( They ( ( can ( be ( used ( to ( ( produce energy ) ( in ( ( the form ) ( of ( ( heat and ) electricity ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (PRP They)) (VP (MD can) (VP (VB be) (VP (VBN used) (S (VP (TO to) (VP (VB produce) (NP (NN energy)) (PP (IN in) (NP (NP (DT the) (NN form)) (PP (IN of) (NP (NN heat) (CC and) (NN electricity))))))))))) (. .)))
They can be used to produce energy in the form of heat and electricity.
(ROOT (S (NP (NNP Heat)) (VP (VBZ is) (VP (VBN produced) (SBAR (WHADVP (WRB when)) (S (NP (NN paper)) (VP (VBZ is) (VP (VBN burned))))))) (. .)))
Heat is produced when paper is burned.