During March, Guy Blanch built a very cute micro-boiler to burn the biogas. This has major advantages over the old gas ring we were previously using: it heats hot water which can then be stored and piped to where it’s needed (i.e. the heat exchanger in the digester, to the water bath to pre-heat the water bath for the pasturiser and eventually come winter through heating pipes to keep crops frost free). The most immediate benefit is that it frees up Yassen’s time – he doesn’t have to keep watch over the open burning ring.
Guy built the boiler using a discarded propane gas cylinder as the body of the boiler.
The gas cylinder was cut in half and the bottom inverted and re-attached (welded) to the top to form a water jacket around the top of the boiler which would catch the heat. An outlet pipe was attached to the water jacket higher than the inlet pipe. The base ring had tabs cut in it to hold a circle of thick steel plate which would act as a heating plate for a kettle or other direct heat function (wonder if it would do toast?). Three feet were made and welded to the bottom – the underneath is open. The outside is painted with matt black stove paint which can withstand temperatures up to 600ºC.
Access to the inside of the boiler is through a little access door which has a simple magnet latch. The Bunsen burner is mounted on a cross brace across the open bottom of the boiler. Attached to the burner is the thermocouple of a Flame Failure Device (FFD) from a domestic cooker. This monitors flame temperature so that if, for any reason the flame goes out, the gas is cut off automatically by the FFD. To override this when lighting the burner there is a switch (under the door to the left on the first picture) which is depressed to allow a gas flow. Above the Bunsen burner on the inside of the boiler is a heat deflector plate – this diverts some of the heat from going straight up the vent to the underside of the hotplate and instead conducts it to the water jacket. There was a fair amount of trail and error with different components before this final arrangement.
The boiler arrived on Cyrenians site with Guy on Wednesday 20th April. Cath had been there the day before to make final arrangements with Yassen (Phil was on a rare holiday) and to source & buy the required components. Having below ground tanks wasn’t ideal for the Cyrenians horticultural team so it was decided that a normal domestic hot water tank would be the container instead – this also has the advantage of being accessible and easily understood for the trainees on the training day. A frame was built using perforated corner angle iron bolted together – we had to buy a bit more for cross-bracing which wasn’t envisaged in the first instance (it’s more difficult to square up & make rigid than you might imagine).
If we could have sourced a double coiled cylinder at a reasonable price in time we would have done, then we would have had separate out puts for the digester heater and the pasturiser pre-heating/polytunnels heater pipes. We might still exchange the single coil hot water cylinder for a double in the future. Because we were unsure of the boilers output we thought it would be safer to use the outer tank of the hot-water cylinder to pipe the boiler hot water circuit through to avoid it getting so hot it boiled the water in the coil and caused a steam explosion. This also has the potential advantage of providing a more even temperature within the cylinder coil for draw off by the digester.
As you can see from the pictures the frame holds the cylinder and the header tanks in place above the boiler. There is valve on the inlet to the boiler so that it can be disconnected without draining the tank. The hot water riser pipe connects with a t-piece from the header tank. This riser pipe will need to be well insulated when the plumbing is finished. The inlet and outlets for the cylinder coil behind the riser pipe are currently unconnected. Yassen had put insulation around the gas pipes during the cold winter to prevent any build up of frozen condensate creating blockages.
We used steel and copper fittings and copper pipe for this but have also bought plastic pipe and fittings for connecting up the digester and pasturiser to the cylinder.
Also have had problems with the pasturiser which has had some reconfiguration by Yassen. The heating element worked well for a while until the screw ring (gland) and element housing broke. The replacement which we put in at the same time as the boiler system was not powerful enough for the job being only 400 W. He reported that it only raised the temperature to 60ºC (needs to be 70ºC for one hour). So we've got a new one on order.