burner under the boiling pot,
right?  If this is the case, it would be difficult to get a proper CO
measurement.  Definitely have some sort of CO alarm in the area
though.  Most of them will sound off at 200ppm or so.  That standard
changed within the last few years and I don't remember the exact point
at which they sound off.

Will you be using natural gas or
LP?  Regardless, the burner appears to operate at 3.5"WC on Natural and
11" on LP.  These manifold pressure will yeild full input of the
burner.  Will you have a gas flow meter in line that you can clock?

Turndown ratios are listed on the Solarflo website.  Don't exceed those and you should be ok.  Rate can be calculated based on manifold pressure.  for example...

SQRT(p2/p1)* q1 = q2

Say
you reduced the manifold pressure with your valve by half, so... the
square root of .5 is .707, so .707 times 50MBH gets you down to
35.36MBH.  Make sense?  Maybe you already know how to do this, so no
offence if I am stating the obvious to you.  This is a basic calculation
and will work for what you are doing.

From the originalCheck Valve
.