In many cases including Latvia, the existing renewable resources potentials could improve significantly the energy supply and decrease the dependence on fossil energy resources. However, the yield of wood wastes (from mechanical processing, cutting, etc.) has its own limitations. Therefore, a considerable increase in energy production from renewables is possible by development of forest plantations and special agricultural crops. 
 The aim of the report is to present the state-of-the-art in the field of energy production from wood and prospects for positive changes in the future. 

 Special attention is paid to the necessity of developing technological processes and products on the basis of wood and wood wastes enabling to obtain: 

• easily transportable solid fuel with a higher heating value (charcoal, fuel briquettes from dispersed biomass, etc.); 
• liquid fuel (ethanol, thermal conversion resins); 
• special auxiliary substances applicable for cutting energy consumption in various power-intensive processes (grinding, dispersion for pumping-over of high-viscous and concentrated suspensions, plasticization, etc.); 
• valuable chemical products (levoglucosan, levoglucosenon, sorbents, active charcoals), using the possibility of combining the processes of their production with energy production (combustion, pyrolysis) and gaining profit for the local energy consumption structure; 
• energy/heat, using different biomass types (sewage sludge, garbage, refuse, etc.) as well as fossil fuels and plastics wastes, ensuring not only energy saving but also improving economic indexes of the process (decreasing NO2 and S contents in gaseous emissions), under conditions of a joint thermal conversion with wood; 
• environmentally friendly products (plant growth activators, etc.  necessary for forest restoration, plantation cultivation, forest management, ultimately, the augmentation of wood stocks and the improvement of wood quality; 
• energy of wood stands/wood containing toxic substances, radionuclides inclusive, thereby decreasing the possibility of environmental contamination occurring in cases of natural calamities. 

Introduction 

Latvia, as the majority of countries in transition from a centrally-planned economy to a market one, is interested in a decreased dependence from the import of fossil fuel by mobilizing and sustainable usage of internal resources. Utilization of wood/plant and different biomass processing wastes open  strong possibilities in the decision of this problem. 

Forests are the main natural resource in Latvia. However, wood is not used enough in the Latvian centralized power system. 

Forests occupy 44.6% of the Latvian area [1]. The main forest-forming species are listed in Table 1. The total current annual increment in Latvia is equal to 16.5 million m3. The allowable forest harvesting volume exceeds 8 million m3, and 85% of this is actually cut at present [2]. It is a wonderful potential basis for the development of the national economy. However, forest utilization in Latvia is currently extensive, and its structure (Table 2), including the export (Fig. 1, Table 3) is  low profitable. Approximately 45% of stocked wood is exported in the form of round timber, chips and firewood. 

Today's Latvia has a project for a kraft pulp mill with a capacity of 600 thousand tons of cellulose per year. However, this project can be realized only provided that foreign investments are attracted. The structure of wood usage should be improved when this mill is put in operation. 

The scheme of wood resources usage in Latvia, taking into account the current consumption structure, is shown in Figure 2 [2]. In the framework of the existing forestry the volume of wood resources for energy production is limited to 3.6 mln. m3 under condition of maintaining the present forest productivity. However, taking into account the potential alternatives for wood waste utilization, the actual potentialities are much lower. 
It should be mentioned that, at the current scheme of export, a considerable part of waste wood, that could be used by the local energetics, replenishes the energy base of timber importing countries. 
 
Energy production by direct combustion 
 
Biomass combustion is the oldest and most popular method for energy production. The advantages of the combustion method for wastes treatment are an approximately 10-fold decrease in the waste volume, the reduction of the risk of soil and water pollution, and the recovery of the heat formed. The problem of increasing the efficiency of chips digesters from 70 to 80% is currently under discussion in Latvia. The solution of this problem will enable to save 0.143 m3 of wood per 1 m3 of fuel [3]. 

The small and medium enterprises are more attractive for Latvia. Such enterprises are characterized by a higher environmental compatibility, especially due to the local nature of renewables stock, and serve as a basic source of a new opportunity for the employment and development of the state infrastructure on the regional level. At present, more than 240 boilers have been reconstructed with the total capacity over 28 MW, and in the nearest few years, the number of boiler-houses will increase up to 300, particularly in small towns and rural districts. The introduction of modern technologies for heat generation from wood in Latvia has been sponsored by international programmes of the Nordic countries, particularly Denmark and Sweden [4]. 

At present, special attention is paid to the production of fuel briquettes from fine waste wood (sawdust, bark, etc.). In the 60s, Latvia had had an experience of the production of fuel briquettes from raw waste lumber. The briquetting of sawdust had not been practically used earlier in the national economy. The output of fuel briquettes is estimated to reach 0.18 million t/year in the case of utilizing the whole sawdust concentrated in saw-mills. 

The utilization of sawdust and bark for energy production is rather promising because up to 0.50 million m3 of sawdust and 0.30 million m3 of bark per year are currently concentrated in saw-mills of Latvia. 

At present, the production of fuel briquettes and granules from sawdust without a binder, satisfying the Standard DIN 51731, with the minimum density 1.0 kg/dm3 and the heating capacity 16.6-18.8 MJ/kg has been started in Latvia [5]. 

Briquettes and granules, including those produced using energy additives (coal siftings, mazut, organic materials) are a more pure fuel as compared to mazut and coal. Co-combustion of waste wood with fossil fuel (coal, oil products) or different organic materials improves the ecological parameters of the process, reducing the content of nitrogen oxides and sulphur compounds in the composition of the gases formed. 
 
There is a possibility of the production of briquettes without additional additives. In this case, wood components, owing to the transformation of their properties during the briquetting process, act as an additive. One of such technologies of fuel briquettes production has been developed by the Latvian State Institute of Wood Chemistry (IWCh). 
 
Mechanical wood processing wastes can be used for energy production in a mixture with activated sludges of water treatment plants. The presence of wood in the mixture enables the combustion of activated sludges with a moisture content of approximately 70%, without preliminary drying. In this case, despite high nitrogen and sulphur contents in the sludges, the presence of wood promotes a decrease of the formation of toxic gaseous products of combustion owing to the activation of oxidation/reduction processes during the catalytic combustion in the fluidized bed reactor [6]. 
 
A considerable share of bioenergy in the total energy balance of many countries is ensured owing to the combustion of lignin-containing wastes of the pulp-and-paper industry. The energy of 1 kg of lignin is equivalent to that of 0.6 kg of oil [7]. For example, from 12% of the energy produced in Finland, a half is obtained as a result of lignin combustion. The sulphate pulp mill, whose construction is conceptually backed by the Latvian Government, will use the same energy source. 
 
It should be mentioned that approximately 10% of the sulphate lignin produced can be yielded from spent sulphate liquors, without any detriment to the power system of the enterprise, and a high benifit can be gained from realizing the products obtained on the basis of the yielded lignin. 
 
Although the direct combustion of biomass for energy production is, undoubtedly, rather attractive, in some cases, a considerably higher economic effect can be reached by a two-stage biomass conversion. According to this scheme, the wood biomass is, first of all, subjected to chemical, thermal or any other type of processing, with obtaining of definite products and subsequent combustion of gaseous, liquid or solid processing wastes (Fig. 3) [6]. 
 
Thermal conversion of biomass for energy production 

The existing technologies of the thermal conversion of biomass for energy production, besides direct combustion, can be divided into the three main groups: gasification, liquefaction, and pyrolysis. The list of the thermal methods for biomass conversion shown in Fig. 4 [6] is not complete and is constantly being extended. 

In comparison with the combustion method, the treatment by gasification has the significant advantages: the gases obtained may be used as the energetical or technological fuel, while during combustion, only the energetical use of heat from the raw material is practically possible [6]. 
 
The recent and on-going fundamental research reveals expanding options for biomass gasification for production of electric power or fuels of higher value. Various pilot and demonstration plants are currently operated all over the world to aid the design and development of cost-effective and environmentally sound technologies for bioenergy production by gasification. 
 
It is difficult to propose the implementation of gasification in Latvia in the nearest future, while power production systems, where biomass gasification is coupled to advanced gas turbine cycles, are at a stage of pilot and small demonstration plants. The location of possible demonstration facilities in Latvia, that occupies a convenijent geographical position relative to other Baltic states and Belarus, could be the best opportunity for commercialization of the  gasification process. 
 
Liquefaction could be an attractive option for feedstocks with a high water content, such as agricultural and domestic wastes or biosludge. A high energy consumption of the liquefaction process complicates its utilization in countries with a deficit in energy production. 
 
The dry distillation method ensures an effective use of organic wastes as a fuel (gas with a high heat of combustion and a solid carbon residue) and liquid products (ethylacetate, methanol). Previous  pyrolysis techniques have a low efficiency, but the pyroligneous liquors derived from such processes have some  commercial value. 

The major product of dry distillation of wood, charcoal, is used as a clean fuel of high calorific value. The charcoal production technology is not complicated and is applicable to small and medium rural enterprises with an output of 500 to 1500 t of charcoal per year, which will process 3200 to 9500 m3 of wood per year [8]. 
 A technology has been developed and recommended by IWCh, in which 4 to 6 apparatuses for charcoal production are combined in units with a joint furnace and a technological wood drier [8]. Volatile products of thermal degradation of wood, gas mixture combustion fumes, are used as a fuel for maintaining the dry distillation process as well as heating and drying of wood. A rational gas supply and a high temperature of combustion in the furnace ensure the conformity of the products of combustion to the standards defined by environmental protection and sanitary supervision organizations. 

Charcoal production for domestic needs is especially advisable in most remote regions, far from ports, autoroads or railroads, since, in terms of mass, charcoal comprises only one third of the wood mass and, in terms of volume, it comprises  60 to 70% of the initial wood volume. From 1 cubic meter of wood with a packing density (compactness of wood) of 0.65, 110 kg of commercial charcoal and 5 to 10 kg of fineness (siftings) (with sizes of below 20 mm) can be obtained. From 1 cubic meter of alder wood, 80 to 85 kg and 5 to 10 kg of charcoal and fineness, respectively, can be obtained. In 1997, 5000 tons of charcoal were produced in Latvia. 
 
According to the results of our work, charcoal siftings can be used for production of active carbon of high efficiency [8]. 
 
Numerous variants of pyrolysis methods (different temperature levels, heating rates, catalysts) provide different orientation and efficiency of the processes. In this case, high-quality products, not designed for energy production, can be obtained. 
 
The quality, properties and yield of carbonaceous products, obtained by way of pyrolysis from various renewables, are determined mainly by the characteristics of the source raw materials. In this case, the yields vary within 15 to 35%. Techniques and regimes of the catalytic low-temperature pyrolysis were developed by IWCh [9-12]. They provide the obtaining of: 

High-quality carbonaceous sorbents with the total specific surface 2000 m-2/g and the ion-exchange capacity 4.6 mg-eqv/g, comparable with the characteristics of the typical commercial  ion exchangers (oxidized active carbons) [56, 67]. The characteristics of the porous structure of the sorbents obtained excell the corresponding parameters of the best samples of commercial active carbons 2-3 times, and are comparable with the properties of expensive carbonaceous fibres. The potential fields of their application are medicine, the food industry and cosmetology [9,10]. 

Iron-containing organomineral sorbents with magnetic properties. The peculiarities of the chemical composition of the given type of sorbent make it possible to recommend them both for gases and air purification from sulphur-containing admixtures and purification of liquid media (for example, from oil contaminations), with subsequent extraction of the spent sorbents in the magnetic field [11,12]. 
 
For production of iron-containing sorbents, activated sludge from wastewater treatment plants [13] as well as the organic residue after biogas production can be used. 
 
Low-temperature pyrolysis, when realized in the fast heating regime, is considered to be rather a feasible method for production of fuel and chemicals [6]. Pyrolysis processes for liquid fuel production from biomass, and several projects are currently at the commercial and demonstration stage. This technology ensures production of bio-oil (a potential substitute for fossil fuel), and charcoal and gases are obtained as subproducts. The implementation of pyrolysis processes for energy production could serve as a basis for a simultaneous development of an operating capacity for production of valuable commodity such as sorbents and chemical substances for pharmacy and organic synthesis. 
 
Oxidative pyrolysis, often being one of the stages of the gasification process, implies thermal degradation of organic raw materials during a partial combustion of volatile products. During oxidative pyrolysis, coke (a solid carbonized residue) is formed, while the mineral products (ash and slag) are the solid residues of gasification and combustion. The carbon product formed during oxidative pyrolysis may be used further as a solid fuel or, after activation, applied as a sorbent. 
 
Pyrolysis regimes, ensuring the turn-over of the process towards the formation of individual chemicals, were realized. An original technology for obtaining of levoglucosan by lignocellulose pyrolysis has been designed at IWCh and tested at a pilot scale [14]. A good levoglucosan yield has been achieved: 20 – 26% from the mass of oven dry lignocellulose or 47.5 – 63% from that of cellulose. Purification of levoglucosan by selective dissolution and crystallization using 90 – 96% ethanol ensuring its content of 95 – 96% in the purified product has been also developed at IWCh. On the basis of levoglucosan, a whole range of valuable chemical products were synthesized and new materials on their basis were created, including ethers and esters, polyurethanes, films, adhesives, UV-polymerized composites, etc. [14, 15]. 
 
Acidically catalyzed pyrolysis makes it possible to obtain another promising chemical product, dehydrated 1,6-anhydro sugar - levoglucosenone (LGS) [16]. Levoglucosenone has proven to be a very convenient "chiral synthon" practically in all the fields of the chemistry of organic synthesis [17], for preparation of a variety of biologically active natural products, e.g. optically active sulphur and nitrogen heterocycles, rare carbohydrates (nonhydrolyzable C-di- and C-trisaccharides - potential enzyme inhibitors), etc. 
 
Studies aimed at the development of LGS production technologies are carried out by IWCh jointly with the Hamburg Institute for Wood Chemistry. The LGS content in volatile products may exceed 70% [18]. 
 
The pyrolysis technology for energy production is supposed to be realized easier in Latvia owing to its lower energy consumption (in comparison with gasification) and a possibility of the production of valuable and promising chemicals using the same equipment, depending on the local and common market needs. 

Energy production and chemical processing of wood  
 
As regards the process of acid hydrolylis of wood with the production of fuel ethanol, then, despite the intensive studies carried out in many countries and the offer of novel improved technologies, their realization in conditions of Latvia is problematic. First of all, this is connected with the fact that, for the time being, the ethanol cost is such that, without state subsidies, it cannot be used as a motor fuel. However, in Latvia's conditions, there is a real chance for implementation of original technology of acid hydrolysis of wood (developed by IWCh) with the production of furfural, that could be widely used both on the internal and international markets. 
 
The considerable lignocellulose residue formed in this case can be processed, for example, thermally to levoglucosan or levoglucosenon. 
 
Based on this residue, according to methods offered by the Institute of Wood Chemistry, high-efficient silica fertilizers, complex fertilizers of the prolonged action and plant growth activators can be produced, and sorbents with a high sorption activity in terms of phenols (including chlorinated ones), aromatic nitrogen-containing compounds, etc. as well as low-molecular organic compounds (sorption in terms of iodine may reach 800-1000 ml/g) may be synthesized [19-21]. 
 
Wood delignification processes besides the main product, pulp (cellulose), yield by-products which may be used not only indirectly for energy production (combustion of lignin-containing liquors, which has been mentioned already), but also for energy savings. Thus, sulphate lignin and tall oil serve as a raw materials for synthesis of the polyol component of polyurethane foams [22-25]. 
 
Polyurethane freon-free foams obtained on the basis of tall oil according to a technology developed by the Institute of Wood Chemistry have been used for 7 years already for the exterior thermal protection of buildings, thereby reducing energy consumption for heating [25]. 
 
On the basis of lignin, products characterized by an enhanced surface activity and dispersive properties have been synthesized, whose application in processes of grinding pumping high viscosity and concentrated suspension and mixing in heterogeneous systems [26]. 

Development of biomass raw materials for energy production  

A considerable increase in bioenergy production is possible by establishing of new plantations on agricultural and pasturelands and promoting the natural regeneration in secondary forests. According to the data available in the literature, poplar coppice provides approximately 6 TOE (tonnes of oil equivalent) per ha, compared with 1 TOE per ha for oil seed rape, widely grown for conversion into "biodisel" in Germany, Austria, Slovakia [27]. However, special subsidy mechanisms are necessary to finance the establishment of wood fuel short-rotation coppices lots. Up till now, Latvia has no comprehensive replanting programmes. 

The forestation of fallow lands, whose current approximate area in Latvia is equal to 500 thousand ha, owing to the inefficiency of agriculture, is estimated to increase up to 1 million ha in the future [28]. 

At present, the area of forest plantations in Latvia is estimated as 7.1 thousand ha and 1.2 thousand ha for spruce and aspen, respectively. In connection with the defeat of coniferous wood stands by the fungi Heterobasidion and Fusaria gray alder plantations have been established recently. At present, 7.0 thousand ha of gray alder plantations are located on defeated forestlands. The growing of species such as alder (Alnus glutinose) and aspen (Populus tremula) is very promising for Latvia [28]. 

An annual increment of wood under optimum plantation conditions would be equal to 6.2 m3/ha and 7.7 m3/ha for alder and aspen, respectively. However, the real current increment, taking into account sanitation cuttings, is considerably lower (Fig. 5). Therefore, in 20 years of plantation growing as of the moment of cutting, the stand will be equal to: 120 m3/ha and 169 m3/ha for alder and aspen, respectively. The productivity of 40-year old plantations is approximately the same as in the case of natural forests (Fig. 6) [28]. 
 

For this purpose, novel lignosilicon-containing products on the basis of lignin, a waste from chemical wood processing, have been synthesized at IWCh and have been tested in a nursery to promote the development of the root system, protect the plant against diseases and stimulate the plant growth. Tests of these novel products of natural origin are rather promising. In one year of growth, these products in a dosage of 1 g per running meter ensure a two-fold increase in the root mass in comparison with the control; the amount of secondary roots increases significantly; the stem diameter increases by 70%. The amount of the plants defeated by Fusaria sp. decreases [29]. 

In many cases, energy production seems to be the only possible method for utilization of the forest biomass grown on soils polluted with toxic pollutants from the metallurgical and chemical industries, radionuclides, etc. Cultivation of quick-growing poplar species on soils polluted with radionuclides allows to extract radionuclides, in particular, 137Cs and 90Sr, from the soil and concentrate them [30]. However, special environmentally sound technologies should be developed to utilize this contaminated wood as a fuel wood [31]. The large oil refinery in Mazeikai, Lithuania, on the border with Latvia, has affected dramatically the forest conditions in our country. 

It has been established that, in 1975-1996, an annual increment of pine wood in forests within the 30-km area around the refinery made up only 55% of the adequate annual increment in similar forests, which had not been subjected to the technogenic influence. The vitality of trees in this area is reduced by 30%, while that in a 10-km zone by 50%. The data obtained allow to forecast a fast drying out of trees in this area during the next 10-15 years. 

The rapid drying out of forests in the zone of the action of the emissions from the Mazeikai oil refinery is accompanied by a profound change in the wood structure,  the reduction of its antibacterial and antimycological immunity and a considerable decrease in the mechanical strength. Hence, the bioenergetic utilization of such wood with the production of two-stage products of thermolysis (levoglucosan, levoglucosenone, etc.) at the first stage of processing, and fuel gas at the second one is the most rational one [32]. 
 
The exclusion of a part of low-quality and plantation wood from the group of direct raw materials resources used for bioenergy production can be compensated by the use of other renewables, including agricultural crops, recycling lignocellulosic materials, municipal and industrial sludges, water treatment sludge, etc. under condition of maintaining the corresponding productivity of forests. 
 
Conclusions  

1. In many cases, the one-stage scheme of wood processing for heat and energy production by direct combustion is less efficient as compared to the two-stage scheme. In the latter case, the mechanical wood working wastes are, first of all, processed chemically, thermally inclusive, or by another way, obtaining definite products, including those for energy needs; then gaseous, solid or liquid wastes are burnt. 

2. Establishment of productions for comprehensive processing of wood with obtaining of not only chemical pulp and secondary products of this production, lignin, tall oil, etc., but also a whole range of unique products from wood bark and foliage biomass as well as different derivatives of secondary products is a promising trend. 

3. The market economy is favourable for realization of new engineering solutions for wood chemical processing enterprises  (including those in Latvia), currently suggested by the world's science. Such an approach, comprising also the engineering solutions developed by IWCh, guarantee competitiveness and can rank Latvia among countries capable of offering promising chemical and biochemical products to the world markets. Such products may be levoglucosenone, levoglucosan, selective sorbents, biologically active substances, high-efficient active carbons, interface additives, etc. 

4. Artificial afforestation of the fallow agricultural land as well as the lands left by the army is a promising trend in the increase of wood production for needs of industry and energetics. The total area of such lands makes up 1 million ha. 

5. To realize all this in Latvia within the framework of the mentioned programmes, involving also the international partnership, the following is necessary: 
 - Evaluation of the economic value and prospects for energy production from by-products; 
 - Valorization of the local potentialities of renewable energy resources; 
 - Comparative evaluation of the economic value of direct energy production from wood biomass and obtaining of market valuable products by chemical processing of biomass with simultaneous energy production. Determination of the plant biomass volume required for this purpose; 
 - Maximum utilization of different renewable domestic wastes, including municipal wastes, water treatment sludge, fractionated garbage, etc. for energy production; 
 - Selection among a great number of alternatives for modern environmentally sound wasteless technologies for energy production to prevent their fast obsolescence. 

6. In Latvian conditions, similar to the North and other countries, a biodiesel, bioethanol and biogas production programme is envisaged. However, at present, taking into account relatively high expenses for biofuel production and low purchasing prices for petroleum products as well as the necessity of investments and low state subsidies, the programme's realization may be real only as a demonstration project. 

7. A definite complex of research and technical measures aimed at the development of the chemical, biochemical and thermal processing of biomass for energy production and different biomass-derived products as well as the manufacture of special chemicals is envisaged. 

References 

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Note: 
Due to space limitation, mainly results of the investigations of Latvian scientists are mentioned in the References list. A careful analysis of the literature sources available all over the world has been done in the aforementioned references.