BIOMASS FOR HEAT PRODUCTION IN RURAL AREAS

Martin Polák, Pavel Neuberger, Vladimír Šleger
Department of Mechanics and Engineering, Czech University of Life Science, Prague, Czech Republic

ABSTRACT

Substitution of fossil fuel with biomass as an alternative is very extending, especially in last years. Besides wood, which is mostly used in small boilers, there are some non-traditional biofuels – energy plants and agricultural by-products. This research is focused first of all on biomass combustion in a local low-power heating systems with output up to 100 kW, which can be taken into consideration for this purpose. Detailed combustion tests with different types of biofuels and burners were done with aim to evaluate both – fuels and boilers. Emission parameters of each fuel obtained during the experiment and many recommendations for boiler designers are mentioned as outcome of this work.

Key words: biomass combustion, emission, boiler.

INTRODUCTION

The most extended way how to use biomass is direct combustion. Technology is well proven in field of big devices (from 500 kW). But situation with small devices (up to 100 kW) is not very satisfactory. Using of boilers which are primary designed for another solid fuel, e.g. coal, etc. is very common. It is necessary to point out, that is not possible to get any way satisfactory results with those boilers. For next research it has important consequence: there is no universal device which can burn out all fuels.

Situation can be solved in two ways:

• use suitable fuel,

• use suitable combustion device or its modification.

As for fuel sources, there is relatively wide range of choice. Besides there are some kinds of agricultural and forestry waste, a special attention could be paid to so called “energy plants”.

MATERIALS AND METHODS

Biomass suitable for energy generation and their specifics
From point of view of yield per acre and plantation expenses perennial plants are more interesting for energy purposes than annual ones. It is not necessary to do cultivation every year and good flourishing vegetation needs only occasional care and harvesting. For harvesting many kinds of plantations it is possible to use mechanization intended for another farm plant. Besides, for energy purposes, some residues from crop plantation can be counted too. For example: cereal or oilseeds straw, fibre plant straw, waste from corn purifying, etc.

For effective fytomass combustion it is necessary to fulfil all specific demands:

• higher content of volatile combustible, which do not burnt on grate, but in space above it;

• higher content of particles, which causes decreasing ash melting point, esp. fytomass;

• lower heat of combustion and consequently lower heat value of fuel;

• possible content of chlorine, esp. at fytomass;

• higher content of nitrogen in fuel;

• higher water content (in some cases);

• possible higher content of heavy metals.

All above mentioned properties mean special demands on burner and whole device:

1. To ensure air intake to all places where there is oxidable combustible.

2. To ensure correct temperature for combustible oxidation. For most of fuels it is temperature in range of 850-1000°C.

3. To ensure enough time to complete all oxidation reactions for flue gases in combustor.

4. To ensure ash removing, especially in fuels susceptible to ash sintering (fytomass).

Regarding above mentioned reasons it is usually necessary to design combustion chamber larger – especially space for volatile combustible burning. This space could not be designed as a heat exchanger, but as a heat keeping space where flue gases are not cooled under required temperature. Besides it is need to exactly locate air intakes, particularly for volatile combustible burning. On that account some constructions are designed with separated fuel gasification and subsequently gas burning.

Experimental combustion
An experimental combustion of fuels and its blends were done with 4 different boilers in 8 modifications, which are indicated A ... H. An optimal setting of boiler parameters was found before each measurement.

All used boilers and their combustion systems with modifications are described below:

VARIMATIK VM 45 is primary designed for brown coal. The fuel slides from a fuel reservoir and it is brought in cycles into the combustion space and burnt on the upper part of rotating cylinder grate. The combustion air is drawn by the help of a fan under the grate. Regulation of fuel supplying cycle (duration of grate rotation and delay) is possible (see Fig. 1).

Modifications are following:
A .... only primary air (I.) into combustion chamber
B .... primary (I.) and secondary (II.) air
C .... primary (I.), secondary (II.) and tertiary air

VERNER V 25 is constructed as a double-chamber boiler for wood chunks and based on the principle of gasification. The fuel is dried and gasified in the upper chamber with the access of primary air. Generated gas goes through a nozzle, where it is mixed with secondary air, into the lower chamber where it is burnt under stabilising ceramic slab. It is possible to control the distribution of an air flow and its division into primary / secondary (see Fig. 2).

Modifications are following:
D .... only secondary air, without primary (predrying) air into upper chamber
E ... secondary and primary air into upper chamber

VIADRUS U 22 is designed as a cast-iron boiler shell which consists of combustion chamber and heat exchanger. Combustion chamber is equipped with bottom fed burner and stabilising ceramic slab above the burner. Fuel is supplied from reservoir to the cast-iron burner through screw conveyor. Primary and secondary combustion air is forced into the body of burner by the fan. Flue gases flow along the plate heat exchanger into the chimney. The regulation of fuel supplying cycle is possible (see Fig. 3).

This device is in following text indicated as F.

VERNER A25 is heat water boiler for pellets which are supplied with screw conveyer through back side of combustion chamber. Plate bottom of chamber is equipped with saw-like grate bar which removes ash in defined cycles. Side and upper walls are covered with ceramic slabs. Air is forced by fan under the grate as a primary and through side walls as secondary (see Fig. 4).

Modifications are following:
G ... original boiler, without any modification
H ... modified boiler, enlarged combustion chamber and redistribution of air inlets

Method of measurement
40 different fuels consisting of 13 primary kinds were tested during the experiment: wood (pressed saw dust), Phalaroides arundinacea, Canabis sativa, Pleuropterus, pine-bark, Miscanthus sinensis, cereal straw, cereal cleaning waste, rapeseed straw, sorrel (Rumex tianshanicus × Rumex patientia), populus, Medicago sativa, brown coal.

Testing equipment
Substantial emission and power parameters were evaluated by TESTO 33 analysis box. Measuring range: CO 0-20000 ppm, CO₂ 0-20%, O₂ 0-21%, Thermometer – thermocouple Ni-CrNi 0-500°C. Continual record was obtained with TESTO 350XL analysis box. Heat power output can be determined from water flow and temperature in the output and input of the boilers.

For A, B, C, D and E modification was used apparatus for one-shot measurement and presented results are arithmetic mean of obtained dates at steady state condition. For modification F, G and H continual records were already done. Resultant value is median with fractile characteristic of gained dates. Dates were processed by means of statistic software R 2.0.1.

All presented results are recounted for 11% O₂ content in flue gases.

RESULTS AND DISCUSSION

On the basis of collected data it is possible to obtain an overview of fuel and emission characteristics of 40 biofuels composed from 13 basic components combusted in boilers with heat output up to 100 kW.

From all tested fuels, wood material comes out the best in all aspects. There are no problems with wood in the case of carbon monoxides (CO) emissions. But in some cases it is necessary to be careful when concerning nitrogen oxides (NO_x) emissions (see Table 1). As the elementary analyses show, the wood material itself contains only a very little amount of nitrogen. The problem is thus in atmospheric nitrogen, which participate in the combustion. In cases of higher values of NO_x, higher temperatures of flue gases were measured. Higher combustion temperature, that causes nitrogen oxidation, can be assessed from this (when taking into account the influence of heat output in exchanger). Thus the solution is to control the appropriate combustion temperature. Next possibility of lowering NO_x concentration is recirculation of part of the flue gases back into the combustion process. From other characteristic of wood, high value of heat of combustion is worth mentioning (best of all tested samples), high content of volatile combustible (also the highest) and on the other hand the lowest content of ash, chlorine and already mentioned nitrogen. Also the combustion stability and the melting point are on a very good level.

The situation with other fuels is similar in many aspects. Satisfactory values of CO concentration is to possible to gain partly by choosing appropriate combustion device and its setting and partly by mixing with other types of fuel. The combination of both has shown as very useful in the case of sorrel. It is very hard to combust only sorrel and its testing proves unsatisfactory results. But if we combust blend of sorrel and canary grass 1:1 in the same device, the results are somewhat better. The best results can be reached if we burn this blend in more suitable device (modification G in our case). By this way it is even possible to fulfil the emission limits. However, if we burn only pure sorrel in modification G, the emissions are very dissatisfactory again. Another possibility how to burn sorrel effectively is to combine it with brown coal. But according to increasing concentration of NO_x and SO₂ with higher proportion of coal it is necessary to consider this possibility. On the basis of received results, fuel containing 10 to 20% of coal can be recommended.

Good results were reached with combustion of hemp, waste from corn cleaning, blend of miscanthus and bark 1 : 1 and blends containing poplar wood. The CO values were satisfactory in all those cases but the values of NO_x overlimited. Here it would be probably suitable to follow the path of combustion device modification (i.e. H). On the other hand, dissatisfactory results were reached with: rape seed straw, lucerne and most blends containing more than a half of sorrel.

On the basis of fuel analyses it is possible to say that heat of combustion of every sample is in the range of 16-19 MJ·kg^-1 and does not differ significantly one from another. Heating value of biofuels will be thus determined mainly by content of water in the fuel. The content of volatile combustible falls into a range of 60 to 75 %. An exception are blends with higher coal proportion, which have low portion of volatile and wood on the other hand, that contains high amount of volatile combustible. Content of chlorine in fuels can be a problem. Chlorine passes over to flue gasses during the combustion. When concerned “only” hydrogen chloride with its negative effects being mainly corrosion of device, the problem would not be so serious. Far more serious are chlorine hydrocarbons, which will be most probably present in flue gasses from burning such fuels. Next item monitored at the samples was so called fuel nitrogen. It is one of the causes of NO_x presence in flue gasses, especially at fytomass that contains higher amount of nitrogen. According to analyses, the biggest amount of fuel nitrogen is present at waste of corn cleaning, lucerne and one of the samples of pure sorrel. Similar as with wood fuel, the solution of decreasing NO_x is in controlling combustion temperature (below 1 100°C) or already mentioned recirculation of part of the flue gasses back into combustion process.

If we want to assess each combustion device, than the best results were reached with the ones that were constructed with adequately large, non-cooled space for burning of volatile combustible and with suitably designed inlets of air. In our case it was modification H. This result is in good accordance with our theoretical presumptions.

CONCLUSIONS

All above mentioned facts indicate that biomass combustion is an issue which is necessary to solve responsibly and meticulously. If biomass is burned in accordance with all its requirements, its utilisation does not burden the environment and its renewability ranks it between fuels that can be count on in the future. But even there still remain many unsolved questions that will be apparently subject of further research in this field.

REFERENCES

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Accepted for print: 1.04.2008

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