EFFICIENCY AND EMISSION OF CROP RESIDUES COMBUSTION FACILITIES IN SERBIA – STATUS AND NEEDED MEASURES FOR IMPROVEMENT

Use of crop res i dues as fuel has a long tra di tion in ru ral ar eas of Ser bia. Nu mer ous bio mass-fu eled fa cil i ties were built dur ing the 80s. Among those were small fa cil i ties for house hold heat ing, with ther mal power from 5 to 50 kW, me dium-size fa cil i ties for farm and green houses, with ther mal power 50 to 1000 kW, and large fa cil i ties for pro cess ing of ag ri cul tural prod ucts, with ther mal pow ers higher than 1000 kW. The re sults showed that the level of bio mass com bus tion fa cil i ties is in gen eral very low. This is es pe cially the case for heat ing fa cil i ties used for house hold heat ing. The mea sures for im prove ment were pro posed.


Introduction
Use of crop res i dues as a fuel has a long tra di tion in Ser bia, es pe cially in typ i cal ag ri cul tural re gion Vojvodina.Tra di tional stoves are still used, orig i nal or re de signed.Newly are used di verse small boil ers, in dus trial prod ucts or self made.Based on au thors ex pe ri ence was setup hy poth e sis that the ef fi ciency of them is still very low, and emis sion of pol lut ants high.
In ten sive con tem po rary use of solid bio mass -crop res i dues, as re new able energy source, be gan in the prov ince of Vojvodina in 1981, af ter en ergy cri ses.First fa cil ities were im ported and used for heat ing of work shops at large ag ri cul tural farms.In the mid 80s first do mes tic de signs for ther mal plants for com bus tion of straw, corn cobs, and sun flower straw and pro cess ing res i dues were de vel oped and con structed.There are two ba sic types of ther mal plants in use: with batch-fired and con tin u ous feed ing of the combus tion cham ber.Mea sure ments have shown that most ther mal plants have low en ergy ef fi ciency, fol lowed by high level of pol lut ants' emis sion [1][2][3].That causes fi nan cial losses and is harm ful for en vi ron ment.Leg is la tion and tech ni cal guide lines for min i mal value of en ergy ef fi ciency and per mit ted emis sion of the pol lut ers are miss ing.Di rect appli ca tion of the legislations from de vel oped coun tries can cause high ad di tional ex penses of pro duc tion and re sult in de creas ing ap pli ca tion of bio mass as an al ter na tive fuel.Follow ing the world trend in de vel op ment, and hav ing in mind do mes tic dif fi cul ties with the in suf fi cient amount of en ergy, it is nec es sary to make the uti li za tion of bio mass widely spread.

Objectives
The main ob jec tive of the in ves ti ga tion was to re view the sta tus of bio mass fa cili ties, stoves, and boil ers, con cern ing: gen eral tech ni cal level, ef fi ciency, and emis sion of pol lut ants.Fur ther ob jec tive was to de fine the needed im prove ment in mea sur ing.That ap plies to small fa cil i ties for house hold heat ing, with ther mal power from 5 to 50 kW and also me dium fa cil i ties for farms and dis trict heat ing, with power higher than 50 kW.

Materials and methods
There were two groups of ac tiv i ties: (1) Analyzing of small scale crop residues heating facilities and assessing their efficiency and emission characteristics based on technical solutions' applied -survey.(2) Testing biomass stoves and boilers following the standard procedures.
The sur vey of heat ing stoves and small boil ers was con ducted on a typ i cal Vojvodinian vil lage.Twelve fa cil i ties were in cluded, from tra di tional Pannonian stoves to hand-made boil ers with ther mal power from 5 to 50 kW.Tech ni cal level of fa cil i ties was eval u ated.Only in few cases, mea sur ing of ex haust gas tem per a ture, con tain ing CO and CO 2 , as a base for ef fi ciency and emis sion of pol lut ants was con ducted.The tests were per formed ac cord ing to the of fi cial pro ce dures: na tional stan dard JUS M.E2 203 [4], JUS M.E6.110 [5], and JUS M.R.4.020 [6], based on DIN 1942DIN (1979)), and sim i lar to ISO R889 [7].For gas test ing Testo 350 was used, and wa ter flow was mea sured by Panametric PT 868-2.For tem per a ture mea sur ing ther mal cou ples NiCr-Ni were used.

Survey
Stoves are stoked man u ally, mostly with corn cobs.Many stoves were built with out pro fes sional su per vi sion.Out dated tech ni cal so lu tions are vastly used be cause the in vest ment value is smaller.Based on the re view ing of twelve small stoves, the follow ing was con cluded.
(1) Designs of stoves are outdated, in some stoves there are no grate, there is no control of combustion air, primary and secondary air supplies are not separated.(2) In some of them the combustion, i. e. combustion air flow, is controlled by opening of ash-tray or combustion chamber door.(3) The chimney designs are not adequate, concerning the dimensions and insulation.
Based on ob served man i folds en ergy ef fi ciency was as sessed to be in the range 30 to 50%.The as sess ment of pol lut ants' emis sion was not pos si ble with out mea sur ing, and can be only con cluded that it is surely over de fined up per lim its.
The re sult of sur vey of bio mass boil ers showed sim i lar re sults.All of them had very sim ple man ual con trol of com bus tion air.High est air con trol level was the use of Burdon's pipe.En ergy ef fi ciency was es ti mated to be 40-60%.
Among the ad vanced farm ers was re corded high in ter est on bio mass use, but the fa cil i ties have to be mod ern ized, of fer ing con tem po rary tech ni cal so lu tions and higher com fort.

Testing
Table 1 shows the re sults of test ing for small and me dium stoves with bio mass com bus tion.From tab. 1 it is ev i dent that there is a low de gree of en ergy ef fi ciency with the small and me dium stoves, even though con trolled stok ing with packed bio mass (straw bri quettes, bales of straw, wood logs) was in ques tion.The value of en ergy ef fi ciency var ies from 26 to 84% or in av er age 55%.The fun da men tal prob lem with these stoves is that there is no pos si bil ity of del i cate reg u la tion of com bus tion.Co ef fi cient of the ex cess air is very high and var ies from 2.6 to 19 or in av er age 10.8.This cools down the com bustion cham ber and causes poorer pro cess of com bus tion, which de creases en ergy ef ficiency and in creases the emis sion of harm ful gases into the at mo sphere.The amount of CO 2 is be tween 0.9 and 10.5% which is the re sult of high co ef fi cient of the ex cess air rate.The amount of CO is up to 5,600 ppm, which is not too high for bio mass com bus tion.N-ox ides were mea sured in only one case, be cause in the pro cess of bio mass com bus tion its amount is very low as a re sult of low com bus tion tem per a ture (600 to 750 °C).

Table 1. Efficiency and emission of pollutants of biomass stoves, [1-3]
From tab. 2 it is ev i dent that there is a low level of ef fi ciency with small and medium boil ers on baled straw.It var ies be tween 31 and 73% or in av er age 52%.A spe cific prob lem with the wa ter heated boil ers is that there is no pos si bil ity of del i cate reg u la tion of com bus tion.Co ef fi cient of the ex cess air is very high and var ies from 1.3 to 8.8 or in av er age 5.1.This re sults in poorer pro cess of com bus tion, which de creases the en ergy effi ciency and in creases the emis sion of harm ful gases into the at mo sphere.The amount of CO 2 var ies be tween 2.5 and 15.3% which is a con se quence of high ex cess air rate.The amount of CO is up to 9,023 ppm, which is high for bio mass com bus tion.The con tent of or gan i cally tied car bon and solid par ti cles in the smoke exhaust was mea sured only in one case.Re sults of the re search show that the real val ues of the char ac ter is tics of ther mal plants de pend on sev eral fac tors: type of bio-fuel, size of chip ping, con tent of mois ture, dos ing bio mass (man u ally, au to mat i cally or me chan ically), type of com bus tion cham ber, mode of pro vid ing air (with or with out a ven ti la tor), place of air in let (be neath the grate and/or above the bio-fuel layer, par al lel with the grate, etc.), air flow con trol in the pro cess of com bus tion (with or with out any kind of lip), the tem per a ture of the com bus tion cham ber, gas pres sure in the cham ber, the tem per a ture of the ex haust gases in the chim ney, amount of phys i cally and chem i cally non-combusted bio-fuel, loss of ther mal en ergy in the en vi ron ment, etc.

Table 2. Efficiency and emission of pollutants of straw bales boilers, [1-3]
Based on test re sults, sur vey of prac tice and ex pe ri ences abroad the fol low ing val ues of ef fi ciency of bio mass com bus tion fa cil i ties have to be stated by low for pe riod of next 6 years: (1) Thermal power 5 to 50 kW, efficiency should be for stoves higher than 50%, and boilers higher than 60%.(2) For boilers with power range 50 to 1000 kW efficiency should be over 65%.
Af ter this pe riod the EU stan dards have to be com pletely adopted.

Conclusions
Hav ing in mind ob tained re sults, fol low ing con clu sions can be made.Na tional leg is la tion on en ergy ef fi ciency and emis sion of bio mass en ergy fa cil ities should reach Eu ro pean level, but step by step.The first next 6 years should be pre para tory pe riod with "milder" leg is la tion -lower ef fi ciency and higher emis sion lim its.After this pre pa ra tory pe riod full Eu ro pean leg is la tive should be ac cepted as na tional.
The first step should be in tro duc tion of oblig a tory test ing of bio mass en ergy facil i ties con cern ing en ergy ef fi ciency and emis sion of the pol lut ants.This should be done by au tho rized lab o ra to ries.Con se quently, one or more na tional test ing lab o ra to ries should be set up and ad e quately equipped.
It is nec es sary to con tinue the ac tiv i ties on im prov ing tech ni cal so lu tions for the ap pli ca tion of re new able solid bio mass with the ob jec tive to ac com plish high level of plant ef fi ciency, lower emis sion and in put of la bor.
There is a need to cre ate clear na tional pol icy on bio mass use as en ergy, with ade quate sup port ing mea sures.

h
-energy efficiency, l -coefficient of the excess air