Hello Ashley, yes upper system bridging is especially a problem with stationary small engine systems.
My own system is small VictoryGasWorks shop 2009-2012 test mule prototype all SS unit.
Was developed around a 6500 watt air-cooled 3600 RPM electrical generator. Up to small four cylinder inline engines. Mine does not have a moisture reducing hopper. Single walled SS. And we did use a lot of conifer woods.
There is the heart of the problems. Too small of engine to make the heats for the tar-ring/tar- cap rising heats melting that the larger engines driving systems can use.
And using a really tarry wood in conifer woods. Even kiln cook/dry confer woods have the wood-sugars still in them that will convert and cook out.
Putting the input wood into the hopper truly forced dry and warmed helps a lot. That means truly dry at 10% or less moisture. Warmed means just out of an engine heats forced air drying. Not bagged wood stored winter outside even under cover that will air re-hydrate back up to 20-25% moisture. Maritine climates do this. The U.K. is certainly maritime climate.
Engine rod shaking works aright. But hard to turn on, and then turn off when not needed . . .then causing more problems in compacting than bridging solving.
My hopper still had an electric motor eccentric weight mounting bracket. Works for command and control. But another electrical system to set up and maintain. And these shaking all eventually metals work harden and crack something. Loosen flange bolts. Vibration work flange seals to leaking. Adaped over motors eat their bearings. Purpose commercial vibrators are expensive.
What also helps is to limit the pyrolysis area volume exposed to core rising heat.
Almost all systems work quite well with small 1/2-3/4 of an hour batch loadings. Even with wettish woods and conifer woods too.
So why the evolved from the side feeders like the GEK systems, the Drizzler systems.
Jonas Hedenberg’s 7 years evolved side auger fed in chipped fuel systems is this too.
Look him up on YouTube by his name and woodgas.
Because he has evolved a neat pyrolysis zone windshield wiper gearbox and motor tampering down systems.
His evolved stationary CHP system was highlight video interviewed by the Omstalliningaresan woodgas promoter fellow here:
Even though not CC enable to be able to translate; you can see Jonas hands describing his tampering system at 7:28 → 8:05.
Jonas’s own videos show this system in action. The Swedish translator guys say he is not timing actuating but needs sensing somehow.
Just be aware Jonas is woodgas fueling four cylinder water cooled Volvo engine as his smallest.
4X the gasifier core heats to work with the versus you and I with our much smaller single cylinder engines.
And this is why BenP’s book system is putting produced gas heat right back into the pyrolysis zone where his experiences have shown you best benefit to prevent tars made gluing bridging.
So lots of approaches possible.
Heat management. Encompassing fuel wood just-in-time dying and warming.
Mechanical. In that is wood bit sizing. Dutch John found that roughly your fuel bits sizing had to be ~1/7 of your downdraft internal diameter. Sheee. Small engine. Small gasifier. Small wood bits making. Most guys refuse this part of it.
Another way will be said is to just go with a divorced process and just go with a wood charcoal system. Actually, valid with a few IF’s.
IF you have wood that makes a good sound handle-able charcoal that can be handled, sieved, crunch sized, sieved, then bagged and stored. Conifers are very poor at this.
And IF your local lets you make the wood charcoal.