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Maximising the thermal substitution rate of alternative fuels in kiln and calciners: Tahir Abbas, Cinar (UK)

Filmed at Cemtech Asia 2015, 21-24 June, Grand Hyatt, Bangkok, Thailand

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Today, I'll just talk about what are the opportunities which we have for burning alternative fuels and what are the barriers and constraints. So we'll go bit more into the flames and how we burn these fules and what are the difficulties. So to start with the introduction, Cinar was incorporated in 1988, so we have been providing services to the combustile industries and cement aligned industries over 25 years.

Some of Cinar's clients among these multinational companies as well as smaller companies For Holcim alone, we have done almost 30 projects for the various kilns and calciners. Including Cilacap and Narogong plants in Indonesia and Thailand, we're working for SCG on their plant Thong Song to improve the thermal substitution and their calciner of biomass and Tyre Derived Fuel.

So, and also I just put this light to show we don't not just provide the advice and the solution to the question which was asked on Sunday, we have a Cinar associates, so we work with fives Pillard Group, Cementis from Switzerland and ALTCrros from States and also PM Technologies from Austria, and with the help of these partners, we're able to provide low-CapEx solutions on permitting issues, and then the business planning, and on burner design and on feeding and dozing system of alternative fuels. And also the upgrades I presented last year, some results on the upgrade projects which we did with PM Tech, so that's now coming back to alternative fuels and biomass. AFR has already mentioned about the waste content, water content of the waste, and the ash content, so these needs to be looked at before we look at the taking any advantages of burning biomass and alternative fuels. And then is the false air and the oxygen level, this could be controlled through the technologies, better technologies, handling systems, and reducing the air leakage, and to combustion and process optimization. So today, it will be more on the the false air reduction and the oxygen. If these are not controlled, then we have heat consumption and also the production losses, the clinker production.

So, that's data coming from Holcim, I will not go through the details. That's a plant, Holcim plant, Portland and U.S., and they were firing 20% of thermal substitution rates of the cheap dice and beyond that they had built up issues, so we worked with them and designed eventually, that's the tyre inlet pipe over there and that is the venturi over the distance of around three meters and that's the computational grid, I'll describe the model briefly later on. So basically, in the riser they had velocity of 28 m/s and that wasn't enough to revert the chips which were falling down two inches by two inches, and that was reduced to give a velocity of around 40m/s over the distance of around 3m. So that's the plant feedback. A refractory throat restriction was added and tyre chip thermal substitution rate was increased from 2 to 8 tons per hour and that is 65% of the calciner thermal input.

And when we look at the price difference, 30 tons, $30 per ton, so the plant was able to save around 1.3 million per year basis and these are the cost. So the study cost was very little and 7% was the implementation cost, and the payback was within a month, but plant implemented these recommendations within a year or so, so now they're able to go even higher around 10 times per hour.

So, simulation tool which we use at Mineral Interactive Computational Flow Dynamics, which is in house to help models which we develop at Emperial College, that's from where the CFD modelling started, and it has multi-fuel firing capabilities, so we can fire coal, natural gas, oil, and all these alternative fuels together, and combustion model is directly coupled with calcination and clinker formation chemistry, and we have applications of around 150 kilns and the calciners.

Also, validation in a Lafarge calciners, it's a four stage calciner with coal burner, fuel coal burner, tertiary coming in, outlet is at the opposite to the tertiary air coming in over here, and we have the outlet over there. And these are predictions and the data on top taken by plants, so we see we have higher oxygen on the left side, and we have lower oxygen as predicted by the model.

So now, AFR in the Kiln Burner. In the kiln burner, we have visible flame so there we can see a ball forming and that could be mineralogious or the kiln burner, or the calciner. So when we modeled the calciner, we found the fuel which was high in carbon wasn't burning, and that was causing these balls, so not the burner problem, but the problem in the calciner which transferred to the kiln. We'll go to the calciners later on, but for the calciner flames, we have invisible flames, so it's very difficult to monitor the flame, to see the flame, and we see just the bright regions because we have combustion taking place under vitiated air conditions, that's very little oxygen coming from the [xx], and we have also the calcination endothermic reaction taking place, so it's very difficult to see if these tyre-chips or fuel, coal is burning or not and where these particles are travelling. That's co-firing, bio-solids that's dried pulverized sewage sludge with 50 micron average size so that burns like a legnite coal, so if we have a good, pulverized fuel then we can burn it quite well, so that is the flame for 30% thermal substitution of this bio-solids. And, we also look at the burner momentum as the momentum of the air, and the fuel injected, also the calorific value, which is a quite standard these days for all the burner manufacturers to have between 7 to 12 Newtons per Megawatt. And that's when the 50 micron bio-solids thermal substitution rate was increased to 75%, we saw the particles were taking longer to burn at 6 N/M, but when the burner momentum was increased to for around 9 Newton per Megawatt, these particles started burning earlier on as compared to before. So that's sustainable, and it is easier if we have a nice biomass or base fuel with smaller particles size, we can go up to 75% thermal substitution rate in the kiln, but when we have, for example, 20% solid recovered fuel with 80% coal, 5/5 Millimeter or 7 Millimeter which we see [xx] motion there, so these SRF particles will just fall down over there, and as a result we have these CO emissions for coal only, 94 ppm and with 20% SRF, we have increased to around 2,000, over 2,000 ppm and that is the limiting factor, so it wasn't possible to increase the SRF in the kiln.

So now coming to the calciners alternative fuels in the Calciners, we can go even 10 times higher particle size with the same density in the calciners as compared to the kiln, and we also have a higher thermal substitution rate, around 60%, or even 100% in some calciners, and we also have several injection locations available. And higher firing rate in the calciner, so around 60% in the calciner as compared to the 40% in the kiln and potential to reduce kiln generated CO and NOx in the calciner where we've secondary fuels, so through re-burn we can reduce emissions of CO and NOx, also to reduce the instability due to the sulphur cycles and build-up issues, but there are also some issues in the calciners, and that's what I'd like to discuss today.

For example, the first problem is the flow stratification, here we're looking at these lines where we have these coal particles injected, so that is the oxygen between zero and 2.3%, so red is more, and blue color is less, and green is almost halfway. So we see all the oxygen is consumed through the release of this volatiles, and when the tertiary comes in these particle, coal particles are pushed to aside and then they take longer, so it's around 6 sec residence time. If we had availability of oxygen earlier on, then we could burn this much earlier and if these particles stay in oxygen region, then we have what is called thermal deactivation of the coal particles where pores of the open up char would be closed down due to the assembling of the heavier tyres.

So that is the oxygen which is available but there's no cross flow mixing mechanism there, and when we look at another configuration where we have tertiary staging, so we have sub-circumetric conditions over there, but when these coal particles travel in this region we've the tertiary coming in, they burn within few meters there. And also, the calciners, they have different sizes, residence time, or retention time, and also the shapes as the upgrades were done, coals and also with the RSP, so it's very difficult to see the aerodynamics, and also we have around 40-45% mass of the air coming through the tertiary air and another 50-55% from the riser duct, so mixing up these two streams in a short time.

So now, I present some of the case studies, Coal and Alternative Fuels in a Inline Calciner. So, that's the plant in UK and we have coal as a primary fuel, and then we've meat and bone meal as grease fuel and also the chip tyres, and aim was to increase the thermal substitution rate of thetyres-chips to around 75% and that's the goal and we look also at the Venturis area, that's a small area to revert the fall in chip-tyre with the upward flow. So, now we look at the primary fuel coal which is the main fuel, and it is important to have a high burnout, the coal, here we're looking at the coal and the oxygen rate is around 21% by volume and 23% by mass. So the burnout is around 82% and that's very low as we would like to burn the fuel in the suspension mode and not been carried over into this cyclones and then creating build-up issues there or coming down with hot meal. But when the coal burner from the side of the calciner was moved into the center axis of the tertiary air or the plant, so we saw the burnout increase from 82-99% and as a result, earlier on we had calcination level of 93% of the meal particles, and there we see the calcination level increase to 98%, which is on the higher side, but we can increase the productions rate of the clinker or reduce the fuel in the calciner. And that's the throat restriction and the chip-tyres of of various sizes. So we are looking at the six sizes below 35 and 35, 10%

and so forth and

75/45, and that's the 8%. And the velocities are in the narrow region around 50 M/S, velocities are quite high but only in a narrow region, but that is not enough to de-accelerate the falling tyre-chip and then accelerate them back after reaching this technician point, and so that is the opening. So now we are looking at the particle burnout or chip burnout of these tyres, so when we look at the smallest size, they're going quite well around 99% and as we go along 96%, 64%, and when we look at these trajectories, we do not see any change in this blue color to red color, so these heavier fraction or heavier size tyre-chip just fall down without burning, and they burn at the kiln back-end, which we do not model that goes out of the computational zone, but they will get mixed up with the hot meal and they burnout would be on the slower side, and that would be limiting factor for CO formation and so forth.

So, as the solution, we suggest to Venturis and just narrow reduction area throat restriction was removed and then two Venturis were modeled and that's giving the pressure drop, because also when we create a Venturi, we increase the pressure drops so the original pressure drop was around 700 pascals and when we designed the other Venturi, that was even less around 600 Pascals with 90% burn of these tyre-chips, and with higher pressure drop which was the limiting factor for the ID fan. Although the burnout increased, but 90% burnout of the tyre chip is good enough because some times these chips would fall not on the side ways, but just vertically, and it's very difficult to control that. So, 90% burnout is quite good enough and that's what plants selected.

So, another example where the burnout of coal was very, very low and the ducts modification of the tertiary air ducts modification was made in a two seconds residence time in a calciner. So that's the two tertiary air ducts, and we've the coal burners over there and these are the two tertiary ducts and then we've a band there and the outlet two second residence time over here.

So, tertiary air modification and that include the burnout in the original case, we had burnout of around 70% which is a very low for a two second residence time in the calciner, and the reason are being the tertiary air flow was forcing the flow down, and we have flow acceleration at the center. So we see the velocity rectors are higher at the center, so when these coal particles which were injected in the riser duct would have less fuel residence time. And that was changed, so the tertiary air inlet was changed to a more like a horse shoe tertiary air and then we had injection of the coal particles at the inlet of this tertiary air which was opened from the inside there. So the burnout increase of coal particles to around 99% and then was designed of the venturi for the thermal substitution rate of the Pro-fuel around 90% and that's the plant feedback coming, and that is before and after this modification so. Clinker production increase slightly and also the thermal substitution of Pro-fuel from 6.8-8.2 and the pre-heater temperature also reduced, and ID fan oxygen reduced, and CO, and also the fuel consumption in gigajoule per ton of clinker slightly reduced, and that was the quite improvement on the burning of alternative fuels as well as overall not loosing the production as ARF mentioned earlier on, and that's not very important, but the quality of pro-fuel was also quite good in terms of low moisture content and the pre-screening done by the plant. So, I'll just show some

results when they try to go to 90% of thermal substitution rate in their calciner. With the 40% here, we're looking at the oxygen and the pro-fuels particle trajectories for 40% pro-fuel, and the next one we're looking at meal particles. We're looking at meal practical in temperature here, so the purple color are the meal particles. We see that the meal particles travel the temperature would reduce accordingly, but when the pro-fuel feedrate was increased to 70% then we saw some of the heavier fraction falling down, and then when they are going up, they are taking another routes, not the previous routes, and that's where we see this area, where these particles are going back and burning and that is increasing temperatures there, so that's the area where we do not have any meal particles travelling, and that's where the temperatures are getting hotter and as that's the refactory issues and the build-up. So for the 90% pro-fuel, it became even worse. So as a solution, the meal had to be splited into two half, one injected at the lower position over there, so now we see the second meal particles are going from there, 50/50% split and the earlier are still at the same location. So we got rid of this high temperature regions which are there.

So, also I'll just briefly talk about the other technologies of our bigger size AFR which can be acquired. So these are the Off the Shelf Approach which may or may not work for particular type of a calciner. So that is FLS HOTDISC which we modeled for clients, and that is the HOTDISC on the left, and we are looking at the temperatures, so we see very high temperatures over there in that area.

Yes, [xx] and we also see the some of these lighter fraction, they were burning there. So we see the red region on the figure on our left around 1,350 Celsius, and that's the area where we have a bend in the calciner and plant was observing deposits there, and the maximum thermal substitution rate with the HOTDISC, although the certain plants, they are around 80% thermal substitution.

But, in this case was limited to 35%. That's Skewers AFS from U.S. and that's burning whole tyres, and the residence time or retention time is around 18 minutes, very high residence time, and the burnout of this whole tyre is around 94%. And the thermal substitution is around 21%, so it's worth going for this installation although we can get around 10-15% by injecting the whole tyres and the kiln back-end. And the reason for that is, we have fuel rich conditioned or staged tertiary air. We have three tertiary air next at the bottom there, and one at the top. So we have the [xx] conditions over there, so that is the wood chips, and the wood chips don't burnout either, and the burnout is lower because we do not have enough air over in this region. The rest of the air is coming there, so most of the tyre chips which do not burn is these are which fall down at the kiln back-end. And that's the compact calciner which we have worked together with PM Technologies, and that is fitted with the tertiary and it can be attached to the riser duct as well, and the principle is to have the cyclonic motion of the particle so that is the coal particles in black and Solid Recovered Fuel particles in purple color. So we see the bigger particles take longer and the smaller particles burn earlier on. So, with the modelling and the initial results are, for the coal particles we've retention time of around .7-1.5 Seconds, which is for typical coal particles. If that residence time is going to increase then we'll have problem with the CO formation for Solid Recovered Fuel 4 mm by 4 mm, we have around 2 second for 99% burnout and for Solid Recovered Fuel, 24 mm by 24 mm, we have around 12 seconds residence

time, and that's quite good for having lower CO and NOx emissions of both of those. Yes, I'll just summarize. On a summary, for higher thermal substitution rate of alternative fuels, one has to look at the, first of all as said earlier, variations in the AFR quality and also the feeding rate fluctuations. It has to be more uniform and continuous feeding for alternative fuels rather than batch feeding, otherwise we'll have VOCs and CO and fluctuations. And then comes the flow startification issues which are both in the kiln and calciner, not so much on the kiln side because there we have external and internal stratification zone and we can control through the main burner. But in calciners that is the main issue and use of venturi for bigger size because if there's no restriction venturi for the area of around 2-3 meters, we will not have higher velocities, and the bigger size chips will fall down, and then create a deposit build-up at the kiln back-end, and we'll have pressure drop anyway. And when we're firing these heavier fractions of alternative fuels, the terminal profile is going to change. So we need to look at the meal split as well to curb the hot-spots, and all these parameters are highly non-linear and competing mechanisms, so for example, radiation to the temperature powerful and turbulence, so we need to use some sort of mathematical modelling approach than taking the head-on or hands-on approach for the installation and see.

So, that's quite inexpensive. Once the model is there, then it's there forever and we can use for other variations if required in future. Thank you, that's all I can say.

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